INFORMAL PRELIMINARY ~mET~NG, SEPTEMBER 4. 49

Therefore be ~t resolved that the International Union of ~eodesy and Geophysics here assembled,

hereby voices its appreciation of the work in this field already done by several geodetic services and other bodies and of the part taken in this work by the submarfnes of the. navies of France, Grec~t Britain, Italy, Japan, ,the Netherlands, the United States of America and the U. S. 2. R. and expresses the hope that this work will be energetically continued when in the future the circumstances will permit to do so.

Signed : R. M. Field, Chairman, J. A. Fleming, Secre tary .

VI. TRAVAUX DE L'ASSOCIATION DE GI~0DP.SIE EN SI~ANCE PLI~NII~RE

PROCI~S-VERBAUX DES SI~ANCES

INFORMAL PRELIiFIINARY MEETING

Monday September 4, 1939, 14 h. 45 m.

An informal pre l iminary meeting of leading delegates of

the In te rna t iona l Associat ion of GeodesF was held in Room

D of the Washington Hotel September 4, 1939.

The following persons were in at tendance : F. A. Vening Meinesz, President ; W. D. Lambert , acting as Secre ta ry ;

O. 2. Adams, K. F. Baeschlin, C. L. Garner, J. de Graaf/ Hunter, W. Heiskanen, H. S. Jelstrup, G. T. McCaw, N. Ogil- vie, J. E. R. Ross, H. Seligmann, W. D. Sutclif/e and C. H.

Swick, Delegates.

Dr. Vening Meinesz s ta ted tha t the purpose of the mee-

t ing was to get things s t ra igh tened out prep~ro tory to the

formal meet ing scheduled for 3:30 p. m. Due to the poli t ical

s i tuat ion, several meet ings of Officers of the Union and the

50 TRAVAUX EN SF.AN~E PLI~NIERE.

A m e r i c a n Organ i z ing Commi t t ee ~Tere cal led p r i o r to the

A s s e m b l y to d i scuss w h e t h e r or not i t should be held. Dr.

Ven ing Meinesz gave a b r i e f synops i s of the r e su l t s of t he se

mee t ings . He s t a t e d t h a t eve ryone was ag reed t h a t the

A s s e m b l y shou ld cont inue as p l anned and i t was s t r e s s e d

t h a t t h e w o r k of the scient i f ic congresses "was needed now

m o r e t h a n eve r before . U n d e r t he c i r c u m s t a n c e s i t was

dec ided no t to d iscuss a d m i n i s t r a t i v e s ma t t e r s , m a t t e r s of

f inance and elect ions, b u t to devote the t ime to d i s cus s ing

scient i f ic ac t iv i t i e s .

Mr. L a m b e r t s u m m a r i z e d the r e p o r t of the m e e t i ng o f the

I n t e r n a t i o n a l A s s o c i a t i o n of Geodesy which was he ld a t

2 :00 p.m. a t George W a s h i n g t o n Unive r s i ty , S e p t e m b e r 1,

1939. D u r i n g the course of t h e m e e t i n g d iscuss ions were

he ld as to w h a t should be done a t the 3:30 m e e t i n g and i t

was dec ided to s t a r t t h e m e e t i ng b y Dr. VENING-MEINESZ

r e v i e w i n g GENERAL PERRIER'S repor t . I t was also dec ided

to call off t he schedu led m e e t i n g of the E x e c u t i v e Com-

m i t t e e S e p t e m b e r 5 and i n s t e a d hold a me e t i ng of the Com-

m i t t e e on Con t inen ta l and Oceanic S t ruc tu re , of which Dr.

R. M. FIELD is Cha i rman . This m e e t i n g is schedu led fo r

10:00 a.m. and wil l no t be l imi ted to m e m b e r s of t he Asso-

c i a t ion of Geodesy , b u t wi l l be open to eve ryone in t e r e s t ed .

I t was po in t ed out t h a t the p r o g r a m would be changed so-

m e w h a t in o r d e r t h a t the people who m u s t leave s h o r t l y

fo r E u r o p e could make t h e i r con t r i bu t ion to t he A s s e m b l y .

The p r o g r a m was gone t h r o u g h e n t i r e l y and the changes

a r e g iven in the r e p o r t of the f o r m a l meet ing .

L a m b e r t commen ted t h a t t h e r e m a y be t imes when the

P r e s i d e n t and S e c r e t a r y could not be p re sen t a t the. mee-

t i ngs a n d s u g g e s t e d p r o v i d i n g subs t i tu t e s . COMMANDER

GARNER a n d Mr. Swlc• were men t ioned as m a k i n g exce l l en t

V i c e - P r e s i d e n t s and t h a t Messrs . JELSTRUP and SUTCLIFFE

would m a k e good sec re ta r i e s .

The m e e t i n g closed a t 3 :20 p.m.

FORMAL MEETING, SEPTEMBER 4. 51

FORMAL MEETING

Monday, September 4, 1939, 15 h. 40 m.

A formal mee t ing of the Associa t ion of Geodesy was held

in Room D of the W a s h i n g t o n Hotel. The meet ing was a t t en-

ded by the fol lowing persons : F. A. Vening Meinesz, Pre-

s iden t ; W. D. Lambert , ac t ing as Secre tary ; O. S. Adams,

H. G. Avers , C. F. Baeschlin, N. Boneff, C. J. Clifford, F. L. Culley, F. W. Darling, J. A. Duerkse.n, C. L. Garner, L. P. Gibbon, J. de Graaff Hunter, W. He'~skanen, H. S. Jelstrup, G. T. McCaw, N. Ogilvie, K. D. P. Ros~n, J. E. R. Ross, H. Zeligmann~ W. D. Sutcliffe and C. H. Swick, Delegates.

Dr. V e n i n g Meinesz opened the meet ing by m e n t i o n i n g

tha t due to the polit ical s i tua t ion it was necessary for m a n y

persons to r e t u r n to the i r coun t ry and therefore the mee t ing

was not as well a t t ended as had been hoped. He said t h a t

several mee t ings of officers of the Un ion and the Amer ican

Organ iz ing Commit tee were held pr ior to the formal opening

of the Assembly to decide whether or not it should be held.

All p resen t agreed t ha t it should cont inue as p lanned and i t

was s t ressed tha t the work of scientific congresses was

needed now more t h a n ever before. I t was resolved to hold

meet ings of the Assembly as scheduled and, in view of the

fact t ha t m a n y persons could no t be present at the meet ing,

it was t hough t well to avoid ma t t e r s of admin i s t ra t ion , fi-

nances, and elections, and to adhere s t r ic t ly to scientific

mat te rs .

Following this Dr. Vening Meinesz read the names of

those who have died since the Edinburgh Assembly and all

present stood and kept a moment's silence in their memory.

Dr. Vening Meinesz gave a summary of GENERAL PERRIER'S

report.

Members were as!~ed to express their opinion on General

Perriers's viewpoints regarding subscriptions. It was mo-

52 TRAVAUX EN S~ANCE PL#.NI~RE.

red, seconded, and carried that General Perrier's suggestion

that the increase to 15,000 French francs of the Associa-

tion's contribution to the International L@titude Service be

approved.

Members were also asked to express their opinion regar-

ding the contribution of 30,000 French francs towards the

w o r k of t he Inst i tute of Isostatic Reduction at Helsinki. All

were a g r e e d to con t inue th i s subscr ip t ion .

Dr . Ven ing Meinesz exp re s sed his t h a n k s to the U n i t e d

S t a t e s Coas t and Geode t ic Survey, in the name fo the Asso -

c ia t ion, f o r t he g r e a t a m o u n t of coopera t ion g iven and es-

pec i a l l y t h a n k e d Mr. SwxcK.

The f inancia l s t a t e m e n t g iven in GENERAL PERRmR'S re-

p o r t was d iscussed , and t h r e e names were p r o p o s e d and ac-

cep ted fo r an A u d i t i n g C o m m i t t e e - t o i n v e s t i g a t e f inancia l

m a t t e r s of t he Assoc ia t ion , namely , Messrs . BAESCttLIN, OGIL-

Vm, a n d SELIGMANN, and al l ~ accepted a ppo i n t m e n t . Some

t ime was s p e n t in d i scuss ing t r a n s f e r r i n g of funds of t h e

A s s o c i a t i o n to some n e u t r a l na t ion in case of a g e n e r a l

wor ld war .

Ba~schl in m e n t i o n e d the Un i t ed S t a t e s as t h e mos t p ro -

bab le n e u t r a l na t i on b u t no f u r t h e r ac t ion was t a k e n in t h i s

connect ion .

The fo l lowing s u g g e s t i o n s were g iven as s u b j e c t s fo r the

fou r g e n e r a l m e e t i n g s of the Assoc i a t i on : T r i a n g u l a t i o n a n d

bases , level l ing , g r a v i t y and i sos tasy , a s t r o n o m i c a l work ,

f igure of t h e E a r t h , etc. I t was po in ted out t h a t some one

wel l a c q u a i n t e d w i t h the sub jec t would be a s k e d to l ead the

m e e t i n g s and t h a t in th i s w a y we could ge t more scient i f ic

w o r k done t h a n a t m a n y o t h e r mee t ings of the Assoc ia t ion ,

w h e r e much a d m i n i s t r a t i v e w o r k had to be done.

The p r i n t e d p r o g r a m was t e n t a t i v e l y rev i sed as fo l lows :

T u e s d a y , S e p t e m b e r 5.

10:00 a.m. (Room 201, School of Government , GWU) mee-

FORMAL MEETING, SEPTEMBER 4. 53

ring of Committee on Continental and Oceanic Structure,

of which Dr. R. IV[. FIELD iS Chainnsm, instead of the sche-

duled Executive Committee l~eeting.

Another meeting of the Association would be held at

2:30 p.m. in the same room with gravity and the figure of

the Earth as the subjects.

Wednesday, September 6.

10:00 a.m. (Room 102, School of Government, GWU) ano-

ther meeting of Dr. FIELD'S Committee to discuss geophysi-

cal work, earthquakes, etc.

At 3:30 p.m. the Asociation meets in Room 201 for dis-

cussion of astronomical work, latitude and longitude, and the

figure of the Earth.

Thursday, September 7.

President VENING 1VIEINESZ'S Presidential Address will be

held, as on the printed program, in I/oom 1, School of Go-

vernment, GWU. The second half of the meeting will be

devoted to isostasy.

Friday, September 8, 9:30 a.m. until 11:30 a.m. (Room

201, School of Government, GWU) meetings of Commissions.

Members will leave promptly at 1 p.m. for planned trip

under the auspices of the United States Coast and Geodetic

Survey to visit the International Latitude Observatory at

Gaithersburg, where a gravity party of the Coast and Geo-

dctic Survey will also be at work.

Saturday, September 9.

9:30 a.m. The question of a 2:30 meeting will be decided later.

Sunday, September 10.

Same as printed program.

Monday, September 11.

10:00 a.m., meetings same as printed program; at 2:30

Commission meetings if desired.

54 TRAVAUX EN SEANCE PLENII~RE.

Tuesday, September 12.

9:30 a.m. Subject will be t r iangula t ion and bases. COM-

MANDER GARNER will lead the discussion.

The 2:30 meeting will discuss questions of sea-level and

E a r t h tides.

Wednesday, September 13.

The Executive commi t tee Meeting at :t0:00 a.m. is aban-

doned. The joint Commission on Ocean Structure will meet

on this morning. The af ternoon will be devoted to an excur-

sion to a t r i angula t ion pa r ty of the Coast and Geodetic Sur-

vey as a r ranged on the program.

Thursday, September 14.

Meeting at I0:00 a.m.

At 2:30 general session of Associat ion of Geodesy. This

meet ing will take up levelling and resolutions of commis-

sions. Tho:e resolutions requir ing action of the ent ire

Union should come before the meet iag of Tuesday morning,

September 12, a t latest .

F r iday , September 15.

Same as pr inted program.

Thanks were given to Mr. LAMBERT for taking the respon-

s ibi l i ty of act ing as Secre tary of the Associat ion in the ab-

sence of General Per r i e r and all approved of his appoint-

ment. COI~MANDER GARNER and Mr. SwIcK were appointed

act ing Vice-Presidents of the Association. Messrs. JELSTRUP

and SUTCLIFFE were chosen as ass is tan t Secretaries.

Professor Boneff presented the following problem which

was re fe r red to General Per r i e r for correspondence with the

countries concerned and to take appropr ia te action.

(< The National Bulgar ian Committee has a l ready

expressed the desire to see fu r the r progress in the

work relat ive to the a l te rna te arc of meridian from

PLENARY SESSION, SEPTEMBER 5. 55

the Arct ic Ocean to Afr ica (see the provisional pro-

g ram of the Seventh Asembly) . I t concerns the a l ter-

na t ive arc which passes through Rumania, Bulgaria,

and Turkey. This a l te rna t ive was adopted in 1933 at

the Lisbon Meeting. I now propose tha t the Union,

having consulted the proper organizat ions of Rumania

and Turkey should draw a defiIiite a l ternat ive scheme

of t r i angula t ion across these three countries. As to

the p~'esence of this t r iangula t ion in Bulgaria, where

the t r i angula t ion of the first order is a continuous

network, i~: might pass e i ther through Sofia or else

through Rouss@. When these three junction points of

this por t ion are once fixed with the Rumanian and

Turkish port ions, the Mil i tary Ins t i tu te of Bulgar ia

will make the necessary addit ional observations. >>

The meeting was closed at 5:30 p.m.

t ' L E N A B Y S E S S I O N

T~esday, September 5, 1939, 14 h. 30 m.

School of Government Building, George Washington Uni-

versi ty.

The following persons recorded thei r a t tendance at the

meet ing :

F. A. Vening Meinesz, Pres ident ; W. D. Lambert , act ing

as Secre ta ry ; Oscar S. Adams, C. F. Eaeschl~n, Walter H. Bucher, D. P. Carlton, Charles J. CIif]ord, F. L. Culley, F. W. Darling, J. A. Duerksen, Owen B. French, W. Heis-

kanen, Paul R. Heyl, J. de Graaff Ht~nter, H. S. Jelstrup, M. Matuyama, Howard S. Rappleye, J. E. R. Ross, H. Selig-

mann, W. D. Sutcliffe, C. H. Swick, C. E. Van Orstrand, C. A. Whi t ten, Delegates.

P a r k e A. Dickey, G. Hess, G. T. McCaw, A. C. Poling,

R. C. Willamson, Guests.

56 TRAVAUX EN SEANCE PLI~NI~RE.

The sub jec t of the mee t ing was Grav i ty a nd Isos tasy , wi th

P r e s iden t VENL~G MEINESZ leading.

The sub jec t was discussed by PROF. MATUYAMA and HEm-

KA~mN and by Mr. SWlCK.

Ma tuyama . The g r av i t y su rvey in J a p a n is car r ied on by

the Geodetic Commission of Japan. There are 124 s t a t i ons

on the four m a i n islands, 30 on Tyosen, 12 on Ta iwan a n d

some 35 in Manchur ia . There are also some 30 m e a s u r e -

men t s over the Nippon Trench of the Pacific Ocean.

Fo r fu tu re operat ions, i t is desired to cover the whole a rea

of Manchur i a on the land a nd the Nippon Sea and o the r

pa r t s of the sea s u r r o u n d i n g the islands. A f t e r t h a t i t is

also hoped t h a t the Mar ianne Is lands, the I s l and of Yap and

the P a l a u deeps can be surveyed.

Prof. M a t u y a m a also descr ibed briefly the charac te-

r i s t ic f ea tu re s of the" d i s t r ibu t ion of g r a v i t y anomal ies for

all the whole m a i n islands, for the Nippon Trench a rea and

for Manchur ia .

H e i s l ~ n e n . I should like to add to the i n t e r e s t i ng repor t

of Prof. M a t u y a m a tha t if the isostat ic anomal ies accord ing

to some local k ind of compensa t ion follows the topography,

t h a t is, i f the anomal ies are posit ive over i s lands and nega-

t ive over the deeps, they can to a grea t ex ten t be e l imina ted

by us ing some sor t of regional i s6s ta t ic sys tem. Pe rhaps

th i s is the case in Japan . Bu t if the isos ta t ic (local) ano-

mal ies do not follow the topography, i t is impossible to

find an isos ta t ic hypothes is t h a t e l iminates the r e m a i n i n g

anomal ies .

In connec t ion wi th the repor t of VENING MEINESZ, I should

on ly say, t h a t we are very g ra te fu l t h a t Dr. VENING MEINESZ

has ca lcula ted the ve ry i m p o r t a n t and labor ious tables for

regional i sos ta t ic reduct ion. The quest ion : local versus re-

g ional compensa t ion mus t be decided by the aid of geophysi-

cal methods. The g rav i ty anomal ies alone conna t decide this .

I n some cases the local me thod gives smal le r anomal ies ;

in o ther cases the regional method gives be t t e r resul ts .

PLENARY SESSION, SEPTEMBER 6. 57

P L E N A R Y S E S S I O N

Wednesday, September 6, 1939, 15 h. 80 m.

This meeting followed a plenary session of the Interna-

tional Union of Geodesy and Geophysics. The Association

met in the School of Government Building, George Washing-

ton University.

The following persons recorded their attendance at the meeting :

F. A. Vening Meinesz, President; W. D. La~nbvrt, acting

as Secretary; Leason H. Adams, C. F. Ba~schlin, F. L. Gul- ley, F. W. Darling, J. A. Duvrksen, C. L. Garner, L. p. G4b- bon, W. Heiskanen, M. King Hubbert, J. de Graaff Hunter, H. Jef]reys, Hans S. Jelstrup, Lewis V. Judson, M. Ma- tuyama, F. A. McDiarmid, A. H. Miiler, Th. N~tha~nmer, John A. Pounder, J. L. Rannie, J. E. R. Ross, H. Seligmann, L. B. Slichter, R. Meldrum Stewart, W. D. Sutclif]e, C. H. Swiek, Chuji Ts~boi, C. A. Whitten, John E. Willis, Dele- gates.

Ray Heredia, G. Hess, G. T. MeCaw, Morris Tepper,

Guests.

The subject of the meeting .was Geodetic Astronomy, with

Mr. C. H: SWICK leading.

Swick. To begin the discussion of geodetic astronomy

I shall first tell about the work of the Coast and Geodetic

Survey along these lines. We have adopted no new methods

or instruments recently. We still use the Talcott method for

latitudes, observe time with the broken telescope t ransi t

for longitude and observe Polaris at any hour angle for azi-

muth. For several years we have made no azimuth and lon-

gitude determinations for other than Laplace control pur-

poses. Latitude determinations are made which are for

58 TRAVAUX EN" SI~ANCE PLI~NIERE.

f i gu re -o f - ea r th purposes , bu t these a r e made cheap ly in

connec t ion wi th the o t h e r a s t r o n o m i c a l de t e rmina t i ons .

A t some f u t u r e t ime we hope to be ab le to d e t e r m i n e ad-

d i t i ona l s t a t i o n s p r i n c i p a l l y fo r f igu re -o f -ea r th inves t iga -

t ions .

W e have made no i so s t a t i c r educ t ions of def lect ion s ta -

t ions fo r s eve ra l yea r s . We have a g r e a t m a s s of d a t a on

h a n d and hope to make c o m p u t a t i o n s of i so s t a t i c def lect ions

a t some t ime in the no t too d i s t a n t fu tu re .

The L a p l a c e s t a t ions used in our 1927 ne t a d j u s t m e n t and

in t he "areas a d d e d s ince then a r e shown on F i g u r e 1 of

Spec ia l P u b l i c a t i o n No. 217, Geodetic Operations in the Uni-

ted States, 1936-38.

A f t e r th i s i n t r o d u c t i o n the d i scuss ion was con t inued b y

Dr. Hans S. Jelstrup, Prof. K. D. P. Ros@n, Mr: J. L. Rannie,

Prof . C. F. Baeschl in , ~h'. C. T. ]~IcCaw, ~ r . F. A. ~ c D i a r m l d ,

Mr. J. E. R. Ross, Mr. A. H. Miller, Mr. J. E. ~u Mr. C. A.

Whi t t en , P ro f . Motonor i ~ t a t uya ma , P ro f . W. H e i s k a n e n and

President Vening Meinesz.

Dr. J e l s t r u p . Gave an accoun t of a s t r o n o m i c a l w o r k in

N o r w a y , S p i t s b e r g e n and Greenland , ca l l ing a t t e n t i o n to the

h a n d i c a p s to long i tude and a z i mu t h obs e rva t i ons due espe-

c ia l ly in coun t r i e s so f a r nor th , bo th to the nea rnes s of the

pole, to the m i d n i g h t sun (or l igh t n i g h t s ) , and to fog and

bad w e a t h e r condi t ions .

The se lec t ion of s t a r s is Often difficult , b u t t h e y have

t r i e d to a p p r o x i m a t e a m e t h o d g iv ing the g r e a t e s t a c c u r a c y

to t he L a p l a c e az imuth . Severa l me t hods have been t r i e d ;

(1) a modif ied ba lance -method , (2) a m e t h o d p roposed b y

Mr. SCHIVE and o t h e r me thods .

A l so i t is diff icult to secure H o r r e b o w - T a l c o t t pa i r s fo r

l a t i t u d e so f a r nor th , b u t the r e su l t s a r e good in any case.

He m e n t i o n e d the obse rva t i ons in S p i t s b e r g e n and Green-

land, and r e m a r k e d t h a t the obs e rva t i ons a t Sabine I s l a n d

were a lso ca r r i ed out for~ de t ec t ing a poss ib le ~vVegener

d r i f t . He s t a t e d t h a t t h e y hoped to m a k e such obs e rva t i ons

a t t he s ame s t a t i on t en y e a r s l a te r . He po in t ed ou t t h a t

PLENARY SESSION, SEPTEMBER 6. 59

d i f fe rences f rom the va lue s ob ta ined by the G e r m a n expe-

d i t ion were pos s ib ly caused b y the i naccu racy of the old

obse rva t ions , or m i g h t a lso be due to a d r i f t or to a v a r i a -

t ion of t he deflect ion.

Mr. McCaw. I n q u i r e d w h e t h e r the ols s t a t i ons on Sab ine

I s l a n d :had been indent i f ied beyond quest ion.

Dr. •elstrup s t a t e d t h a t he h a d . n o t verif ied beyond ques-

t ion where the old Sab ine s t a t i on was, bu t t h a t be had of

course no t cons ide red such old obse rva t ions as those of

Sabine (in 1829) in deduc ing a n y t h i n g .

l~os~n. I n 1901 in t he Swed i sh -Russ i an geodet ic ope ra t i ons

in Sp i t sbe rgen , t h e d e t e r m i n a t i o n of az imuth w a s made in

t he p l ane of Po la r i s . W e could not , the re fore , use a t r a n s i t

a s Dr . J e l s t r u p did. I bel ieve the me thods of Dr. J e l s t r u p

a r e good and i t is s u r p r i s i n g t h a t the H o r r e b o w m e t h o d

could be used there . I have been on Sabine I s land . I t h i n k

t h a t no v a r i a t i o n s of the ve r t i c a l a r e to be expec ted be-

cause of m o v e m e n t s of g lac ie r s , bu t as Dr. J e l s t r u p said,

in G r e e n l a n d such v a r i a t i o n s m a y be possible.

R~nnie . W h a t use is m a d e of az imu th obse rva t ions a t in-

t e r m e d i a t e s t a t i o n s be tween Lap lace s t a t i ons in U. S. Coas t

and Geodet ic S u r v e y prac t ice .

Swiek. F o r s eve ra l y e a r s we have made no a z i m u t h obser -

va t i ons a t i n t e r m e d i a t e s t a t ions . The re a re two p r inc ipa l

r e a sons fo r t h i s change in our f o r m e r prac t ice . F i r s t , t he se

a z i m u t h s were no t cons ide red in t he a d j u s t m e n t of the t r i a n -

g u l a t i o n and t h e y we're o f v e r y l i t t le va lue fo r f igure-of-

e a r t h i nves t iga t ions . Second, t he use of s teel towers , w h i c h

a r e n o r m a l l y l e f t s t a n d i n g only a few days a t a t ime, made

the cost of these i n t e r m e d i a t e az imu ths qui te excess ive

if t he w e a t h e r became unsu i t ab l e for s t a r obse rva t ions fo r

s eve ra l d a y s a t a t ime as f r e que n t l y happens in some sec-

t i ons of t h i s coun t ry .

P ro f . C. F. Baesch l in made a r e p o r t on a s t r o n o m i c a l w o r k

done in S w i t z e r l a n d 1930-39.

Th is inc luded : (1) D e t e r m i n a t i o n of l a t i t ude and long i tude

60 TRAVAUX EN S}~ANCE PLENI~RE.

at the origin-point of Swiss coordinates, at Gurten near

Bern. The latitude work was done by the Horrebow-Talcott

method. For longitude, the difference between the first-order

points Gurten and Bern, was determined by a special simple

method using wire connection between the two paints. (2) In

1939 determinations were made in the southern part of Swit-

zerland (Canton Tessin or Ticino).

Latitude and longitude were determined on our most sou-

them point Generoso; secondary longitude work was done

on a prime vertical section, and latitude determinations to

the southward along our meridian of St. Gotthard, using

the astrolabe.

This work was just starting when mobilization was or-

dered and our engineer was called to the colors.

Willis. A method for determination of longitude, latitude

and azimuth which I used as one of several methods at Can-

ton Island in 1937 seems to have features which may com-

mend its use in surveys in high latitudes. The method

consists in taking the time of transit of each of several stars

over a different vertical circle for each, the azimuth circle

being read and a striding level being read. The telescope

is set a little ahead of the star and the star may be iden-

tified if the vertical circle is also read roughly. If the staPs

are not too close to the horizon, preferably at altitudes

above 40 degrees, there is always a solution. Some stars can

be taken Closer to the horizon to strengthen the azimuth de-

terminations if desired.

Whitten. The U. S. Coast and Geodetic Survey has re-

cently completed the compilation of a table for use in com-

puting an observing program for the method of equal alti-

tude determination of position. This table is constructed for

45 ~ altitude and bearings in the northeast, northwest, sou-

theast and southwest for latitude 0 ~ to 39 ~ An adequate

program can be readily computed with the aid of the Ameri-

can Ephemeris. Since the Ephemeris has more than 800

ava i l ab l e s t a r s i t is poss ib le to ob t a in a se lec t ion of s t a r s

PLENARY SESSION, SEPTEMBER 6. 61

such that eight (two in each quadrant) may be observed in

less than one hour of time. As yet the Coast and Geodetic

Survey has not been able to make field observations of suf-

ficient number to allow any conclusions to be found as to the

accuracy of this method for determining the observer's

position.

Matuyama. In the Sou th Sea Islands we made precise, de-

terminations of longitude and latitude in 1914 and in Saipan,

Palau, Truk and Jaluit in 1915. The points of observation

are well preserved. In 1936 and 1937 at almost the same

seasons and using the same stars similar determinations

were repeated. The result is that the islands of Saipan and

P, alau have shown some slight evidence of being displaced

toward the northwest and the other two toward the south-

east. However, the amount is very near the limit of ac-

curacy and it is not claimed that it is real. I t is, however,

of some interest that the first two islands have deep trenches

to the east.

Swick. In connection with the relatively large deflections

of the vertical in Eastern Canada, I should like to inquire

about the gravity profiles recently determined by the Do-

minion Observatory of Canada across the structural trends

north and east of Lake Champlain. Do they show a conti-

nuation of the strong negative strip .which has been traced

in the United States from Albany, N. Y. to Hagerstown,

Maryland and beyond ?

Miller. In response to Mr. Swick's query, a considerable

amoun*~ of gravi ty work has been done by the Dominion

Observatory in the Appalachian and Acadian regions of

Eastern Canada.

I t is apparent that the general results of the gravi ty

work in the two countries are similar. Gravity axes, if plot-

ted, would roughly parallel structural axes in Canada as in

the United States and continue in the Appalachian region

of Eastern Canada as might have been inferred from their

trends in the United States. This parallelism is not confi-

62 TRAVAUX EN SEAN'CE PLENII~RE.

ned to t h e - A p p a l a c h i a n s , but is also apparent in the

adjoin ing regions to the west.

Je ls t rup. I should like to hear some details of the crysta l

chronometer of which I have read in the The Field Engineer. Might i t not be pract ical for as t ronomical field work

also ?

Is i t avai lable now, and a t wha t price ?

Swick. A crys ta l chronometer was used .on a g rav i ty -a t - sea

expedi t ion to the West Irides iin 1936-37 with s t r ik ing suc-

cess. The chronometer was made by the Bell Telephone La-

borator ies , New York. I t s ra te was so small and so s teady

tha t i t could have been used for the entire expedit ion without

t ime-signal control and without introducing any appreciable

e r ror into the g rav i ty results.

So fax, t h i s par t icu la r type of chronometer has not been

made c o m m e r c i a l l y - a n d at present probably cannot be

purchased. Th i s is unfortunate, as i t is a par t i cu la r ly va-

luable ins t rument for many kinds of scientific work requi-

r ing accura te time control.

Vening Meinesz. There is one instrument, (Bell Labora-

to ry c rys ta l chronometer) now in Holland for a g rav i ty

expedi t ion a t sea tha t has been cancelled because of the

poli t ical s i tuat ion. I t worked very sa t i s fac tor i ly and over

a period of many weeks fluctuations in the daily ra te were

not more than 0.01 or 0.02 second.

P L E N A R Y S E S S I O N S

Thursday, September 7, 1939.

School of Government Building, George Washington

Universi ty.

First Session, 10 h.

The fol lowing persons recorded thei r a t tendance at the

meet ing :

F. A. Vening Meinesz, President ; W. D. Lambert, acting

ABSTRACT OF PRESIDENTIAL ADRESS. 63

as Secre ta ry ; Oscar S. Adams, H. G. Avers, C. F. Baeschlin, F. L. Culley, J. A. Duerksen, E. A. Eckhardt , Owen B. French, L. C. Glenn, W. Heiskanen, J. B. Hersey, Albert J.

Hoskinson, J. de Graaff Hunter, Hans S. Jelstrupj F. A. MeDiarmid, A. H. Miller, R. H. Montgomery, Th. Nietham- mer, John A. Pounder, J. E. R. Ross, Richard Joel Russell,

P. U. Sanchez, H. Seligmann, D. C. Skeels, L. B. Sliehter, 2/1. M. Zlotniek, R. Meldrum St, e~vart, W. D. ~uteliffe, U. H. Swick, Churl Tsubo~, C. A. Whi t ten, John E. Willis, Dele-

gates.

G. Hess, G. T. McCaw, O. S. Reading, A. van Weelden,

Guests.

The first session was devoted to an address by the Pre-

sident of the Association, Dr. F. A. VENING ]VIEINESZ.

A B S T R A C T OF P R E S I D E N T I A L A D D R E S S

B y F. A. Vening Meinesz.

In opening the first, meet ing of the In ternat ional Asso-

ciat ion of Geodesy during the Seventh Assembly of the

In te rna t iona l Union of Godesy and Geophysics at Washing-

ton, a f t e r giving a hea r ty welcome to all present, I should

like to dedicate a few words to the memory of the members

deceased dur ing the past period and express the profound

sorrow of everyone in not seeing back some of our fr iends

and colleagues. Likewise our feelings of condolence extend

to General Pe r r i e r in connection with the cruel loss he has

suffered. The whole Associat ion of Geodesy joins in ex-

press ing sincere thanks to GENERAL PERRIER for his devoted

work for t~e Association, of which during the past period

he has again been the soul.

In welcoming the members of the Associat ion to this

Assembly I hope tha t the meetings will be f rui t ful for

science and tha t they will develop the good fr iendship of the

geodesis ts throug.hout the whole world. May all the meetings

be character ized by the sincere wish to serve science to the

64 TRAVAUX EN SI~ANCE PLI~NIERE.

best: of our ability regardless of personal feelings and suscep-

tibilities, and may they be governed by the desire to be open-

mindad for the points of view of others. May science play

the principal and only part and may administrative, finan-

cial, and s ta tu tory questions be reduced to the utmost

necessary minimum. Our Union has suffered too much loss

of time and energy dedicated to these questions at the

expense of what could be given to our main program - - the

service of the geodetic and geophysical sciences which are

dear to us all. Appreciating the occasion which this opening speech gives

to me, I should like to give a survey of the development of

the geodetic science in the past three years, since the Assem-

bly at Edinburgh, but the subject is too wide to be treated

in the limited time at our disposal. Therefore, I shall limit

myself to only one branch of geodesy which, being the

subject of my own work, I feel better qualified to discuss

than other branches, namely, gravity. I do not wish to give

an account, of the history of our Association in the past

period, because I am glad to leave this to the more able

hands of our Secretary, GENERAL PERRIER, who will present

his valuable report shortly.

Gravity is an extensive and many-sided subject and I shall

have to restrain myself from taking too much of your time.

Impor tant new developments in the apparatus and the

methods of observation may be mentioned. For the deter-

mination of gravi ty on land new possibilities of the greatest

importance have been opened by the construction of gravi-

meters tha t reduce the time of observation to a small frac-

tion of what was formerly needed. They are all founded on

the principle of measuring gravi ty by determining an elastic

deformation which it produces. The first of these gravi-

meters tha t was successful is the HOLWECK-LEJAY inverted

pendulum, which has already come into general use for

geodetic measurements as well as for geophysical pros-

pecting. Others have been developed of which the speaker

ABSTRACT OF PRESIDENTIAL ADRESS~ 65

wishes only to mention a few - - those of ISINGS, of Dr. FRED

WRIGHT, of VON THYSSEN, of GRAF, of WYCKOF, etc. These all

allow a st i l l more rapid determinat ion of gravi ty. The resul t

of these new possibi l i t ies is a rapidly increasing number of

g rav i ty - s ta t ions and in consequence of this the problem is

now shif ted more to the question of how to keep up with

the in te rpre ta t ion of this enormous mater ia l .

Fo r the determinat ion of g rav i ty at sea impor tan t deve-

lopments may likewise be reported. Fi rs t , there is a possi-

bi l i ty in the appara tus of HAALCK which, as is well known,

is founded on the principle of measur ing the pressure of a

definite amount ~f gas by means of the column of mercury

which is in equil ibrium with i t and which, therefore, depends

on gravi ty . We may feel hopeful about its proving to be

successful in meas,uring g rav i ty a t sea on board surface-

vessels which would be a g rea t step forward, a l though a few

dis turbing effects are st i l l encountered.

The mult iple-pendulum method has been grea t ly improved

through the introduct ion by MAURICE EWING of the crys ta l -

controlled t ime-keeper. The laborator ies of the Bell Tele-

phone Company have kindly put the i r r emarkab ly accurate

crystal-chronometer , constructed under the supervisi6n of

W. A. MARRISON, a t the disposal of the << Bar racuda 7> Expe-

d i t i o n of EWINC~ HESS, and HOSKINSON, and a f te rwards at

t ha t of Engl ish and Dutch gravi ty-expedi t ions. The grea t ac-

curacy of the t ime-uni t for the pendulum-observatfons which

has thus been obtained removes one of the two principal

sources of e r ror in these measurements , thus effectively re-

ducing the e r ro r in the results. Another impor tan t new

development is the discovery by B. C. BROWNE tha t two

second-order dis turbance-terms, h i ther to neglected, have an

apprciable effect in many cases, and in some cases when

the sea is rough and the movements of the submarine relat i-

vely large may even a t t a in to values of several tens of milli-

gals. The dis turbances are brought about b y the ver t ical

and horizontal accelerat ions of the pendulum-appara tus and

are propor t ional to the squares of these accelerations. As

66 TRAVAUX EIN- SI~.ANCE PLI~NIERE.

the ~ertical accelerations may be derived from the ordinary

records of the apparatus the corresponding disturbance can

easily be determined, but for the deduction of the second

disturbance-term a special apparatus had to be constructed

allowing the measurement of the horizontal accelerations.

This has been realized by introducing two pendulums of very

long period swinging in vertical planes at right-angles to

e~ch other, each of which allows the determination of the

horizontal acceleration in its own plane. Here I wish to

express my sincere thanks to Mr: BROWNE for his kind coope-

rat ion in the investigation of these problems.

Coming now to the methods of isostatic reduction of the

gravity-results needed for the research on the way isostasy

is realized in the different parts of the Ear th and of the

degree in which it is true, important advance may be reported.

In the first place, we may mention the tables of LAMBERT

and DARLING for the BOWIE or, indirect reduction, which

have been computed for the United States Coast and Geodes

Survey'. These tables make it for the first time possible to

compute accurately the effects on the geoid and on the values

of gravi ty on the geoid which are caused by the removal

of topographic masses and their compensation. The great

importance of these tables for all problems related to the

figure of the Ear th and to the degree in which isostasy is true

for parts of the Ear th ' s surface need scarcely be stressed.

The above effects give a material change of the results for

both problems. Next I wish to mention the foundation of the Interna-

tional Bureau for Isostatic Reduction in Helsinki at the

request of our Association. We are sincerely thankful to

PROF. HEISKANEN for having been willing to undertake this

responsibility, and also to the Government and other insti-

tutions in Suomi which have made this possible by allotting

the funds necessary to augznent the limited amount which

our Association oould put aside for it. At this occasion

sincere thanks may also be expressed to the United States

ABSTRACT OF PRESIDENTIAL ADRESS. 67

Coas t and Geodet ic S u r v e y fo r hav ing fo r a long t i m e

a l lowed i t s I s o s t a t i c Divis ion, under the able l e a d e r s h i p of

Mr. CLARENCE H. SWICI~, to s i m i l a r l y u n d e r t a k e these com-

p u t a t i o n s fo r o t h e r count r ies . I have special r ea sons to be

g r a t e f u l to th i s Div is ion because of i ts hav ing u n d e r t a k e n

the i s o s t a t i c r educ t ion of al l t h e e a r l i e r g r a v i t y - r e s u l t s a t sea.

The a s c t i v i t y of t h e new b u r e a u of Dr. HEISKANEN in t h e few

y e a r s of i t s ex i s t ence h a s been r e m a r k a b l y ex tens ive ; wor ld -

m a p s have been made for the HAYFORD i sos t a t i c r e d u c t i o n

of t he d i s t a n t zones, regi~onal maps fo r a few s o m e w h a t

n e a r e r zones , new r e d u c t i o n - t a b l e s for the AIRY s y s t e m of

r educ t i on fo r t h i cknes se s of the c rus t of 20 and 30 k m and,

i a s t b u t no t leas t , a comple te l i s t of al l t h e g r a v i t y - r e s u l t s

ove r t he whole E a r t h w i th t h e i r i so s t a t i c reduct ion . I shou ld

l ike to c o n g r a t u l a t e Dr. HEISKANEN for these mos t i m p o r t a n t

a ch i evemen t s which a re a g r e a t a s se t fo r all s tud ies concer-

n ing i s o s t a s y and g r a v i t y .

F i n a l l y I wish to men t i on the tab les fo r reg iona l i s o s t a t i c

r educ t i on fo r five d i f fe ren t degrees of r eg iona l i t y of the com-

pensa t i on which I have made dur ing the pa s t y e a r s fo r t he

NETHERLANDS GEODETIC COMMISSION and which have j u s t

been pub l i shed in the Bulletin g4od@sique. I hope t h a t t h e y

wil l a l low the d e t e r m i n a t i o n of the degree of r e g i o n a l i t y in

d i f fe ren t p a r t s of t h e E a r t h ' s c rus t and thus p e r h a p s p rov ide

us wi th a means of e s t i m a t i n g the th ickness of the c rys -

t a l l ine c r u s t in t hese d i f fe ren t a reas . This t h i ckness m a y

d i f f e r f r o m the dep th of the compensatior~ as dcduced b y

m e a n s of HEISKANEN'S tab les a n d m y own tables , because

t h e r e a r e r easons to expec t the s ima t i c or gabb ro i c l a y e r

be low t h e s ia l ic c r u s t to be in a c rys t a l l i ne s t a t e to a c e r t a i n

depth . DALY and GUTENBERG e s t i m a t e th is dep th a t 70 to

80 k m be low sea-level , and p r o b a b l y i t is s l i g h t l y g r e a t e r

u n d e r the oceans t h a n unde r the cont inents , while the re-

verse is t r ue for the s ia l i s c rus t , which p robab ly va r i e s f r o m

be tween 40 and 50 km under the con t inen ts to be tween ze ro

and 20 k m u n d e r the oceans. As the w id th of t he b e n d i n g

of the c r u s t under the load of a t o p o g r a p h i c f e a t u r e t h a t does

68 TRAVAUX EN SI~ANCE PLl~NI~RE.

not break the crust locally m u s t depend upon the thickness

of the crust, we may expect the degree of regionality of the

compensation to give indication about this thickness.

Next let us consider what geodetic conclusions can be de-

rived f rom gravity. I t is well known that STOKES has de-

duced a theorem for determining the figure of the Ear th

f rom gravi ty and in this way has enriched geodesy with

one of its most important possibilities. Up to now, however,

the gravity-determinations were too scarce, for large parts

of the Ear th ' s surfac to allow Stokes' equation to be applied,

but a t length the situation is changing and in this field also

important developments may be expected. In recent years

valuable investigations of this problem have been made of

which I may mention the work of DE GRAAFF HUNTER and of

HmVONEN; the lat ter has made a first at tempt to compute

the geoid from the present gravity-results. Although in

doing this arbi t rary interpolations and assumptions could

not be avoided, the resulting figure of the Ear th is no doubt

already valuable and w.e may look for a steady increase of

accuracy in the future. It appears that our Association has

an important task in this connection. By giving a timely

Iead about the assumptions and the way of interpolating,

unity in future investigations may be furthered. In this

way perhaps different parts of the geoid determined by the

various countries may all fit together into one unique

system. Here again the Isostatic Bureau of Dr. HEISKANEN

may render us great and valuable services, and I am glad

to say tha t Dr. Heiskanen has been found willing to under-

take this important task, continuing in this way the work

already s tar ted by Dr. HIRVONEN.

I now come to the last subject for which I ask your a ttea-

tion, namely, the geophysics/conclusions that can be derived

from gravity-results. I should like to treat this subject

a little more, in detail. Here also great advance may be re-

ported, as well by the important extension of the number

of gravity-stat ions as by valuable theoretical research.

ABSTRACT OF PRESIDENTIAL ADRESS. 69

I am unable to cover the whole field here and so I shall men-

t ion only a few of the areas tha t have been invest igated,

leaving aside many interes t ing resul ts t ha t have been

obtained elsewhere. In the first place, l e t us consider the

areas where g rea t tectonic ac t iv i ty is in progress. The grea t

outs tanding advance of our knowledge in the pas t three

years is .given by the resul ts obta incd by HESS, EWING and

HOSKINSON during the ~< Bar racuda >> Expedit ion in the West

Indies. They have shown a s imi lar bel t of s t rong negat ive

anomalies accompanied by fields of positive anomalies on

both sides as have been found by me in the Eas t Indies and

by I~ATUYAMA near Japan. This makes i t seem ra the r pro-

bable t ha t this fea ture is typical for many if not all areas

where g rea t crustal deformation accompanied by folding

and over thrus t ing of the su r face - l aye r s is occurring. As

suming t h a t these belts are brought about by grea t crusta~

shor tening by compressive forces, the gravity-field in the

Wes t Indies, in the same way as i t is possible for the Eas t

Indies, allows conclusions about the direct ion of these forces

which may lead to valuable ins ight concerning the i r nature.

I f th is research could be extended over all the orogenic

areas where these phenomena are exhibited, we could reach

conclusions about the way t h e forces are d is t r ibuted over

the E a r t h and thus perhaps develop an unders tanding of

the i r causes. Enter ing more in detail about the interpre-

ta t ion of the gravity-fields in the above areas and about thei r

re la t ion to the geological, the seismological, and the, volca-

nological phenomena, mention should be made of the note-

war thy s tudy of HESS on this subject for the West Indies

and the exper iments of KUENE~4 with floating crust-models

under la te ra l compression. We come to the, conclusion tha t

up to now a t l e a s t the explanat ion of these belts by a

downward buckling of the E a r th ' s crust, accompanied by the

folding and over thrus t ing of the surface-layers, is fa i r ly well

covering the facts. The West Indian results, in proving

these phenomena to be present also in tha t area, have given

impor tan t suppor t t~ this hypothesis.

70 TRAVAUX EN SI~ANCE PLI~NIF, RE.

The second f e a t u r e of the E a r t h ' s g rav i ty - f i e ld wh ich

I wish to e n l a r g e on is the d i s t r i bu t i on of pos i t ive ano-

mal ies , in some cases n e g a t i v e anomal ies , o v e r such l a rge

fields that . e x p l a n a t i o n b y p h e n o m e n a in the E a r t h ' s c r u s t

is diff icul t . I f t hese f ields a re r e a l l y dev ia t ions f rom the

i s o s t a t i c equ i l ib r ium - - and more r e s e a r c h is needed in m a n y

cases to m a k e s u r e abou t th i s - - we mus t a s sume t h a t

d e v i a t i o n s of equ i l ib r ium do occur in the p las t ic s u b c r u s t a l

l ayers , which seems to l ead to t he a s s umpt i on t h a t s y s t e m s

of c u r r e n t s a r e p r e s e n t t h e r e . I t is no t necessa ry to men t ion

the, g r e a t i m p o r t a n c e of such conclus ions fo r our gene ra l

i n s igh t in w h a t is go ing on in the E a r t h ; these c u r r e n t s

m i g h t s a t i s f a c t o r i l y exp la in the g r e a t ho r i zon ta l fo rces

w o r k i n g in the c rus t 5y the d r a g which t hey e x e r t on i t

and the deep- focus e a r t h q u a k e s m i g h t p e r h a p s l ikewise find

an exp lana t ion . Close coope ra t ion w i th m a n y o t h e r E a r t h

sc iences will be needed to ob t a in a t r u s t w o r t h y i n t e rp re -

t a t i o n and an u n d e r s t a n d i n g of al l t h e s e phenomena, and so

t he coope ra t ion in the Jo in t ~ Commiss ion on Cont inen ta l

and Oceanic S t r u c t u r e s >> wil l p rove a va luab le h e l p fo r

f u r t h e r deve lopments . As these ex tens ive fields of ano-

mal ies a r e also caus ing t h e ma in dev ia t ions of t h e geoid

f rom t h e equi l ib r ium-f igure , g iven wi th g r e a t a p p r o x i m a t i o n

by the e l l ipso id of revolu t ion , these i nves t i ga t i ons a re l ike-

wise of the g r e a t e s t i m p o r t a n c e fo r the pr inc ipa l p rob lem of

geodesy , t he d e t e r m i n a t i o n of the f igure of the E a r t h ; we

m a y conf ident ly hope t h a t b e t t e r i n s i gh t into the causes of

t he se d i s t u r b a n c e s of equ i l ib r ium in t h e E a r t h wil l f u r t h e r

th i s p rob l em effect ively. W i t h a s t r o n g appea l fo r e f f e c t i v e

c o o p e r a t i o n of t he d i f fe ren t Assoc i a t i ons t h a t f o rm our

Union f o r t h e f u r t h e r i n g of these and o t h e r p rob lems of our

geode t ic and geophys i ca l sciences, I dec la re t h a t the m e e t i ng

of the A s s o c i a t i o n of Geodesy is now open.

P o t g i e t e r l a a n 5, A m e r s f o o r t , Ne the r l ands , J u l y 31, 1939 a.

1. En raison des circonstances, l 'adresse pr~sidentielle pro- nonc~e par le President F. A. VENING :~/I~EINESZ le 7 septembre,

PLENARY SESSIONS, SEPTEMBER 7. 71

Second Session, 11 h.

The following persons recorded thei r a t tendance at the

meet ing :

F. A. Vening Meinesz, President ; W. D. Lambert , act ing

as Secre ta ry ; Oscar S. Adams, H. G. Avers, C. F. Baeschlin, F. L. Cu~ley, J. A. Duerksen, E. A. Eckhardt , Owen B. French,

L. C. Glenn, W. Heiskanen, J. B. Hersey~ Albert J. Hos- lzinson, J. de Graf] Hunter, Ha~s S. Jelst~ntp, F. A. McDiar- ~dd, A. H. Milieu, It: H. Montgomery, Th. Niethctmmvr, John

A. Pou~der, J. E. It. Itoss, Itichard Joel t~usseU, P. C.. San- chez, H. Seligmann, D. C. skeels, L. B. Sl(chter, M. M. Slot- nick, R. Mead,rum Stewart, W. D. S~tclil~e, C. H. Swick, Chuji Tsuboi, A. van Weelden, C. A. Whit ten, John E. Willis,

Delegates.

G. Hess, G. T. McCaw, O. S. 1heading, Guests.

The meet ing was devoted to the: considerat ion of the

Geoid and Isostasy, with PROF. HEISKANEN leading.

Third Session, 14 h. 30 m.

The following persons recorded their a t tendance at the

meet ing :

F. A. Vening Meinvsz, President ; W. D. Lambert , act ing

as Secre ta ry ; Oscar S. Adams, H. G. Avers, C. F. Baeschlin,

F. L. Culley, E. A. Eckhard~t ~ Owen B. French, L. C. Glenn) W. Heiskanen, Albert J. Hoskinson, J. de Graaf] Hunter, Hans S. Jelstrup, F. A. McDiarmid, A. H. Miller, t~. H. Mont- gomery, Th. Nie thammer, John A. Pounder, t~ichard Joel Russeli , P. C. Sanchez, H. SeIigmann, D. C. Sk~els, L. B. Slichter, M. M. SIotnick~ It. Meldrum Stewart, W. D. Snt-

apr~s les d6clarations de g~erre, a pr6sent~ quelques modifications par rapport au texte ci-dessus 4cl~.t le 31 juillet.

72 TRAVAUX EN SI~ANCE PLENI~.RE.

clef]e, C. H. 8wick, Ghuji Tsuboi, C. A. Whitten, John E. W{lZis, Delegates.

G. Hess, G. T. McCaw, A. van Weeldea, Guests.

The subject of the meeting was the Deflections of the Vertical and the Figure of the Earth, with Dr. J. DE GRAAFF

HUNTER leading.

The subject was discusser by Prof. Niethammer, Baeschlin,

Vening Meinesz and Heiskanen; Messrs. Lambert, McDiar- mid, Rannie, Willis and Commander Garner.

Nietlmmmer. Explained how the surface values of g of the

various points of the line of astronomical leveling (deflectior~

of the vertical for determining the geoid) were derived

(without its being necessary to consider individual values of

the rock density) by means of the values of g observed at

the stations of the Swiss gravity net; for this purpose spe-

cial tables are used, making the computation of mean

values as simple as computation of the topographic reduction.

Lambert. Mr. DARLING and I tried to determine the geoid

in the United States from the few available observations of

the defection ,of the vertical. The method used was an

adjustment by least squares, in order that personal judg-

ment might affect the results as little as possible. The

observations were so few that refinements like the reduction

o f the deflection to sea level, of which PROF. NIETHAMMER

has just spoken, were quite unnecessary because of the large

extrapolations tha t we were obliged to make.

We reached, v e r y tentatively, one rather interesting con-

clus~on, namely that on the whole isostatic compensation is

not quite complete or the mass of the compensation is

somewhat less than that of the topographic feature or fea-

tures compensated. Because of the uncertainties due to the

scantiness of the data, we took no steps towards publishing our work.

One c~onclusion forced itself upon us. We needed deflec-

tions very much closer together than we. had them. The

PLENARY SESSION, SEPTEMBER 9. 73

importance of a close spacing for determining the geoid can

hard ly be overest imated. Frequent ly the s ta t ions should be

closer toge ther than the vertices of the t r iangulat ion.

In p a r t of the t r iangula t ion of Finland there is an astro-

nomical s t a t ion a t every ver tex but in generai those who

have a t r i angula t ion to ad jus t do not wish to use too closely

spaced astronomical s tat ions. Not only does the inclusion

of as t ronomical s ta t ions make the numerical v~ork of adjus t -

ment more laborious, but it is believed tha t too many astro-

nomical s ta t ions are actual ly undesirable. In t r iangula t ion

only Laplace s ta t ions are needed.

We have real ly however two different problems : 1) the.

best d is t r ibut ion of as t ronomical s ta t ions for the ad jus tment

of t r i angula t ion ; 2) the best d is t r ibut ion of s ta t ions for

de termining the geoid. I1 will avoid confusion if these two

are considered separate ly .

P L E N A R Y S E S S I O N

Saturday, September 9, 1939, 10 h.

School of Government Building, George Washington Uni- versi ty.

The following persons recorded thei r a t tendance a t the meet ing :

F. A. Vening Meinesz, President ; W. D. Lambert, act ing as

Secre ta ry ; Oscar ~. Adams, C. F. Baeschlin, F. W. Darling, J. ,4. Duerksen, E. Gaviola, W. Heiskanen, J. de Graaf] Hun- ter, Hans S. Jelstrup, L. V. Judson, F. A. M'cDiarmid, A. H. Miller, R. Montgomery, Th. Niethammer, John A. Poundvr, J. L. Rannie, K. D. P. Ros@n, J. E. R. Ross, P. C. Sanehez, W. D. Zutcliffv,, C. H. Swick, Chuji Tsuboi, C. A. Whitten, John E. Willis, Fred E. Wright, Delegates.

G. T. McCaw, A. van Weelden, Guests.

74 TRAVAUX EN SEANCE PLI~NI~]RE.

W i t h the approval of the mee t ing the following appoint-

men t s were made by the P res iden t to represent the Asso-

c ia t ion on three special commit tees of the Union :

1. Repor t of the Genera l Sec re ta ry of the Union. PROF.

C. F. BAESCHLIN.

2. D i s t r i bu t i on ,of Publ ica t ions . Dr. H. S. JELSTRUP.

3. Deposi tor ies for Geophysical D a t a submi t ted by Oil

and Min ing Companies . Dr F. A. VENING I~'IEINESZ.

The scientific sub jec t of d iscuss ion was Isostasy, wi th

PROF. HEISKANEN leading. The sub jac t was discussed by

Mr. C. H . Swick, Dr. Ven ing Meinesz, Mr. Duerksen, Dr. Tsu-

boi, Dr. de Gra~ff Hunter, Mr. Miller, Mr. McCaw and

Dr. Jelstrup. Heiskanen. Day before ye s t e rda y I gave a shor t rSsum~ of

m y R e p o r t on Isostasy, bu t there was no t enough time~ t o

discuss it, so t h a t I shal l open the discussion now. Fi~ ' s t

I should l ike to ask you r opinion about the scope of the Isos-

t a t ic r educ t ion maps. Whe the r for all Zones 13 to 1 we

should make a s ingle m a p only ; or else for the Zones 7 tx) 1

one map, for the Zolies 8 to 10 a no the r map and for the

Zones 11 to 13 a t h i rd map. W h a t do you think, Mr. Swick?

Swick. There is some ques t ion as to how far the ou te r

zones of the Hayfo rd isost:atic compensa t ion should be com-

bined. I f HEISKANEN'S maps are used t hen i t migh t be desi-

r ab le to combine the tab les to correspond to the same divi-

s ion of zones. The chief diff iculty involved is use of the data

f o r o the r methods of reduc t ion such as the AIRY method.

I t is doub t fu l whe the r all the zones, 13 to 1, should be com-

bined.

Ven ing Meinesz. Fo r chang ing over to other sys tems of

compensa t ion , i t will be well to keep a separa te publ ica t ion

of the zones up to 8. I t will be possitJle to combine t h e

zones 7 to 1:

A discuss ion ,and inves t iga t ion is recommended to see

whe the r the i n t roduc t ion of two discont inui t ies , corres-

pond ing to the t r a n s i t i o n of the surface layer to the, in ter-

PLENARY SESSION, SEPTEI~IRER 9. 75

mediary layer and from the intermediary layer to the sub-

crustal layer, would be worth while,.

Duerksen. It seems to me that we should not combine the

outer zones beyond 1 to 7, since the change of compensation

correction from one depth to another would become too

doubtful.

Tsuboi. (No text available.)

de Graaf f Hunter . The relations found by Dr. TSUBOl

between the Fourier coefficients representing the Bouguer

gravity anomalies and the topographical heights over the

U. S. A. give a very concrete answer to the question :

what is the degree of compensation in that area? This is

particularly satisfying to anyone who is in any degree a

doubter. One can now say pretty definitely that compen-

sation exists for the features of great horizontal extent

and that it does not exist for features of horizontal extent

less than 150 kin, except by chance.

I had a slight doubt at first as to the validity of the as-

sumption, made to simplify the computation, that the

earth's curvature may be neglected. The results of the com-

putation, however, seem to me to justify the assumption, for

the'interest centers around the terms of higher harmonic

order . Wi len compensa t ion is non-ex is ten t , as in th i s case,

the h o r i z o n t a l d i s t ances a re suf f ic ient ly smal l to make t h e

c u r v a t u r e effect p a l p a b l y u n i m p o r t a n t .

I w i sh to exp res s m y s incere a d m i r a t i o n for Dr. TSUBOI'S

w o r k and hope t h a t he will find i t poss ib le to a p p l y t h e

s ame p rocess to o t h e r reg ions when g r a v i t y s u r v e y s have been made .

He i skanen . I t h a n k Dr. TsuBoI fo r his i n t e r e s t i n g paper .

In connec t ion w i th i t I would l ike to s ay t h a t the local ano-

mal ies , wh ich have no th i ng to do w i th the compensa t ion ,

can inf luence the s igns of the h i g h e r - o r d e r t e r m s . Also

w i t h o u t a n y deve lopmen t in F o u r i e r ser ies , we can see

w h e t h e r t h e compensa t i on p reva i l s o r not. In t h e f i rs t

case t h e B o u g u e r anomal ies a re s t r o n g l y nega t ive in moun-

t a inous r eg ions ; in t he second case t h e y a re n e a r l y zero.

76 TRAVAUX EN SEANCE PLENIERE.

The development of the anomalies as a function of the ele-

vation of the station gives for the Bouguer anomalies an

elevation term that i s distinctly negative and for the free-

air anomalies a positive elevation term (if compensation

prevails). I t would be better to compute the thickness of

the ear th 's crust for Western America and for Eastern Ame-

rica separately.

Tsuboi. There is an indication, although not very str~ngl

that the depth of compensation depends on the extent or

wave-length of the topo~oTaphy.

.Miller. I should like to ask Dr. HEISKANEN about the

manner in which figures for the annual rate of rising of

Fennoscandia were obtained and to inquire regarding the

magnitude of the gravi ty anomaly over this area and to call

at tention to the negative anomalies outlined by the Domi-

nion Observatory over Precambrian areas in Eastern Ca-

nada.

Iteiskanen. (No text available).

MeCaw. (No text available).

Jelstrup. In accordance with the statement for Finland

given by Dr. HEISKANEN, the existing of typical shoT:e-lines and terraces in Norway show the upheaval of land in geo-

logical time, as in the neighborhood of Oslo.

P L E N A R Y SESSION

Monday, September 11, 1939, 1~ h. 30 m.

School of Government Building, George Washington Uni- versity.

The following persons recorded their attendance at the meeting :

F. A. Vening Meinesz, President; W. D. Lambert, acting

as Secretary; L. H. Adams, Oscar ,g. Adams, Carl L Aslak- son, C. F. Baesehlin, W. H. Bainbridge, P. L. Bernstein,

PLENARY SESSION, SEPTEMBER J.1. 77

F. L. Culler, F. W. Darling, J. A. Duerksen, L. P. Gibbon, W. Heiskanen, H. W. Hemple, Albert J. Hoskinson, J. de

Graaf] Hunter, Hans S. Jelstrup, L. V. Judson, M. Matuyama, F. A. MeDiarmid, E. K. Meade, R. H. Montgomery, H. R.

Morgan, Th. Nie thammer, John A. Pounder, J. L. Rannie,

Howard S. Rappleye. Walter F. Reynolds, J. E. R. Ross, H. Seligmann, James B. Small, Paul Sollenberger, Joseph L. Stearn, R. Meldrum Stewart , W. D. Sutcliffe, C. H. Swiek, C. B. Watts , C. A, Whit ten, Belisario Ruiz Wilches, John E. Willis, R. M. Wilson, Delegates.

C. N. Claire, R. J. de Heredia, H . P. Kaufman, G. T.

McCaw, A. C. Poling, Howard S. Reading, Donald A. Rice,

D. C. Ritchie, M. Tepper, R. O. Williamson, Guests.

The subject of the discussion was Triangulation and Bases,

with COMMANDER GARNER leading.

The discussion was continued by Mr. McCaw, Mr. McDiax-

mid, Mr. Swick, Dr. l~Iorgan, Mr. Rannie, Dr. Veiling Meinesz,

Dr. de Graaff Hunter, Prof. Baeschlin, Mr. Sollenberger,

Prof. Heiskanen and Dr. Jelstrup. Some spoke more than once.

McCaw presented his report on Geodetic Astronomy. Garner. See Appendix, p. 80.

Heiska~en. In connection with the report of Mr. MCC'Aw,

I will add, that in Finland the astronomical latitudes,

longitudes and azimuths have been determined at about

30 points during the last three years. I should like to state

fur ther that Dr. KUKI(AVL~t{I i n Finland at the request of

Prof. BOi'TSDORFF has undertaken a very careful investiga-

tion of lateral refraction. If the path of the ray of light

from one observation tower to another is such that the ray

comes close to the slope of a hill, then since the surfaces

of equal density of air are not quite horizontal, lateral

refraction must arise. Dr. Kukkamaki has tried by the aid

of the topographic maps to detarmine the effect of the la-

6

78 TRAVAUX EN SI~,ANCE PLENIERE.

te ra l ref rac t ion in about 80 t r iangles of the Finnish first-

order t r i angula t ion net. The resul t is tha t near ly all large

closing errors, such as 0".8 to 1".0 are much reduced. The

mean quadrat ic e r ror is a f te r tak ing into account th is effect

only half as large as wi thout it. The account of th is work,

which will be published before long, shows tha t i t is pos-

sible to take in to considerat ion the effect of the la te ra l

re f rac t ion and to get marked ly smaller errors than has been

possible up to now.

Garner. I t would be in teres t ing to compare the t r iangu-

lat ion of our neighbor, Canada, wi th ours.

Rannie. The main differences between Canadian and U. S.

t r i angula t ion is b rought abou+, by more difficult t ranspor ta -

t ion conditions in the former country.

This has led to emphasis being placed on lightness of ins-

t rumenta l equipment, r a the r than on simplicity of design

and ruggedness.

de Gra~ff Hunter . Referred to a new and shor te r method

of ad jus t ing geodetic t r iangulat ion, a m e t h o d devised by

MAJOR G. BOMFORD and applied to the t r iangula t ion of India

in o r d e r to introduce the LAPLACE stations, which did not

exis t a t the t ime of the original adjustments . The work

is published as Professional Paper No. ~8 of ~he Survey of India and is ent i t led The Readjustmvnt of the Indian Trian-

gulation. Copies are obtained from the Director, Geodetic Branch, Survey of India, Dehra Dun, U. P., India.

Ra~schlin. Gave. a shor t r epor t on the lat i tude observat ions

on three s ta t ions of Switzerland : Ziirich, Berne and Gurten,

Berne being the origin of the Swiss plane coordinates. The

observat ions on the three s ta t ions were made by the HOR-

REBoW-TALCOTT method in the well-known manner.

He then repor ted on the second-order longitude work.

While for the f i rs t -order longitude work the reception of the

wireless t ime signals is made by recording them on the

chronograph, for the second-order observations the recep-

PLENARY SESSION, SEPTEMBER 11. 79

t i on is done b y the me thod of coincidence shown b y Dr.

HAENNY. This me thod p e r m i t s the use of a s imple r ad io re-

ceiver. A s h o r t c i rcui t is made by the clock or t he chro-

n o m e t e r to the receiver . F o r some seconds no r a d i o s igna l

is hea rd . The obse rve r has to l i s ten to t he f i rs t Signal

a f t e r t he i n t e r v a l of si lence. By th i s m e t h o d five obse rva -

t ions can be m a d e du r ing the 306 r y t h m i c t ime s igna ls . I f

t h e r a t e of t he clock is zero, al l obse rva t ions g ive the s a m e

seconds . To have a g r e a t e r accu racy the d i u rna l r a t e of t he

c lock is he ld on 29 seconds. Then once in t he se t of t h e

five o b s e r v a t i o n s a j ump of one second is observed. By th i s

m e t h o d the t ime of coincidence is de t e rm i ne d wi th in one-

f i f th of a second. The mean e r r o r of a t ime o b s e r v a t i o n

m a d e b y th i s m e t h o d is n e a r l y the same as by the recor -

cling me thod , b u t much s imp le r i n s t r u m e n t s can be used.

A f t e r w a r d s he men t ioned t h a t a t r i a n g l e in t h e ne ighbor -

hood of t h e L a k e of Cons tance was r e obs e rve d by t h e old

A u s t r i a n , t he G e r m a n and the Swiss Surveys . P r a c t i c a l l y

no d i f fe rences could be found as compared w i t h o lde r obser -

v a t i o n s m a d e n e a r l y 30 y e a r s before . No s h i f t i n g of the

e a r t h ' s c rus t , in th is region, such as our geo log i s t s h a d

expec ted , could be found.

F o r s e v e r a l y e a r s t he N a v a l O b s e r v a t o r y h a s been us ing

q u a r t z c r y s t a l osc i l l a to rs in the t r a n s m i s s i o n of t ime s igna l s .

R e c e n t l y w o r k h a s been u n d e r t a k e n to inc rease the a c c u r a c y

of t he se osc i l la tors . I t has been found t h a t t h e i m p o r t a n t

f a c t o r a p p e a r s to be the m o u n t i n g of the c rys t a l . I t is

n e c e s s a r y on the one h a n d t h a t t he c ry s t a l be he ld r i g i d l y in

place, a n d on the o the r hand t h a t i t s osc i l l a t ion be no t

r e s t r i c t e d . F o r accompl i sh ing th is pu rpose we have found

the bes t r e su l t s to come f rom m o u n t i n g the c r y s t a l on

s tee l wires . The p ropo r t i ons of the c ry s t a l d i m e n s i o n s a r e

m a d e so as to reduce t h e t e m p e r a t u r e coeff ic ient n e a r l y

to zero. The a g i n g of the vacuum tubes does not a p p e a r to

be a n i m p o r t a n t f ac to r . I t is be l ieved t h a t t he se o sc i l l a to r s

wil l equal and m a y even tua l ly become s u p e r i o r to the bes t p e n d u l u m clocks.

80 TKAVAUX EN SEANCE PLENIERE.

Jelstrup. In Norway we have also tried to ,observe in

what may be called << triplets >> of LAPLACE stations, for-

ming triangles of contiguous first-order stations, in order to

check the LAPLACE residuals, computing it for one station

starting from both the other two and taking the mean of

the results.

We would be pleased to know if anyone might suggest a

profitable average distance between LAPLACE points in nets

and chains, 'when one considers the use of these points as a

check on the triangulation in azimuth, as base lines check

i t in length.

A P P E N D I X

TRIANGULATION

by Comdr. C. L. Garner.

The first triangulation in the United States was hegun

by the first Superintendent of the Coast Survey, FERDINAND

R. HASSLER, in northern New Jersey about 15 miles north-

west of the center of Manhattan, in 1817. As a ~natter of

fact, this was the first field work of the then << Coast Sur-

vey >> and was initiated for the purpose of controlling a

hydrograph ic survey of the growing por t of New York which

required naut ical char ts as an aid to mar i t ime commerce

in these ea r ly days.

Both the horizontal and vert ical control work expanded

eas tward ly and southwardly along the coast from New

York as required to coordinate the hydrographic surveys in

those r e g i o n s . Progress in the extension not only of the

control surveys bu t the hydrographic surveys as well was

r a the r slow due to the lack of necessary funds for any large

program. I t is an his tor ical fact t ha t work of th is nature

can only come about th rough the demands of the people for

i t by reason of i t s economic importance, and therefore, i t

has v e r y ra re ly been undertal$en on a large scale and car-

TRIANGULATION BY C. L. GARNER. ~I

ried through to completion in any country before the cur-

rent demands required it.

The triangulation net of the United States grew from its

early beginning in the vicinity of New York City south-

eastwardly along the Appalachian chain of mountains and

consisted first of what is generally known as the oblique

arc of triangulation reaching from New England to the Gulf

Coast in the vicinity of Mobile, Alabama. Coincident with #

this, however, other work was extended westward along

what is generally known as the Trans-Continental Arc of

triangulation which was to follow the 39th parallel of la-

titude from West Virginia to San Francisco. The need for

this arc was 'early visualized as a means not only of coor-

dinating on a single datum the hydrographic surveys along

the Atlantic, Gulf and Pacific Coasts but also of beginning

a framework for mapping and engineering purposes gene-

rally throughout the entire country.

Later on, arcs of triangulation were extended along the

Pacific Coast, along the southern border of the United States

from Texas to California, along the international boundary

between the United States and Canada and in the interior.

Thus by slow stages, depending upon the amounts of appro-

priations, Which were always small, the work progressed to

meet the needs of the times in various sections. From the

very beginning the important thing of course, was to obtain

a network over the country which eventually could be

adjusted into one rigid framework. This, however, could be

accomplished only through a sers of provisional adjust-

ments, as the net grew, designed to furnish the engineering

public and others with the then most reliable values of geo-

graphic positions available.

The accuracy of the work was from the very first based

on high standards, as it was obvious that to be of lasting

value for an aera the size of the United States the control

would have to be very precise to meet the requirements of

advancing property values in years to come. Specifications

for first-order triangulation, then classed as ~ primary >>,

82 TRAYAUX EN SEANCE PLF, NIERE.

required that the average triangle closures should not exceed

I second, with the maximum closure not exceeding 3 seconds.

Fllrthermore, the arcs of first-order triangulation were to be

spaced at intervals of not more than about 100 miles with

the intervening space divided by second-order triangulation

which called for a standard of accuracy of 3 seconds or less

for the average triangle closure, and a maximum closure not

exceeding 6 seconds. Within recent years this standard of

accuracy for second-order work has been greatly improved

without increasing the Cost of the work.

This improvement has been brought about by requiring

that first-order observations be made at all stations where

it is practicable without the necessity of occupying the sta-

tion a second day. I t is well known that the greater part

of the cost of triangulation is due to transportation and pre-

paring for the observations ra ther than to making the obser-

vations themselves. Under favorable conditions, first-order

observations at a station can be completed in from 2 t o

4 hours and second-order observations of the former stan-

dard requirements in probably 60 to 70 percent of the above

time. Therefore, the cost of first-order observations for all

stations where they c a n be obtained during one night 's

work is not more than would be required to meet former

secu1~d-order requirements and experience teaches tha t the

accuracy thus obtained is represented by an average trian-

gle closure rarely exceeding 1.5 seconds, with maximum

closures never more than 5. Our instructions for second-

order triangulation usually contain the following provi- sion :

The observations will be made in accordance with the specifications for first-order triangulation with the exception that no station need be occupied a second night to improve triangle closures which are 5 seconds or less. >>

No limit is specified for the average triangle closure, but it is seldom greater than 1.5 seconds.

With these standards of accuracy now prevailing for the

T R I A N G U L A T I O N BY C~ L. GARNER. 8 ~

t r i a n g u l a t i o n car r ied on in the Uni ted States, there is

every reason to believe t ha t i t will meet all f u tu re requi-

r ements .

Up to 1932 the to ta l first a n d second-order t r i a n g u l a t i o n

in the Un i t ed Sta tes was about 34,000 miles as measu red

a long the axes of the arcs. F rom 1932 to 1935 n e a r l y five

mil l ion dol lars were al lot ted to the extension of geodetic

control , of ~r a subs t an t i a l a m o u n t was used for t r i an -

gula t ion, as a means of a l lev ia t ing the u n e m p l o y m e n t con-

d i t ions a t the t ime. The expanded p rogram du r ing th i s pe-

r iod advanced the control work in th is coun t ry ve ry ma-

ter ia l ly . Thus , du r ing the period of s l ight ly more t h a n

three years, about 31,000 miles of arcs of t r i a n g u l a t i o n

were added to the net . The tota l a m o u n t of first a n d second-

order t r i a n g u l a t i o n in the net today is 78,000 miles.

I t will be noted t h a t there are sti l l m a n y regions in the

coun t ry whe re no t r i a n g u l a t i o n exis ts in an a rea as much

as 200 miles or more in extent . I t is fu r the r no ted t h a t

the sudden t e r m i n a t i o n of the work in 1935 left m a n y un-

closed c i rcui ts so t h a t the work could not be proceeded

un t i l add i t iona l field work was accomplished. All opera-

t ions since t h a t t ime have been devoted to a consol idat ion

of the field work then accomplished in order t h a t the

m a x i m u m u t i l i t y can be made of the data. They are be ing

made avai lable for d i s t r ibu t ion th rough the facil i t ies of our

r egu la r s taff a t t h e W a s h i n g t o n office together w i th two

compu t ing offices in New York and Philadelphia, opera ted

in coopera t ion wi th the Works Progress Admin i s t r a t i on .

F o r t he t r i a n g u l a t i o n in a very large port ion of the coun-

t ry , pa r t i cu l a r l y the eas t e rn section, the BILBY por table steel

t r i a n g u l a t i o n towers have been in use since 1927. These

towers consis t of a double tower, the inner one for the sup-

por t of the i n s t rumen t , the outer one for the suppor t of the

observer , and m a n y of them have been erected a t more t h a n

100 s ta t ions . These towers are of compara t ive ly l igh t ma-

terial , a 90-foot tower weighing about 6500 lbs., and there-

fore ca~ be hauled by a single semi- t ra i le r 3- ton t ruck.

~4 TRAVAUX EN SEANCE PLENIERE.

Various heights of towers have been used since their in-

ception but experience has proved that a 90-foot tower is

most adaptable for general purposes, and although higher

towers are frequently required, these are obtained through

the addition of 10-foot extensions to the top of the tower in

order to obtain the necessary height without the neces-

sity of adding greatly to the weight of the tower. For-

merly the Survey used complete 129-foot towers but without

any top extensions. The lower sections of such towers were

naturally quite heavy and added considerably to the dif-

ficulty and cost of operations. When the top extensions

are used, the towers are generally guyed with steel wire ca-

bles, but otherwise the towers are not guyed. It should

be noted, however, that the towers are not designed to

withstand violent storms. They are placed where people do

not congregate, especially during unfavorable weather, and

our employees are instructed to leave towers during stormy

weather. A number have been blown over during severe

storms but without casualty. Towers designed with the

n e c e s s a r y s t r e n g t h to w i t h s t a n d a n y s t o r m wou ld n a t u -

r a l l y be e x t r e m e l y heavy, and t h e cost of t r a n s p o r t a t i o n

a n d o p e r a t i o n would inc rease co r re spond ing ly .

A 90-foot t o w e r unde r a v e r a g e condi t ions is e rec ted in

a b o u t five o r s ix hour s t ime. U n d e r f a v o r a b l e cond i t ions

i t can be e r ec t ed in cons ide rab ly less t ime, bu t occas iona l ly

a l o n g e r t ime is r equ i r ed due g e n e r a l l y to dif f icul t ies w i t h

t h e founda t ion , a s when the re is rock ou tc rop to be con-

t e n d e d wi th .

T h e PARKHURST 9-inch d i r ec t ion theodo l i t e is g e n e r a l l y

used on t r i a n g u l a t i o n work b y the Coast and Geodet ic

Su rvey . Th i s is a 2 -mic rome te r i n s t rumen t , each mic rome-

t e r r e a d i n g d i r e c t l y to one second. The f i rs t i n s t r u m e n t w a s

des igned in ou r own shops and subsequen t ly m a n y i n s t ru -

m e n t s h a v e been made b y m a n u f a c t u r e r s on the same spe-

c i f icat ions. The ci rc les fo r these i n s t r u m e n t s a r e v e r y ac-

c u r a t e l y g r a d u a t e d b y the N a t i o n a l Bureau of S t a nda rds ,

wh ich h a s coope ra t ed wi th the Coas t and Geodet ic S u r v e y

TRIANGULATION BY C. L. GARNER. 8 5

for a n u m b e r of years in developing these i n s t r u m e n t s a n d

deserves much praise in i ts efforts to ob ta in circles of a

very h igh degree of accuracy.

At all of our t r i a n g u l a t i o n s ta t ions , in addi t ion to the s ta-

t ion m a r k itself, there are two o ther marks , a reference

and an az imuth mark . The az imuth m a r k is visible f rom

the g round and is placed genera l ly abou t 1 /4 mile d i s t an t

bu t w i thou t a n y accura te de t e rm ina t i on of i ts d is tance f rom

the s ta t ion . The purpose of th i s m a r k is to f u rn i s h engi-

neers and su rveyors wi th an accura te az imuth which can

be readi ly used for local projects .

I wish to men t ion briefly the d i s t r ibu t ion of t r i angula-

t ion stations. In the early triangulation of this country,

it was natural that the first objective was to advance the

net over the large area involved as rapidly as possible. The

stations were therefore placed on high mountain peaks

where possible and many of the lines were long. The dis-

tribution of stations thus obtained was not suitable for

genera] engineering and surveying purposes. After the

completion of the main net in the United States as embra-

ced in the readjustment of 1927, it became possible to plan

on a distribution of stations which would better meet ge-

neral needs.

Recent recommendations for a mapping program by the

Federal Board of Surveys and IVfap_s calls for a distribution

of horizontal control stations in the eastern part of the

country, and in the commercially important regions of the

west, such that there will be no point more than six miles

from a control station, while in the rugged mountains of

the west and some other regions, no point will be more

than about 12 miles from a station. Incidentally, the cor-

responding requirement for vertical control for the eastern

portion of the country and commercially important regions

of the west is that no point will be more than 2 1/2 miles

from a bench mark, whereas in the other regions, no point

will be more than 5 miles from a bench mark.

This distribution of stations means that the arcs of trian-

8 6 TRAVAUX EN SEANCE PLI~,NIERE.

gulat ion wilt general ly consist of s ta t ions about 10 miles

apar t . The lengths of lines na tura l ly depend considerably

on the topography of the region. Our reconnaissance par-

t ies are therefore allowed considerable discretion in the

lengths of the lines selected but l imits of 20 or 35 miles are

imposed where practicable. Therefore, while the main

scheme of an arc of t r i angula t ion may have lines from

8 to 25 miles in length, the desired dis tr ibut ion of s ta t ions

wi th in the figures and to the sides is obtained by use of

supplemental s tat ions. In other words, an a t t empt is made

to determine enough stat ions, not only within the main

scheme but also to the sides, to avoid the necessity of

sending a t r i angula t ion pa r ty in to the same region at some

subsequent time.

P L E N A R Y S E S S I O N

Tuesd~zy, September 1~, 1939, 10 h.

School of Government Building, George Washington Uni-

versi ty .

The following persons recorded their at tendance at this

meet ing :

F. A. Vening Meinesz, Pres ident ; W. D. Lambert, act ing

as Secre ta ry ; Oscar S. Adams, H. G. Avers, C. F. Baeschlin,

H. B. Brown, F. W. Darling, J. A. Duerksen, Owen B. French,

W. Hei,';kanen, J. de Graaff Hunter, Hans S. Jelstrup, Lewis

~. Judson, M. Matuyama, Hugh C. Mitchell, L. L. Nettleton,

Th. Niethammer, B. L. Page, John A. Pounder, Donald A.

Rice, J. E. R. Ross, H. Seligmann, W. D. Su,tcliffe, C. H.

Swiek, Belisario Ruiz Wileh.cs, R. M. Wilson, Delegates.

C. N. Claire, C. R. Duvall, G. Hess, H. P. Kaufman, G. T.

McCaw, J. C. Owen, D. L. Pa~khurst , A. C. Poling, D. C.

Ritchie, K. W. Smith, Guests.

PLENARY SESSION, SEPTEMBER 12. 87

First Part of the Meeting. Business : ResoZutions.

The f i rs t p a r t of the mee t ing was devoted to business ,

e spec ia l ly reso lu t ions .

Genera l Se l igmann, C h a i r m a n of Commiss ion No. 19 (Nota- tions) r e p o r t e d fo r t h a t Commission. The Commiss ion h a s

no final r e p o r t to m a k e a t th i s t ime, bu t i t had a g r e e d on

ce r t a i n p r inc ipes and on some few recommended symbols ,

fewer t h a n in the d r a f t r epor t . The symbols a f fec t ing cer-

t a i n l ines of w o r k were no t cons idered because of t he ab-

sence of m e m b e r s especia l ly compe ten t in those fields. The

A c t i n g S e c r e t a r y , PROF. BAESCHLIN, will send the recom-

m e n d a t i o n s of the Commiss ion for comment and c r i t i c i sm .to

m e m b e r s of the Commission, also to o the r s in te res ted .

A r e so lu t i on commend ing the work of the I so s t a t i c Ins-

t i t u t e a t He l s ink i was adop ted and recommended to t h e

Union fo r adop t ion . (See p. 137.)

The o r d e r in which the va r i ous sub jec t s h e r e a f t e r dis-

cussed were t a k e n up was in gene ra l t h a t of the numer i ca l

d e s i g n a t i o n of t he Commiss ions concerned. (See B u l l e t i n g4oddsique n ' 62) . The page re fe rence are to t he Edition d~finitive. )

Commission No. 2 (Bulletin g~od~s~que, n" 62, p. 386.)

The p roposa l of PROF. H. SCHMEHL (Btdletin g~od~sique, n ~ 62, p. 390), w a s considered.

Mitchel l . T h e F e d e r a l B o a r d of Su rve ys and Maps has

e s t a b l i s h e d a Specia l Commi t t ee on Defini t ions of S u r v e y i n g

and M a p p i n g Terms , w i th a r e p r e s e n t a t i v e m e m b e r s h i p

which expec t s to cover~ the en t i re field of s u r v e y i n g and

m a p p i n g . As c h a i r m a n of t h a t spec ia l commi t tee I have al-

r e a d y done cons ide rab le w o r k in the field of geodesy, the

one in which I a m pe r sona l l y mos t interested~ and I a m

i m p r e s s e d wi th the i m p o r t a n c e and need for the work , a~d

~8 TRAVAUX EN SEANCE PL#,NII~RE.

also with its size. It is not something to be done in a hurry;

1 estimate that it will be several years before my committee

will be ready to make a full report. It is growing in im-

portance because of the adoption of older technical terms

to supply terms of somewhat different meanings in the ne-

wer branches of the art of surveying. Some confusion is

sure to result if such terms are not gSven standard defini-

tions. I am sure that if the Association undertakes this

work, the Committee of which I am Chairman can be of some

service to it and I shall be glad to undertake to provide all

possible cooperation,

It was decided that in view of existing conditions, no ac-

tion was desirable at this time.

Commission No. 3. The proposal of the Amer ican Geo-

physica l Un ion (Bulletin g~od~sique, n ~ 62, p. 394, see p.

142) r e l a t i n g to the effect of f ros t on bench m a rks was

t a k e n up and explained by Mr. SwmK. I t was approved in

pr inciple and re fe r red to the Pres iden ts of the Commiss ions

mos t d i rec t ly concerned, Commission 3, (Triangulation) a nd

Commiss ion 5, (Precise Leveling) for f u r t h e r s tudy.

Commission No. 4. The proposa ~- of the Repor te r on Pro-

ject ions, MAJOR TARDI, (Bulletin g~od~sique, n ~ 62, j p. 407)

for a more accura te table of mer id ional pa r t s ( increased la-

t i tudes) and isometr ic la t i tudes , was refer red to the nex t

assembly of the Associat ion.

Commissions Nos. 3 and 5. The proposal of the Amer i -

can Geophysical Union for the s tudy of ea r th movemen t s

(Bulletin g~oddsique, n" 62, p. 393) was adopted and recom-

mended to the Un ion for adoption.

Commission No. 8. The ques t ion of the in te rva l be tween

worldwide longi tude opera t ions was considered. (Bulletin g~od~sique, n" 62, p. 552, see p. 101.) I t was agreed to

approve the decision reached by the I n t e r n a t i o n a l As t rono-

PLENARY SESSION, SEPTEMBER 12. 89

mical Union in Stockholm on this point, making the interval

between longitude operations ten or fifteen years.

The proposal of the American Geophysical Union in re-

gard to determining the vertical gradient of gravity (Bul- letin g@od4sique, n" 62, p. 554, see p. 135) was .discussed

by Garner and Lambert and approved.

The proposals of the American Geophysical Union urging

further gravity work for the determination of the geoid and

other purposes (Bul~gr g@oddsique, n ~ 62, p. 555,

see p. 133 to these minutes) were taken up and discussed.

Dr. de Graaff Hunter, proposed three resolutions,, namely,

Recommendation for more Gravity Determinations, Recom- mendation for Determining The Geoid by Astronomical Ob- servation, and Recommendation for Torsion Balance Obser- vations; see, p. 133, 138 and 134), which were adopted

as covering not only the same general ground as the

proposals of the American Geophysical Union but also the

interrelation of gravity observations and astronomical

observations. The three resolutions were discussed by

Prof. Baesehlin, adopted by the Association and recom-

mended to the Union for adoption. (Approved by the Union September 15, 1939.)

Commissions Nos. 9 and 11. The proposal of PROF. TO-

MASCHEK and LETTAU for the establishment and operation of

trifilar gravimeters (Bulletin g~od~sique, n ~ 62, p. 560 and

567, see p. 140) was submitted to Prof. Bseschlin, adopted

by the Association and recommended to the Union for adop-

tion. (Adopted by the Union September 15, 1939). Par t b)

since it involved financial questions, was not considered.

A letter from PROF. SCHEPERS regarding the continuance

of the Batavia latitude station was read by Mr. Swick. I t

was suggested that PROF. SCtIEPERS take up the question

raised directly with PROF. CARNERA, Director of the Interna- tional LatitudQ SQrvice.

90 TRAVAUX EN SEANCE PLENIERE.

The ques t ion of officers, committees, and repor te rs was

pos tponed un t i l the mee t i ng on Thursday , September 14,

1939.

The o ther proposals f rom the var ious commit tees were

passed over for f inancial and other reasons.

Second Part of the Meeting. Scientific Part.

The scientific pa r t of the session was opened by Dr. Wil-

chos, Di rec to r of the Mi l i t a ry ~eographie I n s t i t u t e of Bo-

gotfi, who spoke on the work of the Mil i tary Geographic Ins-

t i t u t e of Bogot's and Deflections .of the Vert ical in Colom-

bia, see p. 91 (texto espa~ol) and p. 94 (english text). The pape r was discussed by Prof. Baeschlin and Dr. de

Gra3ff Hunter .

Baeschl in. (No text avai lable.)

de Graaff Hun te r . The large deviat ions of the ver t ical me t

wi th in Colombia p resen t a problem in an agg rava t ed form,

bu t one which is ac tua l ly p resen t in every detached sur-

vey. I n each case it is necessa ry to choose values of la t i tude ,

longi tude and az imuth a t the or igin of a survey. Genera l ly

the observed as t ronomica l values are used. There is no as-

su rance t h a t these agree with a preconceived reference

figure, f rom which there is a lmost cer ta in ly a deviat ion,

la rge or small . The only method avai lable for ca lcu la t ing

such dev ia t ions is provided by STOI~ES' theorem, for which

cons iderable knowledge of the g rav i ty field is needed, a nd

prac t ica l ly i t is un l ike ly t h a t a precision of 1" of ,arc in

the d i rec t ion of the computed vert ical will be ol~tained by

th is means .

I t is ve ry desirable t h a t t r i a ngu l a t i on of a ny c oun t ry be

connected wi th t h a t of ne ighbor ing countries, by which

mean~ the a r b i t r a r y choice of devia t ion a t an or igin po in t

is called for only once for the whole connected sye tm in-

s tead of for each separa te country .

EL I N S T I T U T O GEOGR~tFICO MILITAR DE BOGOTA

Y LAS DES~IACIO[(ES DE LA VERTICAL E:~ COLDMBIA. 9 1

A P P E N D I C E I

Texto espa~o~.

]EL INSTITUTO GEOGR~FIC0 MILITAR DE ~OGOTA Y LAS DESVIACIONES DE LA VERTICAL EN COLOMBIA

pot Belisario Ruiz Wilches.

Colombia se p resen ta por p r imera vez en las Asambleas de

la UniSn geod~sica y geofisica in te rnac iona l y el I n s t i t u t o

geogr~.fico mi l i t a r de que soy Director ha emprendido t ra-

ba jos de Geodesia y Geofisica sSlamente de t res afios pa ra

ac~. E1 informe que tengo el honor de p resen ta ros da

cuen ta del resul tado de tales t r aba jos y en ~l se hace men-

ciSn de que en el escaso lapso de t iempo a que he hecho

referencia se hart hecho t r iangulac iones que cubren una

extens iSn aprox imada de 16000 kilSmetros cuadrados, apo-

yadas en diez puntos de LAPLACE.

E! a ju s t e de es tas redes de t r iangulaciSn, a jus te cuyo

detal le est~ en el informe, da resul tados de plena confianza

pa ra los t r a b a j o s de Geodesia ya que alcanza una d i ferencia

de sSlamente ochenta y cinco milimetro~ en t re la base me-

dida d i r ec tamen te en Car tago y la longi tud de dicha base

ca lculada a t raves de la red desde la base de Prado, es decir

a t r aves de m~s de 300 ki lSmetros de red. E s t a d i ferencia

en t re la base medida d i rec tamente y calculada ~ t raves

de la red es sSlamente de 25 mi l imetros pa ra la base de

Car tago si el c~lculo se hace par t iendo de la base de E1 Sa-

lado, sSlamerzte. P a r a todos los c~lculos se ha empleado

el elipsoide in t e rnac iona l de HAYFORD y paxa las operaciones

de t e r reao se haa empleado pa ra la medida de las bases cin-

ta~ de i a v a r debfdamente pa t ronadas en el Observa tor io de

Tedd ing ton haciendo uso de los aparato~ coaocidos pa ra

92 TEAVAUX EN SEANCE PLENII~RE.

ob tene r una tens ihn cons t an t e ; los ~ngulos se han medido

con el i n s t r u m e n t o WILD T-3.

L a ob tenc i6n de las pos ic iones a s t r on6mi c a s de los pun tos

de LA.PLACE ~e ha hecho p a r a las l a t i t udes por el m~todo de

TALCOTT con i n s t r u m e n t o s t ipo BAMBERG de 54 mm. de abe r -

t u r a y 50 cm. de d i s t anc i a focal y el e r r o r p robab le ha s i d o

el1 t o d o s los casos i n f e r io r ~ 1" de a rco ; el n~mero de p a r e s

de e s t r e l l a s no h a s ido i n f e r io r s 10. L a long i tud se h a

ob ten ido po r recepc i6n de sefiales i na l s de las ~mi-

t i d a s p o r los O b s e r v a t o r i o s de W a s h i n g t o n (Es tac i6n de

A r l i n g t o n ) , L a P l a t a (Monte G r a n d e ) , Rio de J a n e i r o y P a r i s .

L a ho rn se ob tuvo s i emp re por el m~todo de aleuT.as igua les

de e s t r e l l a s (m~todo de ZINGER) escogidas de cats pre-

p a r a d o s p a r a las b a j a s l a t i t udes de Colombia por el I n s t i t u t o .

E1 e r r o r p r o b a b l e p a r a la long i tud en ]a red cen t ra l no ha

p a s a d o de 0, 05 s. As i pues se puede t ene r t ambihn confianza

en los r e s u l t a d o s de las operac iones p a r a ob tene r las coor-

d e n a d a s geogr~f icas p o t A s t r o n o m i a .

Pe ro lo ex6t ico de los r e su l t ados , p r e c i s a me n t e lo que mo-

t i va e s t a expos ic i6n que os hago, soYL los va lo res excep-

c iona lmen te fue r t e s que r e s u l t a n p a r a la desv iac i6n de la

ver t ica l , , t o m a n d o como zero ~ Bogots va lo res a lgunos de

el los t a n g r a n d e s como no los hemos v is to f igu ra r en nin- grin o t ro es tud io .

Vease, po r e jemplo , en el i n fo rme la desv iac i6n de E1

Sa l ado en donde a l canza cas i un minuto , lo que hace que

la d i s t a n c i a e n t r e las dos es tac iones ca l cu lada por las posi-

c lones a s t r o n 6 m i c a s de e l las y con el auxi l io de las t a b l a s

del e l ipso ide de HAYFORD dif iera de la med ida d i r e c t a m e n t e

y ca l cu l ada po r los p roced imien tos geod6sicos sobre el mi smo

el ipsoide, en una c a n t i d a d m a y o r de mil ochoc ien tos me- t ros .

P a r a d a r s e cuen ta del porqu~ de este cur ioso fen6meno

b a s r a c o n s i d e r a r que el t e r r i t o r i o de Colombia es t s a t r a v e -

sado de S u r a N o r t e po r t r e s g r a nde s mac i sos de la Cor-

d i l l e r a and ina , mac i so s cuya a l t u r a s o b r e p a s a en muchos

p tmtos lo~ 5000 me t ros , s epa rados po t los val les de los r~o~

EL IN.STITUTO GEOGR~.FICO MILITAR DE BOGOTA

Y LAB DESVIACIONES DE LA VERTICAL E N COLOI~BIA. 93

Cauca y Magda lena cuyo nivel en la par te t r i angu lada .no

a lcanza a 200 m. La t ea de t r i angu lac i6n a r r a nc a de Bogota

que est~ colocada en la ver t i en te occidental del m~s orien-

tal de los t r e s r amales de la Cordillera, v a a pa r a r al fondo

del valle que separa el r amal cent ra l del ramal occidental.

(V~ase las f iguras.)

E n es tas condiciones todos los pun tos que est~n sobre la

ve r t i en te occidental de la Cordil lera or ienta l tendr&n des-

v iada la ver t ica l con respecto de los pun tos que estAn en la

pa r to or ien ta l de la Cordi l lera cent ra l y la suma de las

desviaciones de la ver t ical en los dos pun tos es la que apa-

rece como desviaci6n de la ver t ical en el cuadro del in forme

y por t an to aparece con un valor exajerado. E x a m i n a n d o la

f igura en la que se han hecho f igurar los diez pun tos de

LAPLACE que se han tomado para la red puede verse que lu-

gares que t i enen posiciones an~logas t i enen el mismo valor

pa ra la desviaci6n de la ver t ical como aparece en t re Bogota

y La S i rena ; en cambio el ~ngulo de las vcrt icales es mAximo

en t r e pun tos que est{m en s i tuac i6n inve r sa con relaci6n a

la Cordil lera, por e jemplo La Espe ranza y E1 Salado.

Es indudab le que la ver t ica l de BogotA, que pa ra efectos

de comparac i6n se ha tornado como no d e s v i a d a debe e s t a r

desviada t ambi6n no pudiendo t ener un pun to de re ferencia

pa ra ob tener valores absolutos de la ver t ica l sino cuando lle-

gue a red geodSsica a la Costa At l~n t i ca en donde puede

suponerse que un punto cerca de la ori l la del m a r y prece-

dido de una g r a n l l anu ra como por e jemplo Puer to Colombia

t enga una direcciSn de la g ravedad sens ib lemente igual a la

n o r m a l a la superficie de equil ibrio del geoide al nivel del m a r

supues to el geoide homogeneo y po t cons iguiente m u y cer-

cana de la n o r m a l al elipsoide de comparaci6n en ese p u n t o ;

pero adn fa l t a mucho t r aba jo pa ra l legar con la red del in te-

r ior a Ia costa y mien t r a s tanto , t a n to las la t i tudes como Ins

longi tudes ob ten idas a s t ron6micamen te se r ian i lusorias pues

cuando se ha tenido la sa t is facci6n de reducir el e r ror pro-

bable en el t r aba jo de As t ronomia a u n valor insignif icante,

7

94 TRAVAUX EN SEANCE PLENIERE.

se sabe que dent ro de esos valores hay causas de e r ror t a n

super iores que a fec tan la posiciSn geogr~fica muchis imo m~s

de lo que pud ie ra a fec ta r la un t r aba jo de As t ronomia de

campo aunque fuese b a s t a n t e descuidado y que tales e r rores

por su n a t u r a l e z a no son f~ciles de el iminar .

E n s in tes is la posici6n de Bogota, que se ha tornado como

fundamen ta l , es b ien seguro que no sea la real sobre el elip-

soide de comparac i6n ya que sus dos coordenadas pueden es-

t a r a fec tadas de er rores que se desconecen; pero por el mo-

mento no t iene el I n s t i t u t o geogr~fico mil i tar , m a n e r a de

l legar a u n me jo r valor de tal posici6n. Es te problema, j u n t o

con el que envuelve el hecho de habe r encontrado en Colom-

bia t a n al tos valores ,para la desviaci6n de la vert ical , valo-

res cuya explicaci6n pudiera set so lamente la que he esbo-

zado en es ta exposiciSn, es lo que he querido somete r a vues-

t ro es tudio y a vues t ro i lus t rado criterio.

A P P E N D I X I

(English text.)

T H E M I L I T A R Y G E O G R A P H I C I N S T I T U T E O F B O G O T A

A N D D E F L E C T I O N S O F T H E V E R T I C A L I N C O L O M B I A

b y B e l i s a r i o R u i z Wilches.

Colombia is represen ted for the first t ime at a mee t ing of

the I n t e r n a t i o n a l Un ion of Geodesy and Geophysics, and the

I n s t i t u t o Geografico Mili tar , of which I am the director, un-

der took geodetic and geophysical act ivi t ies only three yea rs

ago. The repor t which I now have the honor of b r i n g i n g

before you gives an account of the resul t s of those act ivi t ies ,

and in it you will find reference to the fact t ha t shor t period

t r i a n g u l a t i o n s have been carr ied out which cover an ap-

p rox ima te a rea of 16,000 square ki lometers, and which are

suppor ted on t en Laplace points .

The resu l t s of the a d j u s t m e n t of these t r i angu la t ions ,

THE MILITARY CEOGRAPHIC INSTITUTE OF BOGOTA. 95

the actual detail of which can be found in the report, are

so ~atisfactory, in spite of the very difficult topographic

and climatic conditions encountered, that a difference of

only 85 ram. was found between the direct measurement of

the Cartago base, and its calculated length as computed

through more than 300 kilometers of triangulation from the

Prado base. This difference becomes one of only 25 ram.

if we compute the Cartago base from the length of the in-

terrnediate base at El Salado. The tables of the HAYFORD

international ellipsoid were used in all the calculations. In-

vat tapes calibrated at the National Physical Laboratory

at Teddington, England, and standard tensiorz equipmerLt

were used for the measurement of base-lines and WILD T-3

theodoli tes for t ha t of angles.

The la t i tude determinat ion of the LAPLACE points was af-

fected by the TALCOTT method, using at least ten s t a r pa i r s

i~ each case, the instruments employed being of the B~-

BERG type, with an aperture of 54 ram. and a focal length of

60 cm. The probable error has in all cases been inferior to

I" of arc. The wireless signals for the determination of

longitude were those of the observatories of Arlington

(Washington) , Monte Grande (La P la t a ) , Rio de Janeiro,

and Paris . Time was determined by the ZINGER method, the

s t a r pa i rs being selected from the special catalogues pre-

pared by the Ins t i tu te Geogr~fico Mit i tar for the low lat i-

tudes of Colombia. The probable e r ror in longitude for

the central network has not exceeded 0.05 s. These out-

s tanding resul ts are a proof of the re l iabi l i ty of the as t ro- nomical work.

Now let us come to some very s ta r t l ing resul t s obta ined

by the methods jus t outlined, and which refer to ex t raor -

d inar i ly high value~ encountered in the deviat ion of the

plumb line from the true vertical, deviat ions which were

computed taking the deviation in Bogota as negligible. These

resa l t s const i tute the very reason for this exposition, and

are la rger than any we have hi ther to come across.

in the report , for example, the deviation at E! Salado near

96 TRAVAUX EN SEANCE PLI~,NI~RE,

Ibague, the capital of the department of Tolima, which is

situated just at the foot of the eastern slopes of the Cor-

dillera Central is given as being nearly one minute. Thus

the distance between the stations at Bogota and E1 Salado,

as obtained from the astronomical fieldwork, using the tables

of the HAYFORD ellipsoid differed from the corresponding

distance, as measured directly by geodetic triangulation and

reduced also to the HAYFORD ellipsoid, by over 1800 me-

ters.

This curious fact arises because the terri tory of Colom-

bia is traversed from south to north by the three mountain

ranges of the Andes, of a height greater than 5000 meters at

many points, which are separated by two very deep valleys,

tha t of the Magdalena River with only 200 meters of ele-

vation to the east, and that of the Cauca to the west of

the central Cordillera. The network of triangulation which

starts from Bogota, on the western slopes of the eastern

Cor.dillera, ends at the bottom of the Cauca Valley, be-

tween the central and western Cordilleras, (see the figure

p. 97)

Thus our s tar t ing point, BogotA, being on the western

slopes of the eastern Cordillera, has a deviation in an oppo-

site sense to that of any point situated on the eastern slo-

pes of the central Cordillera, and the sum of the deviations

is tha t which appears on the report. This accounts for the

large values obtained. In looking at the figure, in which

the ten LAPLACE points appear, it can be seen that places,

such as Bogota and La Sirena, having similar positions with

regard to the mountain ranges, have analogous values for the

deviation from the vertical; while the difference in deviation

is greatest at places such as La Esperanza and E1 Salado,

which are inversely located with regard to the mountain ranges.

I t is evident that the vertical at BogotA, to which the

other verticals have been referred, must be deviated; but

it seems impossible to obtain a true vertical till the geo-

detic network reaches the Atlantic Coast, where it may b~

THe, MILITARY GEOGRAPHIC INSTITUTE OF BOGOTA. 97

~.~

~ o.~

0

O, 0 0

0 0 0 0 0 0 0 0 0

�9 ~ ; ~ u~ "H

~ o

98 TRAVAUX EN SEANCE PLENIERE.

supposed t h a t g r a v i t y in a p lace on the sea - shore oppos i te

a l a rge p l a in a c t s a l o n g a l ine n o r m a l to t he equ i l ib r ium

su r f ace of t h e geoid. Bu t th i s ex tens ion of t he t r i a n g u l a t i o n

en t a i l s a l a r g e a m o u n t of f u t u r e work, and meanwhi l e the

pos i t ion of t h e LAPLACE po in t s r e m a i n s u n t r u s t w o r t h y , even

when t h e p r o b a b l e e r r o r of the a s t ronomica l d e t e r m i n a t i o n s

fo r these p o i n t s has been r e duc e d to negl ig ib le values , and

those pos i t i ons a r e u n t r u s t w o r t h y because we know nosy

t h a t e x t e r n a l causes of e r r o r ac t in such a w a y as to in t ro-

duce d i s c r epanc i e s f a r l a r g e r t h a n those t h a t would r e su l t

f r om ca re less t echn ique in the a s t ronomi c a l obse rva t ions .

To s u m up : The p r o v i s i o n a l pos i t ion of BogotA, which

has been used a s or ig in , wil l c e r t a i n l y not be the rea l one

on the HAYFORD el l ipsoid , s ince i t s two coord ina te s m a y be

a f fec ted b y e r r o r s whose m a g n i t u d e is unknown; b u t for

t h e t i m e b e i n g t h e I n s t i t u t o h a s no means of a r r i v i n g at

a more a c c u r a t e value. This problem, t o g e t h e r w i th t h a t

involved in t h e f ac t t h a t such h igh va lues fo r the dev ia t ion

f rom the v e r t i c a l were found, and whose cause could pre-

s u m a b l y on ly be t h a t s k e t c h e d ~ i n th i s paper , is w h a t I

w a n t to s u b m i t to y o u r s t u d y and exper ienced judgznent .

A P P E N D L X H

A C T I O N T A K E N B Y T H E I N T E R N A T I O N A L A S S O C I A T I O N

O F G E O D E S Y ON V A R I O U S P R O P O S A L S

International I s o s t a t i c Bureau a t Hels inki .

The Internationed Union of Geode'sy and Geophy~;.cs

expresses its high appreciaticm of the scientific worTc c~lready done by the International Isostatic Inst~t~,te at Hels~nki undvr the leadership of Dr. W. HEISI,:ANEN and expresses

the opinion that the co~tin~ation of this bt~eau is of the

ACTION TAKEN BY THE INTERNATIONAL

ASSOCIATION OF GEODESY ON VARIOUS PROPOSALS. 99

greatest importance for all the sciences engaged in the s tudy of the Earth 's crust 1.

Adopted by the Associa t ion September 12, 1939 and re-

commended to the I n t e r n a t i o n a l Union for adoption. Adop-

ted by the I n t e r n a t i o n a l Un ion September 15, 1939.

CO1KMISSION 2

Bulletin g~od~sique n" 62, p. 390.

Propose ! of Prof . Dr. H. Schmehl .

a) Eine E n t s c h l i e s s u n g der I n t e rna t i ona l e n Geod~itisch-

Geophys ika l i schen Vere in igung (IGGV) ;

Die I.G.G.V. hdlt die Herstellung eines Fach-W6rterbuches tier C-eoddsie in deutseher, englischer, franz6sischer und ~talienischer Sprache f~r eru~nseht.

b) Beratungen dari~ber, wie die Verwirklichung diese~ Wunsehes am schnellston zu erreichen ist.

Cons idera t ion postponed to a more favorable occasion.

C O M M I S S I O N $

Bulletin gdod~sque n ~ 62, p. 394.

Proposa l of the Amer ican Geophysical Union.

The Asociation of Geodesy wishes to encourage experi- mental test in the protection of survey markers from frost action under various climatic conditions.

Approved in pr inciple by the Associa t ion a nd re fe r red to

the P re s iden t s of Commissions 3 (Triangulation) and 5

(Precise Leveling) for f u r t h e r s tudy.

1: See p. 139 the Resolution concez~ing the periodic calculation of the form of the geoid.

100 TRAVAUX EN SEANCE PLEIqTERE.

COMMISSION 4

Bulletin g~od~siquv n" 62, p. 407.

Proposition du Rapporteur g~n~ral des Triangulations.

~ur l'opportunit~ de publier avee le nombr~ de d~eimaZes voulues une table des lar croissante~s et des latitudes isom$triques concernant les trois ellipso~des de Bessel, Clarke

(1880) et Hayford. Une pareil le table a ~t~ publi~e r~cem-

ment mais avec une precision insuffisante pa r le BUREAU

HYDROGRAPHIQUE INTERNATIONAL DE IVIOlqACO.

COMlVIISSION 5

Bulletin g~oddsique n ~ 62, p. 393 et 427.

Proposal of the American Geophysical Union.

The Association of Geodesy wishes to emphasize the im-

portance of accurate measurements of crustal movements in both a vertical and a horizontal direction. A knowledge

of these movements is not only important as affecting Che per~nanence of geodetic surveys and markers but is of much value to seismologists in investigation,s of the oecurence of earthquakes.

Adopted by the Associat ion and recommended to the In-

t e rna t iona l Union for adoption. Adopted by the In te rna-

t ional Union at i ts Plenary. Session. September 15. 1939.

COMDIISSION 5

Bulletin g~od~sique n ~ 62, p. 427, 533.

R~duction du nivellement g~om~trique.

E tude d'une mode de r~duction correct du nivellement

g~omdtrique, compte tenu des valeurs r~elles de la gravitY.

ACTION TAKEN BY THE INTERNATIONAL

ASSOCIATION OF GEODESY ON VARIOUS PROPOSALS. 101

Rapport de TH. NIETHAMMER, avec un p r o j e t de ~Qeu (p. 533),

a y a n t le m~me o b j e t qu 'un p r o j e t de vceu pr~sent~ p a r H.

SCH~mHL (p. 427) , a ins i con~u en vue de comple t e r l e s

nive l l ements g~om~tr iques p a r des mcsures de g rav i t~ :

D~e lnternationale Geodiitische Geophysikalische Verei-

nigung (I.G.G.V.) spr~cht den Wunsck aus, dass die Nivelle- ments-Linien zur Verbindung der griisseren Meere zweeks

Ermitt lung orthometrischer HSkenunter~chiede systema-

tisch mit Sehwerestationen besetzt werden.

The p roposa l s of P r o f e s s o r s N i e t h a m m e r and Schmehl

we re a p p r o v e d in pr incip le . The fo rmu la t i on of a r e so lu t ion

combin ing the two p roposa l s was r e f e r r ed to t he Bureau

of t he Assoc ia t ion .

C O M M I S S I O N M I X T E 8

L ' o r d r e du j o u r de la Commiss ion mix te 8 peu t ~tre a ins i

fix6 :

2. - - Approbation de la d4cision vot4e & Stockholm d'ajour-

ner la discussion d'une reprise @ventuelle des operations ~, de

fa9on qu'un ddlai de dix ou quinze ans s'4co.ule entre deux

operations successives. . . . . . . . . . , �9

The A s s o c i a t i o n of Geodesy concur red in th is decis ion

(Sep tember 12, 1939).

C01WM'ISSION 9

Bulletin g~od~sique n" 62, p. 554.

P r o p o s a l of the A m e r i c a n Geophys ica l Union.

a) The Association of Geodesy calls attention to the known

variability of the vertica~ gradient of gravity and expresses

1. De longitudes mondialea.

102 TRAVAUX EIq SEANCE PLENIEt%E.

the hope that it may be studied with the aid of recently

developed sensitive gravimeters.

Approved by the Association of Geodesy September 12,

1939.

C O M M I S S I O N S 9 AND 2Q

Bulletin gdoddsique n" 62, p. 555.

Proposa l of the Amer ican Geophysical Union.

b) The Association of Geodesy wishes to express anew its

desire that Resolution X X on tke Determination of the

Geoid (Union g6od6sique et g~ophysique internationale;

sixi~me Assembl~e g6n6rale r6unie ~ ]~dimbourg, 14-25 sep-

tembre 1936, Camelot Press Limited, London and Southamp-

ton, 1937, p. 62-63) be put into effect. In addition to the

observations recommended in thi8 resolution the Association

desires an increase in the number of observations with the

torsion balance to be correlated w~th other observations in determining the geoid.

c) The Association of Geodesy deems it desirable for tkr

various countries to establish comprehensive nets of gravity

reference ~tations as rapidly as possible in order that local

geophysical surveys related to the gravitationa~ field may be coordinated whenever this is feasible.

The substance of these two resolutions, and more also,

is contained in the three resolutions on this same general

subject adopted by the Association September 12 and appro-

ved by the International Union September 15, 1939.

1. - - R e c o m m e n d a t i o n for more Gravi ty Determinat ions .

The InternationaT, Union of Geodesy and Geophysics

wishes to express anew that it is of the highest importance

ACTION TAKEN BY THE INTERNATIONAL

ASSOCIATION OF GEODESY ON VARIOUS PROPOSALS. 103

not only for geophysical research but also for the determi- nation of the geoid, that gravi ty work should be extended, especially in regions where no such work has been done.

Adopted by the Association of Geodesy September 12,

1939. Adopted by the International Union in Plenary Ses-

sion, September 15, 1939.

2. - - ~ecommendation for Detel~fining the Geoid

by Astronomical Observations.

The International Association of Geodesy, recognizing the

importance of the method of determining the form of the geoid by integration of values of the deviation of the ver- tical, e~ther along lines or over networks, expresses the hope that all countries will carry out such work in the way best suited to their special circumstances. The work of this nature mentioned in the Rapport sur les d~viations de la

verticale to the Washington meeting, provides examplvs of valuable progress in this field, particularly in India and Switzerland.

Adopted by the Association of Geodesy September 12,

1939. Adopted by the International Union in Plenary Ses-

sion, September 15, 1939.

3. - - Recommendations for Torsion Balance Observations.

The International Union of Geodesy and Geophysics wishes to recommend that, in addition to other observatione for determining the gvoid, also observations wi th the torsion balance should be made at suitable stations for this pur- pose.

Adopted by the Association of Geodesy September 12,

1939. Adopted by the International Union in Plenary Ses-

sion, September 15, 1939.

104 TRAVAUX EN SEANCE PLENIERE.

C O M M I S S I O N M I X T E 11

Bulletin g~oddsique n ~ 62, p. 567.

Proposals of PROF. R. TOMASCHEK and H. LETTAU

through the Deutsche Vereinigung fiir Geod~sie und Geophysik.

Die internationale geodiitisch-geophysikalische Vereini-

gung hiilt die Aufstel lung weiterer Trifilar-Gra':L~,c.~er und Horizo~tal-Doppelpendel in verschiedenen Erdte.ilen und mi t verschiedenen Ki2stenabstand fur n6tig zum Studium der

elastischen Gezeiten der Erdk6rpers und der Verschieden- heiten der EIastizitiit der den Erdmantel zusammensetzenden

Gesteine. Translation.

The International Union of Geodesy and Geophysics devm~ the establishment of fur ther stations provided wi th trifilc~r- gravimeters and horizontal double-pendulums and at diffe- rent distances from the coast to be necessary for the s tudy of the elastic earth tides and of the variations in ,the elaz- t ici ty of the rocks forming the earth's crust.

Adopted by the Association of Geodesy, September 12,

1939. Adopted by the International Union in Plenary Ses-

sion, September 15, 1939.

P L E N A R Y S E S S I O N

Th~r3day, September 14, 1939, 14 h. 30 ~ .

School of Government Building, George Washington Uni-

versity.

The following persons recorded their attendance at the

meeting :

F. A. Vening Meinesz, President; W. D. Lambert , acting as

Secretary; Oscar S. Adams. C. F. Baeschlin, D. H. Baldwin,

PLENARY SESSION, SEPTEMBER 14. 105

Charles J. Clif]ord~ F. W. Darling, J. A. D~erksvn, Maurice Ewing, H. W. Hemple, J. de Graaf] H~nter, Hans S. Jelstrup, Lewis V. Judson, P~I. Mat~yana, Charles McCombs, Delmas B. Newlin, Ernest J. Parkin, John A. Po~nder, Howard S.

Rappleye, M. Rosenman, J. E. R. Ross, James B. Small, W. D. Sutcl~f]e, C. H. Swick, Chuji Tsuboi, Dr. R. Vaj~:, John

E. Willis, R. M. Wilson, Delegates.

H. Bouchard, C. N. Claire, Jesse Hi!l, G. T. McCaw, O. S.

Reading. Donald A. Rice, Paul A. Smith, Guests.

First Part of the Meeting. Business.

The meeting began with some items of business not dia-

posed of on September 12, 1939.

Mr. Willis asked considerat ion of a resolution recom-

mending the wider use of the photo~~ zenith tube for

as t ronomical observations. The meeting approved the reso-

lution in principle and asked the Pres ident and Secre tary

to formula te the exact wording.

The Union was asked to approve the resolution (adopted

by the Union, September 15). For the tex t of the resolu-

t ion as approved by the Associat ion of Geodesy and by the

In te rna t iona l Union, and for Mr. WmLIS'S explanat ion and

Dr. JELSTRUP'S opinion see Appendix ! to these minutes.

In accordance with the decision of the In te rna t iona l Union,

no elections of officers are to be held a t this meeting, the

present officers continuing to serve until thei r successors

are chosen by postal vote, or otherwise. This decision does

not apply to members of scientific commission nor to re-

por ters .

The Association voted, however, to continue the member-

ship of scientific commissions unchanged, wi th the understan-

ding t ha t any delegate interested in a subject could be en-

rolled as a member of the corresponding commission on ap-

plicatio~ te the Gener~l Secretary.

106 TRAVAUX EN SEANCE PLF.NtERE.

Dr. de Graaf f H u n t e r a sked for the fo l lowing add i t i ons

to t he m e m b e r s h i p of Commiss ions 18 and 20.

Commiss ion 18 (Arc of meridian through Siam and adjoi- ning countries and the junctions of the Malay Archipelago with Australia and the Philippines) : COMMANDER C. L. GAR-

NER (U. S. A. ) .

Commiss ion 20 (Geoid) : Mr. F. A. ~V[CDIARMID (Canada ) .

These add i t i ons were approved .

T h e A s s o c i a t i o n f u r t h e r vo ted to cont inue the R e p o r t e r s

g e n e r a l in office. The E x e c u t i v e Commi t t ee w a s a u t h o r i -

zed to n a m e s u b s t i t u t e R e p o r t e r s in cases of necess i ty .

Second Part of the Meeting Scientific Part. Precise Leveling.

The scient if ic po r t i on of t he me e t i ng fol lowed, n a m e l y

a d i s cus s ion of P rec i se Leve l ing led by Mr. H. S. RAPPLEYE

a n d Mr. C. J . CLIFFORD. See Appendix II to these minu tes ,

V~'tical Eart~ Movements by CHARLES J. CLIFFORD.

The s u b j e c t w a s f u r t h e r d i scusse r by Prof . Baeschl in ,

Dr. Tsuboi , Dr. Vening [~einesz, Dr. de Graaf f Hun te r . and

o the r s .

Baesch l in . To the ques t ion w h e t h e r o r not changes of th~

bubble a r e obse rved whi le t he . d r u m of the Z~,Iss leve l l ing

i n s t r u m e n t is t u rned , the a n s w e r is t h a t no change of t he

bubb le can be observed.

I n Swi t ze r l and losses of bench m a r k s by the r econs t ruc -

t i on of r o a d s occur, i f t he bench m a r k s a r e in a r o a d o r

s t r e e t . F o r t h i s r e a s o n we now place the bench m a r k s in to

~he wa l l s of old s tone houses . (B lackboard ske t ch of t y p e s

of bench m a r k s ) . I n the town of Zi l r ich changes of the

h e i g h t of the bench m a r k s a re observed, i f new blocks of

l a r g e bu i ld ings a r e cons t ruc ted . The level ing of the town-

sh ip is r e p e a t e d e v e r y fou r or five years .

Tsuboi . We have re leve l led sca rce ly a s ingle l ine in J a p a n

w i t h o u t f i nd ing some change in the e l eva t ion of the bench

PLENARY SESSION, SEPTEMBER 14. 107

m a r k s on it. I t would be ra the r too long a s tory to tell

here all t ha t we have found. The fact t ha t in one ease the

ver t ica l change t ak ing place dur ing the work of level ing

could not be neglected will i l lus t ra te how great the defor-

ma t ions are in Japan.

A n art icle by TsuBoI about these ma t t e r s appeared in the

recent issue of Erg,ebnisse der Kosm~schen Physik. Yening Meinesz. W h e n repea t ing the leveling of the Ne-

t he r l ands a f te r fifty years, unex3ected va r ia t ions were found.

In one ins tance a change was found of as much as 30 era.

on uns t ab le g round (15-30 meters of pea t ) ; this is a t t r i bu -

ted to changes of the bui ld ings bear ing the bench marks ,

bu t we are no t sure about it.

I t is t hough t in Hol land tha t the l imit of accuracy has

now been reached because of the dai ly changes in the ver-

t ical and the effect of the ea r th t ides; i t is believed t h a t

f u r t h e r re f inement in i n s t r u m e n t s or methods b r ings no

appreciable advantage .

de Graa f f -Hun te r . - - In Ind ia for a good n u m b e r of

years pr ior to 1926 an e r ror of about 0.5 foot in re la t ive

he ights of two points of ele;cation difference about 6000 feet

could not be explained. I t was finally a t t r i bu t ed to d iu rna l

changes in length of the wooden levell ing s taves. I t had

been the pract ice to s tandard ize them peroidically, bu t i t

h~.d been assumed tha t there was no d iu rna l change in

staff l engh t due to t empera tu re and to exposure to the sun.

W h e n tests a t var ious hours of the day were made i t was

found t h a t the changes t h rough the day were suff icient to

account for the e r ror found, and la ter observa t ions made

wi th inva~-s t r ip s taves e l iminated it. Such e r ro rs are made

no tab le when the general level of a line does not va ry much,

bu t become conspicuous on lines on which there is a sys te-

ma t ic error . Perhaps a s imi lar exp lana t ion would apply to

the resu l t s in Canada ment ioned by Dr. Ross.

108 TRAVAUX EN SEANCE PLI~-NI~RE.

A P P E N D I X I

Le t t e r of Mr. JOHN E. WILLIS, Naval Observatory,

to MAJOR W. BOWIE and Dr. W. D. LAMBERT.

September 13, 1939.

See Commission mix te 6 : Lati tudes, p,

Opinion of Dr. Je ls t rup on the Zenith Tube.

September 14, 1939.

The President asked the opinion of Dr. JELSTt~UP as to the proposal of Dr. WmLIS of the Naval Observatory for recommending the s ta r t ing of a new international coopera-

tion in longitude, latitude, and other astronomical mat ters , using the photographic methods emphasized by the Naval Observatory.

Mr. Je ls t rup s t rongly recommended this work f rom a scientific point of view, and only hoped tha t the present crisis would not upset the s ta r t ing of this task too much.

i~so lut ion .

September 14, 1939.

The INternational Association of Geodesy expresses the opinion that ~t would be of great importancv to geodesy and geophysics ta extend the use of photographic zenith tube fo a number of ~tations throughout the world.

Adopted by the Association of Geodesy September 14, 1939. Adopted by the Internat ional Union September 15, 1939.

VERTICAL EARTH ~'IOV~]MENTS, BY CH. J. CLIFFORD. 1 0 9

A P P E N D I X II

VERTICAL EA~T. '-I M D V E ~ E ~ ' T S

by Ch~r lzs J. Clifford.

At this time I intend to discuss the sometimes abrupt, so-

metimes slow, vertical movements of the earth's surface

resulting from earthquakes and similar causes, and not

those slow osciUations of the earth's crust caused by earth

tides, changes in gravity, and similar speculative causes.

One of the first studies of vertical movements made by

the Coast and Geodetic Survey concerned an area in southern

California in the vicinity of the-Salton Sea. The original

leveling through this area was run from San Diego, Califor-

nia, to Yuma, Arizona, in 1920 and 1927. In 1928 another

line was run nortluvard from El Centro to Colton. Portions

of both of these lines were releveled in 1931, after an earth-

quake had occurred in h,larch 1930. I will now pass around

this blueprint showing the results of this investigation.

The upper line of the diagram shows the elevations of the

bench marks above mean sea level. The vertical scale is

I to !0,000, and the horizontal ~cale, 1 to I00,000. The

lower part shows the divergence .of the 1931 leveling from

the original. In this part the vertical scale is I to i, sho-

wing the true divergence, while the horizontal scale still

remains I to I00,000. It will be seen that the maximum dis-

placement of about 7 centimeS_ors took place in the vicinity

of Holtville.

The region around Los Angeles is an earthquake area

with one or two fault zones. A r.et of rather closely spaced

lines of leveling had been run in this vicinity. ~J!ovement

had long been suspected because of the necessity of rerun-

ning small portions of the lines. In order to lay the ground-

work for future study the entire net wan releveled in 1931

and I~32, and sever~.l nov; lines were added to strengthen

3

110 TRAVAUX EN SEANCE PLENI~RE.

t he net . The computa t ion and ,adjustment of this work

had ba re ly been completed when the ea r thquake of

March 10, 1933, occurred. The grea ter pa r t of the ne t

was releveled immedia te ly and one new line was run. This

re leve l ing showed t h a t the m a x i m u m movement occurred

a t and n e a r Sig-nal Hill in the n o r t h e r n par t of Long Beach,

where the e levat ions are approx imate ly 0.7 mete r h igher

t h a n before the ear thquake . This work has no t ye t been

s tud ied and therefore no definite conclusions have been

reached. However, the resul t s of the leveling can be made

avai lab le to anyone who wishes to s tudy this case.

At Helena, Montana , an ea r thquake occurred in October,

1935. The f i rs t -order line th rough Helena was releveled in

the sp r ing and s u m m e r of 1936, toge ther wi th some second-

order l ines in the vicini ty. Several new lines were r u n as

a bas i s for f u r t h e r study. Only s l ight differences were

ob ta ined on th is releveling, and they were not definite enough

to ind ica te movement .

In wes t e rn Nevada a severe ea r thquake took place in

December, 1932. A first- order l ine was run in 1933 across

a f au l t which developed. This l ine has not yet been re-

run .

In 1911 a f i rs t -order l ine of levels was rune wes tward

f rom Br igham, Utah , toward San Francisco, California.

Along th i s l ine beach marks were placed a t in te rva l s of

th ree to five miles, as was the cus tom at t ha t t ime. On

March 12, 1934, there was an ea r thquake which developed

evidence of f au l t i ng across th is line nea r Kosmo, Utah. A

p a r t y work ing in t h a t region was ins t ruc ted to r e run the

leveling, ob t a in ing checks both to the eas t and to the west

of the faul t . D u r i n g the course of this level ing new bench

m a r k s were es tab l i shed at one-mile intervals . The resul t s

show t h a t the m a x i m u m movemen t of - -0 .3903 mete r oc-

cu r red a t F 10 abouts 0.2 mile eas t of Kosmo.

A n o t h e r region of possible seismic ac t iv i ty where level-

i ng has been done is nea r New Madrid, Missouri. Severe

oa r thquakes took place here in 1811 and 1812, bu t there

VERTICAL EARTH ~,IOVErvIENTS, RY CH. J. CLIFFORD. lil

has been none of importance since then. Several closely

spaced second order lines hhve been run in this neighborhood,

but they have not yet been rerun for investigation. Por-

tions have been releveled to eliminate errors in the original

work,

In California, lines af first-order levels have been run

across 8 known faults from Los Angeles eastward. These

lines were connected to leveling already run. For a distance

of five miles on each side of these faults bench marks were

placed very closely. In the first mile they are I00 feet

apart; in the second mile, 200 feet; increasing the distance

I00 feet for each mile, until in the fifth mile they are 500

feet apart. Several of these lines have been releveled, but

the work has only recently been completed, so that there

has been no opportunity to study it.

Although the region near San Antonio, Texas, is not very

subject to seismic activity, yet some movement has occurred

there. In 1917 a first-order line of levels was run through

San Antonio and then in 1932 another was run southward

from Brady to San Antonio. It was necessary to level over

a large number of bench marks before a check was obtained.

Because of the discrepancies developed, it was decided to

relevel over all the bench marks in San Antonio and its

vicinity, obtaining checks, of course, with the older leve-

ling. It seems peculiar that the results of this leveling seem

to indicate that a bench mark placed in a large stone rail-

ro~d station has risen 25 millimeters. No explanation of

this has been made. because the leveling has not been stu-

died.

In San Jose, California, there has been the greatest set-

tlement that has ever been observed in this country. In

1911 and 1912 a line of first-ordre levers was run from

Brigham, Utah, to San Francisco, California, through Ni-

les, San Jose, and Redwood City. In 1920 another line

was run southward from San Jose ta Santa Ann.

The leveling from San Francisco to San Jose was rerun

in th~ winter of !931-32. Boneh marie P 7 on the Hall of

112 TR~'VAUX EN SEANCE PLI~NIERE.

Records in San Jose was found to haw settled 3.7 feet or

1.13 meters since 1912. The following winter releveling

was again extended from San Francisco through San Jose

and Niles to firm ground. This releveling was also conti-

nued from San Jose southward to Santa Margarita. The

results showed that P 7 had settled an additional 0.4 foot,

'making the settlement since 1912, 4.1 feet or 1.25 meters.

A check was obtained about 12 miles south of San Jose at

Coyote, where the difference between the leveling of ~920

and 1932-33 amounted to 0.4 foot. This axnount is believed

to represent the amount by which P 7 had settled between

1912 and 1920.

A_ close network of levels has .been established around

San Jose, and some lines have been leveled as many as

seven times. The settlement continued until it amounted to

about 5 feet, but it has now apparently ceased, for some

increase in elevation was noticed on the last releveling.

While this area has not yet been studied, it is my belief

that this settlement has been caused by excessive pum-

ping of ground water, which caused the clay subsoil to

compact.

Now I come to a type of problem for which we have not

obtained a solution. ~ great many times, when we tie new

lines to lines which were run 40 or 50 years ago, we find

that we must relevel considerable portions of the old work

before we obtain checks. Often we secure cheeks in two

groups of marks, and yet the two groups do not check

each other. We do not know whether we should attribute

these failures to check to ground movements or to errors in

the old leveling. From the frequency with which these ca-

ses occur, we are inclined to believe that they are caused

by movements of groups of marks.

A typical case is the one in which we attempted to tie a

second-order line to the first-order line between Crookston

and Stephen, ~{innesota. It was necessary to level over

about half of the old line to Stephen, where a check was

s e d u r e d . T h e d i s e r e ~ a n c l e s w e r e so l a r g e t h a t i t wa~ d s e i d e d

JOINT MEETING OF OCEANOGRAPHY,

GEODESY, METEOROLOGY, HYDROLOGY. 1 1 3

to relevel the en t i re f i rs t -order l ine f rom Crookston to Ste-

phen. This was done in 1938, and some very er ra t ic move-

men t s of marks were disclosed. These movements r anged

f rom a few mi l l imeters to about 15 cent imeters . This l ine

is about 80 k i lometers in length. An in te res t ing fea ture is

the fact t h a t a second-order l ine f rom Stephen to E m e r s o n

in Mani toba checked very closely a line which had been r u n

m a n y years previous ly by the Geodetic Survey of Canada.

J O I N T M E E T I N G O F T H E ASSCCIA~O~*S O F O C E A N O G R A P H Y , GEODESY, 5].~T]~OXOLOGY

A N D } I Y D ~ 0 L � 9

T~esday September 12, 1939, I~F h. 45 m.

The fol lowing persons were in a t t endance ~ :

D. La Cour, P res iden t ; O. S. Adams, P. J. Aguerrevere ,

A. R. V. Arrel lano, C. F. Baeschlgn, H. C. Barlcsdale, L. V.

Berkner , W. Bowie, M. L. Brashears , H. B. Brown, D. J. Ce-

ders t rom, L. A. Cole, E. B. Collins, L. H. N. Cooper, F. L.

Culley, W. Darling, J. A. D~erksen, J. Egedal, V. "W. E k m a n ,

M. Ewing, R. M. Field, U. C. Fishel, W .Gorczynski, B.

Gutenberg , G. M. Hall, B. HetIand-Hansen, J. B. Hersey,

K. Hidaka, C. R. Home, A. J. Hoskh~son, M. K. Hztbbert, C.

Iselin, H. S. Jelstrup, A. B. Jones, IV. D. Lambert, E. W.

Lane, R. M. Legette, L. Lek, J. M. LeRoy, H. A. Mariner ,

M. Matuyama, tv. E. Matthes, F. G. McEwen, O. E. Meinzer,

A. H. M~Iler, R. B. Montgo~.cry: d. /~. Morgtm, ~N. _&. I'~elson,

L. L. Nett leton J. A. Potender, J. H. Pra t t , J. P roudman ,

R. Revelle, K. Ros~n, R. J. Russell, P. Schureman, H. SebIat-

nigg, L. B. Slichter, P. Sollenberger, F. M. Soule, H. T. S get-

son, W. T. Stringfield, W. D. Sz~tcli~e, H. U. Sverdrup, C. H.

Swicl~, C. V. Theis, U. T. Tl,mm. D. G. Thompson, C. Tsuboi,

S. B. Utories, T. W. Vaughan , F. 4. Ve",2ing ;Ieinesz, A. Vine,

L. K. Wenzel, J. E. Willis, R. ,]f. VTifson, G. P. ~Vootlard.

1. Lea norris en itahques sent eeux des g6ed~smns.

114 TRAVAUX EN SEANCE PLENIERE.

O c v a n o g r a p h y . - - Dr. Proudman gave the report of the

Committee on Mean Sea-Level and i~s Variations. He repor -

t e d t h a t he now had p r o o f - p a g e s of Monthly and Annual

Mean Heights of Sea-Lwel (Up to and inc lud ing the y e a r

1936) w h i c h he hoped would be pub l i shed a t an e a r l y date .

He a l so r e p o r t e d t h a t a p a m p h l e t g iv ing d a t a fo r the y e a r

1937 a p p e a r e d a t t he beg inn ing of 1939 and t h a t one is t o

be p u b l i s h e d each year . He a d d e d t h a t i t w a s t he in ten-

t ion o f t h e A s s o c i a t i o n t h a t a t t he end of each f ive-year

p e r i o d t h e s e p a m p h l e t s 'would be bound t o g e t h e r and publ i shed .

Dr , Bowie r e m a r k e d : << It. wou ld be of i n t e r e s t i f t he

n a t i o n a l l eve l ing ne t s of E u r o p e a n count r ies could be com-

b ined in a s ingle a d j u s t m e n t in o r d e r to d i scover t he e x t e n t

to wh ich m e a n sea- level a t t he t ida l s t a t i o n s dev ia t e s f r o m

an e q u i p o t e n t i a l sur face . Th is is a p rob lem of i n t e r e s t to t he

A s s o c i a t i o n s of P h y s i c a l O c e a n o g r a p h y and Geodesy. The

a d j u s t m e n t of the combined ne t s of Canada and t h e Un i t ed

S t a t e s showed t h a t mean sea- level of t h e Pacific is h i g h e r

on the a v e r a g e t han t h a t of the A t l a n t i c and the Gulf of Mexico >>.

Dr . Ven ing Meinesz s t a t e d t h a t th is m a t t e r would be

t a k e n up a t a l a t e r date .

I t was a g r e e d t h a t the Commi t t ee be r e a p p o i n t e d w i th the

a d d i t i o n of Prof . JACOBSEN.

Dr. R. H. F l e m i n g r e a d Dr. LA FOND's p a p e r Variations

of sea-lever on the Pacific Coast of the United States.

Dr. H. A. Mar ine r commented on th i s p a p e r as fo l lows :

<< T h e i n t e r e s t of the r e l a t ions of sea- level to the va r ious

o c e a n o g r a p h i c f e a t u r e s 'g iven in Mr. LA FOND'S p a p e r is

ev ident . W h a t I w a n t to call a t t e n t i o n to a t the p r e s e n t

t ime is to t he f a r - r e a c h i n g uses to which sea- level d a t a m a y

be put . The geo log i s t a s su re s us t h a t changes in t he re la-

t ion o f l and to sea of g r e a t m a g n i t u d e have t a k e n p lace in

t he pas t , and t h a t the re is e v e r y reason to be l ieve t h a t

changes in the r e l a t ive e leva t ion of l and to sea a r c t a k i n g

JOINT MEETING OF OCEANOGRAPHY,

GEODESY, I%IETEOROLOGY, HYDROGRAPHY. 115

place now, bu t t h a t the ra te of change is so slow as to escape

detect ion by geological methods. The va r ia t ions in sea-

level f rom year to year, if cont inued for a sufficient n u m -

ber of years, will fu rn i sh us definite quan t i t a t i ve da ta of

th i s r a t e of change. In ano the r fie,'d, too, these sea-level

changes fu rn i sh da t a of interest , namely, in c l imatology - -

for i t is clear tha t if the c l imate is becoming more genial ,

there m u s t follo~v a recession of the glaciers and a d iminu t ion

of the ice caps. This added water will make i tself ev ident

in the sea-level da ta >>.

Dr. He l l and-Hansen s ta ted : ~< The seasonal va r i a t i ons

in m e a n sea-level a long the Cal i fornia Coast showed m i n i m a

in abou t March and November while on the At lan t i c Coast

of the Uni ted Sta tes m a x i m a occurred in the same months .

In both cases the va r ia t ions are connected wi th va r i a t i ans

in the cur ren ts . The difference of phas$ seems to be an

i m p o r t a n t fac t which may lead to general conclusions of

considerable value ~.

Geodesy. - - In view of Dr. ECKHARDT's i l lness Dr. Net t le -

ton p resen ted his paper on t idal va r i a t i ons of g rav i ty .

Dr. L a m b e r t presented his Rapport sur les Mar~es de

l'Ecorce terrestre and called a t t e n t i o n to the fac t t h a t co-

pies of this paper were now avai lable for d i s t r ibu t ion in

Room 105, Bui lding B. He also s ta ted t ha t this was a pre-

l i m i n a r y vers ion and conta ined var ious e r rors which should

be corrected in the definitive text. This definitive text will

be publ i shed if funds are available. Au tho r s whose work

is summar ized , and those whose work is omit ted, are re-

quested to send in addi t ions and correct ions to the Re- porter .

Prof . Russel l p resented a paper Isostatic e f fec ts of Mis-

sissippi River sedimentation by himself and H. N. FISK 1.

1. See Appendix. p. 117.

11~ TRAVAUX EN SEANCE I~LEI~$IERE.

Dr. Thorn r e m a r k e d t h a t he w i s h e d t h e o p p o r t u n i t y to

h e a r a n d d i scuss f u r t h e r t he i n t e r p r e t a t i o n t h a t sed imen-

t a t i o n is inducing depress ion r a t h e r t han vice versa .

Dr. F i e l d made the, fo l lowing c om me n t on th i s l~aper :

<~ F r o m t h e geologica l po in t of v iew i t is c e r t a in ly no t y e t

c l ea r a s to w h a t a geosync l ine r e a l l y is. I t s eems h i g h l y

p robab le , however , t h a t the t ype of geosyncl ine of the Ap-

p a l a c h i a n s (The Va l l ey and Ridge Prov inces ) i t no t a typ ica l

geosync l ine in t e r m s of or ig in . I t is a l so i n t e r e s t i n g to no te

t h a t t h e t e r m sync l ino r ium was o r i g i na l l y p roposed fo r the

d e f o r m e d sed imen t s which fill the <~ geosync l ine >>. U n f o r t u -

n a t e l y t h i s t e r m has s ince been m i s i n t e r p r e t e d so t h a t

we now do no t have a n y t e r m for the v e r y p h e n o m e n a upon

wh ich t h e ~< geosync l ine >~ was o r i g i na l l y p o s t u l a t e d >>.

Meteoro logy and Hydro logy . - - Dr. Theis p r e s e n t e d his

p a p e r E a r t h t ides expressed in f luc tuat ions of w a t e r level

in ar tes ian welZ,,, in N e w Mexico.

Mr. L a m b e r t commented on t~fis p a p e r as fo l lows : (( I

have d i scussed Dr. THEIS'S c o m p u t a t i o n s wi th h im and wi th

off icials of the Geological Su rvey and we do no t see eye

to eye. The o r d i n a r y t ida l p roblem, uncompl i ca t ed b y aqui-

fers , is d i f f icul t enough, as is seen b y t h e r e f e r ences con-

t a i ned in the b i b l i o g r a p h y of m y repor t . W h e n aqu i f e r s a r e

in t roduced , t he complex i [y is a lmos t inconceivable . I wish

to r eco rd a d i s sen t f rom the ca lcula t ion , t h o u g h w i s h i ng

to exp re s s m y app rec i a t i o n of the w o r k >>.

Dr. S t e t s o n commented as fol lows : ,~ F o u r m e t h o d s of

observation contribute to the question of tidal deformation

in the Earth's crust : (I) geodetic observations for varia-

tions of the vertical with respect to sta:'s; (2) tilt measure-

ments; (3) gravimeter data and (4) changes of water-level

in deep wells. The importance of the use of all of these

in conjunction was stressed, and also the desirability of those

in charge of oil fields to watch for um~sed wells in different

localities where similar observations could be made ~.

ISOSTATIC EFFECTS OF MISSISSIPPI

RIVER DELTA SEDII%IENTATION. 117

Drs. L a m b e r t and Field read announcemen t s of in te res t

to those present .

P res iden t Dr. la Com" expressed his appreciat ion, on behal f

of the Associat ion, to the var ious speakers for the va luable

con t r ibu t ions made.

A P P E N D I X

I S O S T A T I C ] E F F E C T S O F M I S S I S S I P P I R I V E R D E L T A S E D I ~ I E N T A T I O N

by" R i c h a r d J o e l R u s s e l l , a n d H a r o l d N. F i s k .

The dai ly load of sediment deposited in the lower del ta

r e , o n cf the Mississippi 'River averages about two mil l ion

tons. I f d i s t r ibu ted over such depths as exist in the v ic in i ty of the r ive r ' s mou ths this amoun t of inorganic ma te r i a l

should cause an accret ion of about 30 square miles of l and

each year . In fact the accumula t ion of locally derived organic

ma te r i a l s should accelerate the gain far beyond t h a t rate. A

compar ison of surveys about a cen tury apart , however,

indica tes no t the addi t ion of 3000 or more square miles of

land surface in sou the rn Louis iana but ne i ther appreciable

gain nor loss.

Localized gains of land in such places as Garden Is-

land Bay. where the bounda ry between land and wa te r

has been dr iven Gulfward at an average ra te of 1262 feet

per y e a r since the year 1890. or a long the sides of

act ively advanc ing pass channels, have been offset by

erosional losses along all coasts not immediately associated

with active deposition. The failure of the lower delta to

make an impressive gain in area suggests either that the

bulk of the sediment is being swept away and deposited far

offshore or that its weight is suf~cient to depress underlying

materials. Consideration of settling velocities, the feeble

currents in the Gulf of Mexico, and existing physical offshore

118 TRAVAUX EN ~EANCE PLENIERE.

b o t t o m condi t ions r ende r the f irst a l t e r n a t i v e e x t r e m e l y

un l ike ly . I t a p p e a r s t h a t mos t of the sed iment a c c u m u l a t e s

n e a r t he r i ve r ' s m o u t h s to th icken downward r a t h e r t h a n

bu i ld ou tward . Th~ e a r t h ' s c rus t a ppe a r s to y ie ld under

t h e load.

S tud ie s on the th ickness and ex ten t of the Recent

geo log ica l f o r m a t i o n in Lou i s i ana by Mr. JOHN W. FRINK

and the w r i t e r s confirm the conclusion jus t s t a t ed . Basa l

g r a v e l s were depos i ted in e a r l y Recent t imes, when g lac ie r s

were s t a r t i n g t h e i r l a s t m a j o r r e t r e a t f rom the con t inen t s

in h i g h e r l a t i t udes and seas were some 300 to 400 fee t lower

t h a n a t p resen t , pos s ib ly some 30,000 to 40,000 y e a r s ago.

These g r ave l s have been t r aced doyen the Miss i ss ipp i Val ley

f r o m Cai ro a t such dep ths as 200 fee t wi th g r a d i e n t s not

d i f f e r ing m a t e r i a l l y f rom those a long the va l l ey floor.

T o w a r d the coas t the s lopes of these basa l g r a ve l s s teepen

a b r u p t l y and lead to the b o t t o m of an immense bas in filled

b y a l luv ium and loea ted in the v ic in i ty , of New Or leans

d i r e c t l y unde r the reg ion where d r a i n a g e p a t t e r n s ind ica te

the Miss i s s ipp i R i v e r has been d i s c h a r g i n g the bulk of i ts

s e d i m e n t du r ing the l a t t e r p a r t of Recent t imes . Some

s ix o r seven m a j o r courses of the r i ve r are c l ea r ly vis ible

on t o d a y ' s sur face . These a re all ~< modern >> in a geologica l

sense and r e p r e s e n t only the l a t e s t episodes of << Reeen t >>

h i s t o r y . Al l lead to mou ths w i t h i n a s emie i r cu l a r a r ea

h a v i n g a r ad iu s of 80 miles s l i gh t ly to the sou th of New

Or leans . The b a s a l g rave l s of the Recent f o r m a t i o n a r e

deepes t in e x a c t l y th i s s ame region, a t t a i n i n g a m a x i m u m

d e p t h s l i g h t l y in excess of 3000 feet . The shape of the

Recen t f o r m a t i o n in sou the rn Lou i s i ana is e s sen t i a l l y t h a t

of a huge , sha l low ladle whose bowl centers nea r New

Or l eans a n d whose th in handle ex tends up the a l luvia l

va l ley . The bowl is abou t 150 mi les wide and abou t three-

f i f t h s of a mile deep, so is qui te f lat when cons idered in

n a t u r a l scale. Even th i s f l a t t ened bowl, however , lends

s t r o n g s u p p o r t to the idea t h a t w i th in Recent t imes the

ba se of t he depos i t has been depressed more t h a n 3000 fee t

ISO~TATIC EFFECTS OF MISSI~SIPI~I

RIVER DELTA SEDIMENTATION. 1 1 9

under the accumula t ing load of sediment . P r e s u m a b l y all

c rus ta l ma te r i a l below has shared in the depression.

The Pleis tocene of Louis iana consists of four format ions ,

each of which is character ized by basa l gravels over la in by

finer sediment~. The pa t t e rns of long abandoned Mississippi

River courses are c lear ly displayed on the upper surfaces of

at least the younge r three of these format ions , the record

being clearest i_~ the case of the Prair ie , la tes t Pleistocene. Recent condit ions are duplicated in each of the Pleis tocene

format ions . Each is re la t ive ly th in as i t extends up and

along old val leys in land. Each th ickens ab rup t ly toward the

coast. The th ickes t pa r t of the Pra i r i e fo rmat ion centers in

a bas in about 3000 feet thick and located immedia te ly west

of the Recent basin. The next older Pleistocene format ion ,

the Montgomery, th ickens to at least 2200 feet in a bas in

somewhat no r thwes t of the Pra i r i e basin. The older

Ple is tocene bas ins occur beyond the l imits of Louis iana .

Such da ta as we have a t hand indicate the s t rong p robab i l i ty

of a th ick Bent ley bas in in sou theas t e rn Texas and t h a t the

oldest Pleistocene, Wil l iana , forms a th ick basin in sou the rn

Alabama, to the east of Louis iana. Each Pleistocene bas in is

located where relict d ra inage pa t t e rn s show the mou ths

of e i ther the Mississippi or its Pleis tocene equ iva len t s to

have been. The th ickness and ex ten t of Pleis tocene forma-

t ions sugges t two impor t a n t genera l iza t ions : (1) deltaic

s2diment seems capable of accumula t ing a t a given site un t i l

some such th ickness as 3000 feet has been reached, and (2)

a new accumula t ion occurs a t a new site r a the r t h a n di rec t ly

over an old << ladle bowl. >~

The r ema in ing pa r t s of the or ig ina l surface of each

Pleis tocene fo rma t ion cons t i tu te an al luvial terrace. The

four Pleis tocene ter races in Louis iana now cover abou t one-

qua r t e r of the area of the ent i re state. Each extends in l and

a long the val leys of s t r eams tha t existed prior to and du r ing

the t ime of i ts deposition. Each b roadens coas tward where

val leys widen and each develops, sou thward f rom its fluvial

120 TRAVAUX EN SEANCE PLENIERE.

elements, a broad coastal zone. Each Pleistocene period of

depositioff, caused by r is ing seas as glaciers waned, yielded

a surface in al | respects s imilar to the Recent al luvial

surface, extending up valleys as s t r ips of flood plain and

broadening along the nor thern side of the Gulf of Mexico

as del taic coastal plain. The Pleistocene surfaces differ

f rom the Recent chiefly in mat te r s of gradient and

deposit iona! inactivity. The Recent alluvium is stil l active

f rom a depositional s tandpoint and its surface gradients

are those required by today ' s s t reams. The Pleistocene

surfaces have been lifted inland and t i l ted cSastward in

increased amounts in proport ion to age. They are now

mainly inactive from a depositional s tandpoint and are being

eroded.

The average airl ine gradient down the Recent alluvial

val ley of the Mississippi is about one-third foot per mile.

The Pra i r ie terrace in central Louis iana is only s l ight ly

steeper. The adjacent h~Iontgomery terrace slopes at about

two or three feet per mile and the older terraces slope at

from si~ to ten feet. Terrace gradients i nd ica t e . a t i l t ing

throughout the Pleistocene aud Recent. The older ter races

have shared longest in the t i l t ing and have a t ta ined s teepest

slopes.

The direction of terrace t i l t ing brings out the impor tan t

general izat ion that gradients have been established toward

the posi t ions of subsequent deltas. Slopes on the la tes t

Pleistocene, Prairie, terrace form a pa t te rn concentric

around the major site of Recent deposition ra the r than

toward the center of the Pra i r ie lzasim Contours on the

Pra i r ie terrace s t r ike nor theas tward in the t e r r i t o ry

nor thwest of the Recent delta and nor thwestward in the

t e r r i t o r y nor theas t of New Orleans. This appears to be

excellent evidence tha t subsidence in the Recent delta region

is warp ing the Prair ie surface. This warping has resulted

in the abandonment of many of the original stre~m courses

on the terrace for routes leading more directly down the

newly establ ished slopes.

ISOSTATIC EFFECTS OF LIISSISSIPP!

RIVER DELTA SEDi~IENTATION. 121

Terrace slopes steepen appreciably in the vicinity of

subsequent deltas and flatten, tending to assume gradients

similar to those on the Recent alluvial surface inland.

Along the sides of the ~,~lisMssippi River alluvial valley north

of Louisiana the gradients of all Pleistocene terraces are

about those of the active flood plain, so that the vertical

separation between terraces remains quite constant over

long distances. Conditions are similar to those so widely

noted in France by DE LA~:IOTHE. Divergences, resulting in

such contrasted gradients as those just cited for central

Louisiana, are established only as terraces approach the

coast. The width of the zone of divergent terrace gradients

is about 200 miles in Louisiana. The sharpest divergences

in gradients occur just north of the active delta, in the

Florida Parishes of Louisiana. Recent valleys have

floodplain gradie.nts of about two feet per mile in this region.

The Prairie terrace slopes at ire feet per mile, the Mont-

gomery at eight, and the 2entley at 25 feet per mile.

Divergent terrace gradients exist not only as topographic

features on today's surface but also in the basal gravels.

These are exposed on valley walls inland but toward the

coast the evidence must be obtained from oil wells and other

borings. With some 30,000 vcells available to study, the

subsurface of southern Louisiana is reasonably well known.

Basal gravels of terrace formations have been followed

coastward for considerable distances from the southernmost

points of surface outcrop and thicknesses of individual ter-

race formations are known beneath much of the deltaic

coastal plain. Dips on basal gravels of each Pleistocene

formation are steepest on the flanks of its own depositional

basin and values reach as high as'IS0 feet per mile. Followed

upslope and inland each basal gravel rises tot he existing

floodplain level where it begins to outcrop as a surface

feature. Gravels of the Vr formation lie below those

of other Peistocene formations beneath the coastal plain

but in land a t t a in elevat ions above those of other

i22 TRAVAUX EN SEA~4CE PLI%[IERE.

gravels, so tha t in nor thern Louisiana they cap many

of the highest hills. Bentley gravels come next to at-

ta in ing Wil l iana depths coastward and next to a t ta in ing

Wil l iana elevations inland, a pa t t e rn repeated in the next

younger formations, so tha t the Pra i r ie comes closest in all

regards to the Recent. With reference to Recent gradients

it is clear t ha t there has been an actual elevation of each

Pleistocene format ion inland and an actual depression

coas tward to depths far lower than those of ice-age seas.

The t i l t ing of Pleistocene formations results both f rom

coastal subsidence and what appears to be a compensating

uplif t inland.

The most s t r ik ing example of iniand uplift occurs in

southwestern Mississippi, where a highland has been created

with maximum elevations in excess of 500 feet above sea

level. The culminat ing summits lie not fa r over 60 miles

nor th of the sea level waters of Lake Pontchar t ra in . On

the south slopes of this highland are the sharpes t divergences

"noted between terrace gradients in Louisiana. Immediate ly

south of these slopes are the s teepest dips as yet" found

in Recent gravels and these lead direct ly to the deepest pa r t

of the Recent basin of deposition. The southwestern

Mississippi highland area thus appears to be the direct

resul t of the subsidence occurring immediately to its south

during the las t 30,000 years or so. North of this highland

the Wil l iana terrace slopes not southward, toward the Gulf

and Mississippi River del ta but gent ly northward, toward

central Mississippi. N o antecedent s t ream has been able

to main ta in i ts course across this highland. Pearl River

flows around i ts subdued eas tern end and the Mississippi

t runcates i ts western extremity. A change in the pa t te rns

of active channels of the Mississippi in this vicinity is. pos-

s ibly re la ted to the uplift.

Te r t i a ry h is tory appears to duplicate the Quaternary

throughout the Gulf Coast record. A detailed survey of

Midway and Wilcox (Eocene) conditions by the junior au thor

reveals a basin of deposition about 3000 feet thick and with

ISOSTATIC E F F E C T S OF M I S S I S S I P P I

RIVER DELTA SEDIMENTATION. 12~

an e x t e n t s im i l a r to t h a t of the Recent , cen te r ing a b o u t

150 miles , to the nor th . This s t u d y shows c lear ly t h a t t he

Midway-Wi lcox th ins G u l f w a r d f rom the cen t ra l p a r t of i t s

basin. S l igh t sugges t ions of Gu l fward th inn ing were no ted

in our s tud ies of t he Q u a t e r n a r y bu t the absence of deep

wells well ou t in the Gulf of Mexico prec ludes an unqual i f ied

s t a t e m e n t to the effect t h a t the t h inn ing is as r a p i d as t h a t

inland. Even the Miocene bas ins a re too f a r south, to r evea l

much evidence of G u l f w a r d t h inn ing of ind iv idua l f o rma-

t ions. Many of the deepes t wells in Lou i s i ana reach only

the u p p e r and middle p a r t s of the Miocene a t dep ths of f rom

12,000 to over 13,000 feet . The basa l Miocene a p p e a r s to be

loca ted be low reasonab le d r i l l ing dep ths a long much of

coas ta l Lou i s i ana . T e r t i a r y subsu r face r ecords ind ica te t h a t

the t i l t i ng obse rved in P le i s tocene depos i t s is on ly p a r t of a

gene ra l p rocess t h a t has been ac t ive t h r o u g h o u t the Ceno-

zoic.

Deposition along the northern side of the Gulf of

Mexico since late Mesozoic time has resulted in the accumul-

ation of a thick belt of sediment whose longitudinal trend

parallels the coast and to which has been given the name,

Gulf Coast geosyncline. As the geology of this belt is more

and more understood it becomes increasingly evident that

individual formations in this << structure >> thicken toward

basins, each of which presumably marks the site of most

active deposition during the time it accumulated. The axial

trend of the belt as a whole appears to reflect that of

Cenozoic shores. Crustal behavior during the Tertiary

appears to have been similar in all respects to that during

the Q u a t e r n a r y .

Geo log i s t s have w r i t t e n much concern ing the << s t r e n g t h >>

of the e a r t h ' s crus t . Does evidenqe f rom the Gulf Coas t

ind ica te a << s t r o n g >> or a << weak ~ condi t ion? F r o m the

s t a n d p o i n t of y i e ld ing and subs id ing under s e d i m e n t a r y

load ing the c rus t a p p e a r s ~< weak >> indeed. Here, as in

a p p a r e n t l y all o the r places where vas t th i cknesses of sedi-

124 TRAVAUX EN Sl~ANCE PLENII~RE.

m e n t have accumula ted , the depos i t s a r e al l those

o r d i n a r i l y obse rved to a c c um ul a t e a t about sea level. This

s u g g e s t s e x t r e m e l y easy y ie ld ing . Beach sands w i th e x a c t l y

the same shel ls t h a t a re now found a long t o d a y ' s beaches

of the Gulf of Mexico in L o u i s i a n a have bee~n encoun te red

r e c e n t l y in the v ic in i ty of New Or leans a t a dep th of s i l g h t l y

more t h a n 3000 feet . F r a g m e n t s of cypress wood f r e sh

enough to whi t t l e a re commonly encoun te red a t s imi l a r

dep th s in oil wells. T h r o u g h o u t the en t i r e r ecord of

a ccumula t i on no f ac t a p p e a r s to con t r ad i c t the opinion

t h a t subs idence took p lace a t e s s e n t i a l l y the same ra t e t h a t

t he depos i t s th ickened . The y i e ld ing of the c rus t in a

v e r t i c a l sense a p p e a r s to t a k e p lace wi th a s t o u n d i n g ease.

On the o t h e r hand i t is a p p a r e n t t h a t a r e l a t i ve ly la rge a r e a

is a f fec ted a round the m a r g i n s of the subs id ing area . T h a t

the ef fec t of s u b s i d e n c e ex tends b r o a d l y is an ind ica t ion of

a c rus t << s t r o n g >> in a tens i le sense. The cen t r i pe t a l s lopes

obse rved on t e r races , d o w n w a r d ~yarping t o w a r d s i tes of

subsequen t de l ta depos i t ion , d e m o n s t r a t e the ab i l i t y of l a rge

a r e a s of e a r t h ' s c rus t to h a n g t o g e t h e r much in the w a y t h a t

a f i r eman ' s ne t is able to s u s t a i n a h e a v y we igh t p laced in

i t s center .

The << weakness >> o f . t h e c rus t in y i e ld ing to load in a

v e r t i c a l sense is m a t c h e d by the ease w: th which i sos ta t i c

r e a d j u s t m e n t s p roduce upl i f t . The h i g h l a n d in s o u t h w e s t e r n

Miss i s s ipp i is def in i te ly y o u n g e r t h a n the close of the

W i l l i a n a because the W i l l i a n a t e r r a c e is f lexed ac ross i t

as the su r face of an an t i c l ina l arch. F r o m the close

p r o x i m i t y of the h i g h l a n d to the bas ins of l a t e s t Q u a t e r n a r y

l oad ing i t appea r s t h a t the g r e a t e r p a r t of the up l i f t has

t a k e n p lace ve ry recent ly . The Recen t lens of s ed imen t is

a b o u t 150 miles long, abou t ha l f as wide, and abou t 3000 feet

th ick . The h i g h l a n d r e sembles i t in oval shape , in or ien t -

a t ion, b u t i t s d imens ions are smal le r , abou t 100 b y 60 miles .

I t s eems p robab le t h a t excess m a t e r i a l d i sp laced as the

Recen t lens g rew h a s been d i s t r i b u t e d more wide ly and is

in p a r t respons ib le fo r the epe i rogen ic up l i f t t h a t ha s ra i sed

ISOSTATIC EFFECTS OF ~%IISSISSIPPI

RIVER DELTA SEDIMENTATION. 125

Pleistocene surfaces farther inland and that accounts for

part of the divergences in gradients noted between individual

terraces.

The << strength >> of the earth's crust suggested by warped

terraces is also conspicuously displayed around the margins

of the Mississippi River delta where delta-flank depressions

form deep embayments and low territories extending far

inland from the site of most active deposition. Delta-flank

depressions are almost universally associated with large

active deltas. The compound delta at the mouth of the

Rhine displays an extensive lowland and depressed Zuider

Zee area to the north and the correspondingly depressed

area of Zeeland to the south. The Po and various other

large rivers display equally striking delta-flank depressions,

areas carried down faster than alluviation has been able to

neutralize from the topographic standpoint. Among streams

not exhibiting delta-flank depressions are those that have

recently changed their courses radically. The Hwang-Ho

prior to the .year 1852 led to a delta with excellent flanking

depressions but the course established then leads to a mouth

not yet sufficiently developed to exhibit either conspicuous

alluvial deposits or delta-flank depressions.

On a smaller scale the Adour, of southwestern France,

exhibits exactly the same contrast between old and new

courses. Streams emptying into long estuaries are in an

unfavorable position to create delta-flank depressions but

the weight of their deposits undoubtedly tends to maintain

inland waters beyond such times as their deltas might

otherwise be supposed to reach the adjacent sea. This is

shown by the inward dips of marginal formations toward

even such a small estuary as that of the Mersey and toward

such recently filled estuaries as those of the Rh6ne, Salinas

of California, or the grander deltas of southern Burma.

Though the sediments of soutern Louisiana are readily

faulted, a condition exhibited conspicuously in association

with th~ rising" cores of p rac t i ca l ly all sal t domes, t he re

9

126 TRAVAUX EI~ SI~,ANCE PLI~NIERE.

has been little or no evidence found as yet to the effect that

faulting has developed in connection with the regionally

broader warpings caused by the deposition of great lenses of

sediment in basins or along the margins of the associated

areas of uplift. There is considerable difficulty in demon-

strating the presence of such faults and it is possible that

the future will find our present conclusion reversed in this

regard but we feel that there is sufficient evidence to

justify calling attention to at least the strong probability

t h a t all c rus ta l de format ion associated wi th Mississippi

R iver s ed imen ta t i on has been accomplished by wa rp ing

r a t h e r t h a n f rac ture .

I n p r e sen t i ng th is paper before a group of ~eophysicis ts

the au tho r s have purpose ly l imi ted i ts scope to ma te r i a l s

they r ega rd as demons t rab le fact. They hold no br ief for

a n y pa r t i cu l a r exp lana t ion of isostasy. They regard as

unsound, however, any proposed .mechanism not capable of

exp l a in ing such deformat ion as occurs in Lou i s i ana and

ea rne s t l y hope t ha t this paper wil l s t imula te in te res t t h a t

m a y lead to a more sa t i s fac tory exp lana t ion of i sos tasy t h a n

appea r s to exist today. M a n y of the fac tual da t a presen ted

are t r ea ted more thorough ly in the recent Bulletins of the Louisiana Geological Survey and m a n y of them are

summar i zed f rom a more geological viewpoint in Quaternavnj History of Louisiana, p. 1199-1234, Vol. 51 (1940), Bulletin of the Geological Society of America, where a b ib l iog raphy

of abou t e ighty pe r t inen t papers is presented.