Voi,. 7, 1921 
GEOLOGY: W. BOWIE 
25 
parts of the earth and for various types of topography, is very close to the 
ideal or true system. While the isostatic investigations have been con- 
fined to only a portion of the earth's surface it seems probable that all 
land areas will be found to be in isostatic equilibrium. 
A test was made to show whether the compensation of small areas of 
topography could be ignored without seriously affecting the results. 
Forty-two stations having high elevations were selected for the test. 
The elevations of these stations varied from 3000 to 14,000 feet. When 
the compensation of the topography for a circle with a radius of 17.9 
miles was ignored the gravity anomalies became much larger and 37 
of the 42 anomalies had negative signs. When the area of the circle was 
given a radius of 36.5 miles the anomalies were still further increased and 
all of the anomalies except one had the negative sign. With the small 
circle the mean anomaly with regard to sign is — 0.037 dyne while with the 
larger circle the mean anomaly with regard to sign is —0.072 dyne. 
When it is considered that the mean anomaly with regard to sign for these 
42 stations under consideration, all at high elevations, is only +0.005 
dyne, it is realized that the ignoring of the local compensation is not justi- 
fied. The table preceding shows these results. 
This test shows that, for even very small areas, the topography is at 
least largely compensated. This is a most important conclusion for by 
analogy no such mass as the sedimentary material forming the delta of a 
large river entirely escapes isostatic compensation as has been held by some 
investigators. 2 
There are very definite relations between the gravity anomalies and the 
Cenozoic and Pre-Cambrian formations. The Cenozoic stations have 
negative anomalies in nearly all cases, and the largest anomalies are found 
at stations located on this formation. The anomalies at the stations on 
the Pre-Cambrian areas of limited extent are nearly all positive. 3 
The writer made an investigation of these relations and arrived at the 
conclusion that the negative sign of the Cenozoic anomalies is due in great 
part to the abnormally light material of that formation. Of course it is 
necessary to consider the effect of the Cenozoic material which extends 
below sea level. Nearly all Cenozoic formations are at comparatively 
small elevations and the abnormal density of the material which may be 
above sea level could account for only a very small part of the anomalies 
at Cenozoic stations. 
In a similar manner it was found that the existence of abnormally dense 
material in the Pre-Cambrian formation will account for the positive sign 
of the anomalies at stations on that formation. It will be necessary, as in 
the case of the Cenozoic formation, to consider the material that extends 
below sea level. 
These are logical conclusions for it is not conceivable that with a whole 
country (the United States, for example) in practically perfect isostatic 
