552 
GEOPHYSICS: W. BOWIE 
Proc. N. a. S. 
from constant or systematic deviation from normal conditions in the im- 
mediate vicinity of a station. 
The gravity observations not only give a means of determining whether 
there are regional deviations from normal densities in the outer portion 
of the earth under the continents and oceans to cause equilibrium at the 
depth of compensation, approximately 96 kilometers, but they also afford 
an opportunity to detect abnormal densities in the materials of the earth 
which are close to the surface but below sea level. Recent investigations 
in the United States and in India show a decided relation between the 
value of gravity and the Cenozoic geologic formation. This formation 
is approximately 10% lighter than the material in the older geological 
formations. Pendulum observations may be a means of indicating the 
approximate depth to which the Cenozoic formation extends at any place. 
Gravity observations also indicate a relation between the value of . 
gravity and the Pre-Cambrian formation. In this formation the ma- 
terial is, in general, somewhat heavier than normal, and when a gravity 
station is established at or near Pre-Cambrian rocks the value of gravity 
is found in most cases to be greater than normal. It is not improbable 
that a knowledge of this relation between the Pre-Cambrain formation and 
the gravity anomaly may make it possible to discover crystalline rocks or 
others that are extra heavy which may be covered by materials of normal 
or lighter densities. If this proves to be the case, it may be possible to 
locate areas underlaid by dense rocks close to the surface of the ground, 
which may have an economic importance in connection with boring for 
oil. Much money has been lost by oil prospectors in drilling in regions 
which have been found by them to be underlaid by crystalline rocks. 
It is seen that gravity determinations have much more than a purely 
geodetic value. The results obtained from them will undoubtedly have 
a very great effect on the science of geology. 
There are geophysical problems that would be considerably cleared up 
if the value of the intensity of gravity over the oceans could be determined 
accurately. Such observations would make it possible to determine 
whether the outer portion of the earth under the ocean is in the same iso- 
static condition as has been found under the continents. It would seem 
probable that isostasy obtains to the same extent under the oceans as 
under the land, but it has not been possible to prove this in the absence 
of accurate gravimetric determinations on the ocean. 
It is true that gravity observations have been made over the ocean, 
but the apparatus, in each case, lacked the necessary precision to give 
the value of gravity with a sufficiently high degree of accuracy. It is 
probable that improvements will be made in the apparatus for deter- 
mining gravity at sea and when one has been perfected to such an extent 
that it will give results of adequate accuracy, a long step forward will 
have been made in geodesy. 
