RELATIONS OF GEOPHYSICS TO GEOLOGY 9 
portant for the interpretation. In areas where there are a number of 
anomalies close together, it therefore results that the effect of each 
anomaly is obscured by effects from adjacent ones. 
Potential function measurements can be made of numerical value 
where there is a large effect of a body of a known geometrically simple 
form near the points at which measurements are made. The successful 
contouring of the top of cap rock in some of the shallow salt domes by 
Barton is the most brilliant example of this method. 
The geologist can aid very much in the interpretation of the po- 
tential function type of geophysical survey by obtaining as much 
information as possible regarding the type of structure to be expected 
in a particular area and by regional studies to indicate to the geo- 
physicist the most probable locality for investigation for anomalies. 
By the seismic method, it is possible theoretically to make accurate 
determinations from a few observations close together. This method, 
therefore, has an advantage over the potential function methods in 
requiring very much less areal extent to be investigated. However, 
in actual field surveys, the problem is not as simple as indicated by 
theory. Corresponding with the bending of light waves through the 
air over a desert, we have the bending of sound paths underground, 
due to irregular distribution of elasticity and density in sedimentary 
rocks. These variations are in part a result of depositional conditions 
and as such show a general stratification parallel to the bedding; and 
the geophysicist should call on the geologist for aid in studying these 
variations of sedimentation. But, in addition to such variations, there 
is some evidence gradually accumulating that uplift will effect changes 
in the elasticity and density sufficient to require consideration in the 
accurate determination of depths which are now being attempted with 
the reflection seismograph. This is one of the most important studies 
which geophysicists must make before contouring can be precise, and 
it is one where the aid of the geologist is especially important. At the 
present time we are assuming, for lack of complete data, that forma- 
tions arched over a structure maintain the same physical constants 
as when they are in a normal position. 
How can the geologists and the geophysicists set about finding the 
variations in physical character of the rock caused by structures of the 
type we are studying in the oil fields? In the first place, we can use 
wells which have already been drilled on other structures and study 
variations reported by the drillers. So-called concretionary zones in 
shales and gumbos, and indurated sand members, may be due to the 
uplift of beds containing solutions into an environment of less pres- 
sure and temperature, thereby causing precipitation. Folding and 
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