Vol.. 7, 1921 
GEOLOGY: W. BOWIE 
23 
SOME GEOLOGIC CONCLUSIONS FROM GEODETIC DATA 
By William Bowie 
Division of Geodesy, U. S. Coast and Geodetic Survey 
Communicated by A. O. Leuschner. Read before the Academy, November 16, 1920 
For a number of years geodetic data were collected for the purpose of 
controlling surveys and maps and for the determination of the shape and 
size of the earth. But in recent years they are also extensively used in 
investigations dealing with geological and geophysical problems. These 
investigations have included data for the United States, India, Canada 
and a small part of Europe. 
The investigations in isostasy have proved that for the areas considered 
there is approximately the same mass in each column of unit cross-section 
extending from the surface of the earth down to a depth of,. say, 75 miles. 
This statement is justified regardless of what the depth of compensation 
actually is, for the deficiency or excess of mass in a few miles at the depth 
of approximately 75 miles is a small percentage of the mass of the whole 
column. With regard to depth of compensation it may safely be said 
that there is no sharply defined surface which is at uniform depth below 
sea level, limiting the isostatic compensation. It is probable that there 
is a zone, rather than a surface, which limits the compensation. 
We have no evidence as to the manner in which the compensation is 
distributed vertically, but the uniform distribution, which was adopted in 
order to make the computations more feasible, is as logical as any other 
simple method. Regardless of what the method of distribution is actually, 
the conclusions reached from the isostatic researches cannot be seriously 
affected. 
What is the area of the cross-section of the unit column that may be in 
equilibrium is a matter which is in doubt, but as data are accumulated 
the area of the cross-section is successively lessened. 
It has been found that large areas are in almost perfect isostatic balance 
and it seems to be very probable that an area of about 70 miles square is 
very closely compensated. This is the area of one square degree at the 
equator, or very nearly so. 
It has been found that the distribution of the compensation of a topo- 
graphic feature horizontally to a distance of about 60 kilometers from the 
feature gives as consistent results as local compensation, but when the 
compensation is distributed horizontally to a distance of 170 kilometers 
the results are not so accordant as with local distribution of the compensa- 
tion. 1 
It must be held that any system for correcting geodetic data for the 
effect of topography and isostatic compensation, which makes the com- 
puted values of gravity agree closely with the observed values in different 
