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GEOLOGY: W. BOWIE 
most geophysical problems. The work done in the Survey^- ^-^ shows 
that, at some depth below sea level (of the order of 100 kilometers) the 
pressure of any unit column is very nearly equal to that of any other 
unit column. For instance, the pressure exerted on a square mile of 
the imaginary surface at a depth of say 96 kilometers^ below sea level 
at the sea coast or under the plains is about the same as the pressure on 
an equal area at the same depth below sea level under the Rocky Moun- 
tains or under any other mountain masses. 
If we assume that the equalization of pressures at the supposed 
depth, called the depth of compensation, is perfect, then we must con- 
clude that the land masses are counterbalanced by deficiencies in den- 
sity of the materials below sea level, under the topographic features. 
It is not probable that the pressures are exactly equal for small areas of 
the surface at the depth of compensation. It is very probable that 
they are practically equal for areas of the order of ten thousand square 
miles. It is one of the important problems of the geodesist to collect 
sufficient data to show the minimum area which may be in a high state 
of isostasy. Another problem for him to investigate is the distribution 
horizontally and vertically of the deficiencies in mass, which balance 
the material which is above sea level. 
For the purposes of making the computations, the compensating 
deficiencies of density or mass are supposed to be uniformly distributed 
directly under the topographic feature from the earth's surface to the 
depth of compensation. And it is also assumed that the negative masses 
exactly equal the positive masses which are above sea level. 
We cannot tell from the data now at hand just how near the truth 
are these assumed conditions. But we do know that they are very 
much nearer the truth than those conditions based upon a rigid earth, 
with the topographic features held up by the strength of the earth's 
outer material. The assumed isostatic conditions are also shown to 
be much nearer the truth than those based upon the theory that the topo- 
graphic features should be ignored. 
It will be interesting to consider briefly the results of the latest inves- 
tigations by the Coast and Geodetic Survey upon the subject of gravity 
and isostasy.^ 
We may assume that the method of reduction which shows the small- 
est effect of systematic or constant errors is the nearest the truth. For 
instance, there may be used in the tests five classes of topography (the 
sea being ignored, as we have no very accurate gravity observations 
at sea). They are indicated in the tables below. The isostatic or 
Hayford method of reduction was used with two depths of compensa- 
