DEPTH OF ZONE OF FLOW IN THE EARTBH’S CRUST 103 
these conditions really are. And here we are confronted with 
difficulties at the outset—for neither the pressure nor the heat 
existing as successive depths within the earth’s crust can be 
calculated or otherwise determined accurately. 
In order, however, to obtain some approximate measure of the 
pressure which exists at successive depths below the surface of the 
earth—and it is only the outermost portion of the crust extending 
to a depth of say 50 miles that comes into consideration for the 
present purpose—two assumptions, among others, will be made: 
(a) that the rocks composing the earth’s crust are incompressible, 
and (6) that they are free from tangential stresses set up by the 
contraction of the earth’s interior through secular cooling or by 
other causes. As a matter of fact, however, the rocks composing 
the crust are compressible and they are probably in most parts of 
the earth’s crust in a state either of compressive or tensile stress. 
The first factor of compressibility, however, probably does not 
introduce any very serious error if omitted from consideration, but 
the second factor of tangential stress is more important and prob- 
ably varies greatly not only at different points at or near the 
surface but at different depths below the surface. The data in 
our possession are inadequate to enable its influence to be calcu- 
lated, but the investigations of Chamberlin, Woodward, Hoskins, 
and others' go to show that above the ‘‘Level of No Strain” great 
arches, if formed in the relatively cold surface crust, would be 
incompetent to sustain more than a small fraction of their weight 
and would thus collapse, tending to equalize the stresses in each 
successive zone. 
For the purpose of this inquiry, the pressure at successive 
depths below the surface of the earth is assumed to be the weight 
of a column of rock equal in height to the depth in question. 
The specific gravity of the rock is taken as 2.8. These pressures 
are shown in Fig. 1. 
It is also impossible to arrive at an accurate knowledge of the 
temperature within the earth’s crust at successive depths below 
the surface. As is well known, the temperature gradient within 
the earth’s crust shows considerable variations. 
1 See Chamberlin and Salisbury, Geology, I, 553 ff. 
