326 
Since the pressure is supposed to take place in a horizontal 
direction, it will not have any direct effect to raise the centre 
of gravity of the portion of the crust under consideration ; So 
that, if the layer in question rest upon a liquid substratum, 
we may expect some portions of the disturbed crust to dip into 
the superheated rocks. But in that case a corresponding 
volume of such subjacent rock must rise into the anticlinals. 
Hence, = (@) == (Oe 
And the equation becomes 
kle=% (a) — = (0). 
Extending the inquiry to any area of the surface of length J and 
width w, the equation becomes 
2 klwe = & (A) —& (B), 
where & (A) and & (B) are the volumes of the elevations above, 
and of the depressions below, the “datum level.” 
The whole surface of the globe being next taken into 
account, the relation becomes, 
Area of the Globe x 2ke = (A) — 3 (B). 
It is important to understand what is meant by the “datum 
level.” It is an imaginary surface, which occupies the position 
which the surface of the crust would occupy at the present 
time, if it had been perfectly compressible, so that no corruga- 
tions would have been formed in it by lateral compression. 
For it would in that case have become simply more dense, 
without being disturbed in position. 
The above relation is applicable to the earth’s surface, 
although that is not strictly regular in its general form, and 
may contain local elevations and depressions affecting its mean 
figure,—that is, its mean figure as uninfluenced by lateral 
compression. For these inequalities, though of small amount 
as compared with the dimensions of the globe, may be large in 
