THE EARTH, A FAILING STRUCTURE. 63 
6. As stated in Darwin’s paper itself, if the earth is not 
solid throughout, if the outer solid crust is less than two or 
three hundred miles thick, the stress-difference must be much 
greater at some points within the crust than the value com- 
puted by him, namely, 4 tons per square inch. Now, in so 
far as any effect on the stresses above that depth is con- 
cerned, the part of the earth below a given depth is an exact 
equivalent of a liquid, if at that depth hydrostatic equilib- 
rium exists — that is, if the pressure in all three directions 
is the same at each point at that depth. Now, many different 
men, reasoning from various points of view, have each 
reached the conclusion that a liquid condition, or its equiva- 
lent in the sense just indicated, is reached at a depth less 
than 200 miles. 
For all these reasons, then, it appears probable that under 
the stress-differences within the earth due to the weight of 
the continents and mountains, the material must slowly 
yield, the continents slowly sink downward and the ocean 
bottoms rise. 
One apparently invincible argument against this conclu- 
sion is thus stated in Darwin’s paper: “The interior of the 
earth must be in a state of stress, and as the land does not 
sink in, nor the sea bed rise up, the materials of which the 
earth is made must be strong enough to bear this stress.” 
But the premise of this argument ignores the overwhelming 
mass of evidence accumulated by the geologists, that large 
portions of the present surface of each continent have at 
some past time been at or below sea-level. Changes in eleva- 
tion in the material composing the continents are put in evi- 
dence everywhere by geologic structure and topographic 
forms. The continents are in the habit of sinking and ris- 
ing. We must therefore draw the conclusion, apparently 
contradictory to Darwin’s, that the materials of which the 
earth is made are not strong enough to bear the stresses to 
which they are subjected. 
This conclusion is not necessarily contradictory to Dar- 
win’s conclusion, for the reason that the stresses which cause 
