452 T. C. CHAMBERLIN 
the different ocean beds, although the conclusions about to be 
urged would be strengthened if such common action could be 
demonstrated. But quite apart from this, it is believed the 
following argument rests upon rather firm observational and 
inductive grounds except in the matter of two fundamental 
postulates which are almost universally assumed by geologists. 
These are as follows: (1) It is assumed that the earth was at 
first a nearly perfect spheroid, the surface being essentially 
plane. (2) It is assumed that the great movements of the 
earth’s crust have consisted fundamentally of shrinkage. Prob- 
ably no serious geologist maintains that the earth has enlarged 
its average diameter during geological history by expansive 
action, whatever he may hold respecting local expansion. 
These two propositions being accepted, it follows that the 
radial shrinkage of the ocean bottoms has surpassed the radial 
shrinkage of the continental platforms to the average amount of 
some 10,000 or 12,000 feet. This excess of radial shrinkage is 
to be multiplied by four to measure the excess of volumetric 
shrinkage, since the area of the ocean bottoms is about four times 
the area of the continental platforms. The master factor, there- 
fore, in the surface movements of the earth has been the sinking 
of the ocean bottoms and the formation of the great oceanic 
basins. Most geologists will probably agree that the continental 
platforms have also sunk, in the sense that they have shortened 
their radial distances from the center of the earth. Opinion 
seems to be divided, however, on the question whether there 
have been actual epeirogenic uplifts or not. Probably most 
geologists would regard the rising of the Thibetan plateau in 
late Tertiary times as involving an actual increase of radial dis- 
tance. Probably very few geologists would question the abso- 
lute elevation of the crests of the loftier mountain ranges. 
However, the question of absolute as distinguished from relative 
sinking does not seriously affect the question in hand. If the 
earth has absolutely grown smaller by some notable amount the 
average ideal ocean has, as a consequence, grown deeper (if its 
volume has remained constant), for its circumferential expanse 
