SOURCE OF EVOLUTION OF PROVINCIAL FAUNAS 599 
may be said that the widest departure of the average land sur- 
face from this ideal summit level has probably at no time 
exceeded 25 or 30 per cent. of the whole height of the platform, 
while at times of greatest approximation to the theoretical sum- 
mit level through base leveling, its departure has probably not 
reached 10 per cent. of the whole height of the platform. The 
conception of a continent, therefore, as a platform maintained 
against deforming agencies by constant truncation of the pro- 
truding portions and by constant upbuilding about its borders is 
not seriously vitiated by the inequalities which crustal readjust- 
ments force upon it from time to time. 
The ocean basins, considered as inverted plateaus or anti- 
plateaus, have no analagous agency for the reduction of their 
bottoms to an ideal plain, and their inequalities are greater (in 
their broad features, but not in detailed accentuation), and yet 
here is a reasonable approximation to a bottom plain, for more 
than half of the oceanic bottom lies between 12,000 and 18,000 
feet below sea level. But the variation of 6000 feet included 
between these limits would be regarded as very large if it were 
a land surface. 
More or less warping of the surface of the solid part of the 
earth is doubtless in progress at all times, but there is much 
concurrent geologic evidence to the effect that the really 
mportant changes are periodic rather than uniformly progres- 
sive. The most important items in this evidence are the great 
base levels and the great epochs of mountain making, the former 
pointing to long periods of relative quiescence, the latter to 
exceptional periods of disturbance. In the larger conceptions 
of the earth movements, the minor warpings may be ignored, 
but in the interpretation of the details of the earth’s history they 
play a not unimportant part. The degree of importance of this 
part is dependent upon the critical relationships which the warp- 
ing may bear to sea-level relations. The present study is con- 
cerned with such relationships in their bearing upon the progress 
of marine life. 
The discussion proceeds upon the following general concep- 
