210 T. C. CHAMBERLIN GROUNDWORK OF EARTH S DIASTROPHISM 



On the other hand, the planetesimal view brings into play a group of 

 effective processes which tended to cumulative differentiation of the ma- 

 terial of the earth from the earliest stage of accession throughout the 

 whole period of growth, though these were also antagonized by some 

 counteracting agencies ; but the limit in this case was very much broader. 

 As a result, the full-grown earth inherited profound differentiations of 

 material. These had a broad regional distribution correlated with the 

 currents of the atmosphere and hydrosphere as well as the main lines of 

 early diastrophism. From these fundamental differentiating agencies 

 there arose segments of lighter and heavier material of continental and 

 oceanic dimensions. These diversities are held to be the specific ancestors 

 of those that now manifest themselves in the great reliefs of the earth. 

 It is the prodigious task of surface agencies and of diastrophism, working 

 together, to bring about, in the course of coming ages, a closer equation 

 between these segments of lighter and heavier matter. This will require 

 a vast lapse of time ; but we have no wish to hasten its completion. The 

 perpetuation of land life hangs on the extent of the delay in its accom- 

 plishment. 



Probably one of the greatest contributions of isostasy, when its values 

 shall be fully realized, will be its testimony to the significance of those 

 wide and deep inequallities of substance which make possible the great 

 reliefs of the present earth's surface. 



