312 H. O. WOOD SOME CONSIDERATIONS TOUCHING ON ISOSTASY 



shearing as well as flow and fracture, accords well with Leith's observa- 

 tions and interpretations of rock cleavage. 



Effects of the growth of Batholiths 



However, it must also be pointed out that the observed occurrence of 

 batholiths suggests a proximate cause and mechanism which may be ap- 

 pealed to quite independently of tangential pressure, as well as concomi- 

 tantly with it, in explaining mountain building in harmony with isostatic 

 adjustment. But in this case the cause of batholithic melting must be 

 sought. 



Localized modification of Strength in Surface Shell 



We come now, thirdly, to a consideration of the weak prisms in the 

 outer envelope, where yielding under accumulating tangential stress may 

 begin and go on, accompanied by subterranean density changes, until a 

 mountain system has been built and the strain in the surface shell tem- 

 porarily relieved. It is the overwhelming testimony of geology that the 

 places of weakness which subsequently are folded up into mountains are 

 the great synclinal troughs in which sedimentary material has been de- 

 posited until series often many thousands of feet in thickness have been 

 accumulated. These light, unconsolidated, and uncemented materials 

 thus come to form the upper part of earth prisms in places where earlier 

 in the cycle there was stronger, denser rock, cooled by the blanket of 

 water above it, which later subsided under the accumulating load into 

 regions of higher temperature. These prisms are thus weakened in their 

 upper, more competent portions. 



So, if we start with a mountain region adjoining the sea, the ordinary 

 action of erosion will remove what may be relatively weak matter from 

 the mountain upland and deposit it, as a still weaker aggregate, in the 

 adjoining trough of the sea. If we then grant that the outer shell can 

 not sustain much change of stress in the vertical direction, a partially 

 compensatory uplift of the mountain block and subsidence of the sea 

 trough will ensue; and this will go on, strengthening the column of 

 denudation by bringing up stronger rock to replace the weaker rock worn 

 away and weakening the column of deposition in the manner already in- 

 dicated, until baseleveling has advanced very far or gone to completion. 

 This action demands a readjustment of matter in the yielding layer be- 

 neath the region affected which may be visualized as a slow shifting" of 

 matter from under the loaded region to beneath the unloaded region. I 

 purposely avoid here the word undertow on account of a connotation of 



