ISOSTASY 347 



beneath the land. In his view this lateral transfer was a complete cause 

 for laterally folded mountains. 



That such a tendency to lateral transfer exists can not be doubted, but 

 the question arises as to the amount of this transfer. Is it sufficient to 

 cause the great lateral shortening observed in folded mountain ranges? 

 Button's theory was deduced largely from his observations in the high 

 plateaus of the west, in which the element of lateral shortening was so 

 slight that the lateral motion due to isostatic adjustment might perhaps 

 have caused the observed geologic relations. The situation, however, is 

 far otherwise in the case of the Appalachians and even of the eastern 

 folded ranges of the Eocky Mountains. In fact, the difficulty of deriving 

 sufficient motion from the differences in gravity due to unloading some 

 thousands of feet from the land have been recognized by most students 

 of the matter as very grave. This is so decidedly the case that most 

 students have considered that, while isostasy may account for the broad, 

 continental, or epeirogenic upswellings, it is inadequate to explain the 

 great folded chains. Button's view was exactlv the opposite, however, 

 for he concluded that isostasy did account for laterally folded mountains 

 but not for continental uplifts. For the latter movements he found it 

 necessary to postulate an unknown cause, pei:haps expansion or contrac- 

 tion of deep-seated bodies of magma. It is important to note that he 

 considered some other factor to be operative in addition to isostasy. 



In very recent years there is a revival of this theory and even a decided 

 expansion of it. Bowie in 1920 goes so far as to say that the theory of 

 tangential thrust must be given up in favor of one more modern and 

 more in accord with the facts, and that the mountains resulted from 

 vertical movements due to expansion. He also accepts the existence of 

 other means of deformation than isostasy. The most recent advocates 

 of isostasy claim that its action is complete and that the earth densities 

 are perfectly and continuously compensated. It is not possible to discuss 

 in this paper the mathematical and physical basis of the theory; it has 

 its strong points, but also its weaknesses. It must, however, meet various 

 conditions established by the visible data of geology, and by them must 

 be gauged its success in explaining mountain structures. 



Looking at the process of isostasy from the theoretical standpoint, one 

 is led to question whether actual movement did result from these 

 pressures due to differences in load, and, also, if such motion took place, 

 in what part of the crust it would be — in its upper portions or very 

 deeply buried. The probability of actual movement is affected funda- 

 mentally by the nature of the materials themselves. If the crust had 

 the fluidity of water, adjustments would be almost instantaneous, and 



