HYPOTHESES OF MOUXTAIX-BUILDIXG 281 



.assumptions. These are extrapolations on a large scale from the results 

 of exjDeriments which may not be directly applicable to the earth. Al- 

 though the preliminary test of this hypothesis with the Sierra IN'eyada 

 Mountains and Coast ranges appears to him to have been successful^ it is 

 thought that the conditions in the northern Eocky Mountains, where the 

 ^reat overthrusts have large horizontal components, will prove less favor- 

 able to it, and that it may not be widely applicable. 



Thom's hypothesis postulates that compressive movements are active 

 in the deeper and more rigid parts of the shell beneath the overlying 

 sediments, and that the deformation of the latter is directed or controlled 

 in part by the deformation of the former. This view appears sound. His 

 statement, however, that the intense compression in the outer portions 

 of the crust was dissipated by overthrusts, but that the deeper part con- 

 tinued to be deformed, suggests that the structures which he describes are 

 younger than the overthrusts. It has been shown on page 272 that the 

 development of overthrusts was probably progressive, both in space and 

 lime, and that the easterly overthrusts were the latest formed. It was 

 .also suggested, on the basis of the apparent depth of the folds, that the 

 zone of compression, as it became more intense, may have migrated out- 

 ward toward the surface. If these suggestions are well founded, it may 

 be that the structures described by Thom, both the deep-seated and the 

 more superficial, were formed before the overthrusts occurred, and that 

 they may have played an important part in localizing the overthrusts. 



Causes of Mouxtaix-buildixg 



The primary cause of mountain-building is unknown. However, the 

 abundant evidence of compression found in most mountain regions im- 

 plies a shrinkage of the earth to a smaller volmne. ^N'umerous agencies 

 have been recognized as contributor}- to this result, among which are 

 •cooling, crystallization, progressive condensation under gravitative influ- 

 ences, molecular and subatomic changes, redistribution of internal heat, 

 .and perhaps other causes. Gravitative readjustments take place peri- 

 odicalh', as a result of accumulating stresses, rather than continuously. 

 The origin of igneous magmas is closely interwoven with these readjust- 

 ments, and the movements or intrusions of these magmas play an im- 

 portant part in the distribution of temperatures and in the application 

 or localization of stresses within the earth's crust. The net result of 

 these various agencies, so far as mountain-building is concerned, is to 

 produce the compressive stresses that in turn produce the folding and 

 overthrusting. 



