334 s. bubrard folding of mountain ranges 



Mountain Formation by vertical Expansion 



This difficult question has been recently discussed by Dr. William 

 Bowie, in his paper, "Relation of isostasy to uplift and subsidence." 

 The conclusions which Bowie has drawn from a study of the Appalachian 

 system are a helpful contribution to the understanding of the Himalayas. 

 Bowie writes : 



'"We are led to the conclusion that the cause of the mountain formation is a 

 local one. and the only local cause seems to he a change in density in the 

 column. This change must have heen the result of a local expansion in the 

 isostatic shell under the sedimentary material which was thrown up to form 

 the mountain mass." 



This explanation appears very applicable to the Himalayan system. 

 The primary feature of mountains is their height, and the highest points 

 of the Himalayas are mainly granitic. Sir H. Harden, the late Director 

 of the Geological Survey of India, writes : 7 



"Almost all the peaks of 25,000 feet and more are composed of granite, 

 gneiss, and associated crystalline rocks. The great peaks lie on continuous 

 elevated zones of granite and crystalline rock. The great central gneissose 

 rocks of the Himalayas were proved by General McMahon to be an intrusive 

 granite foliated by pressure'' ; 



and again the same eminent authority writes: 



"During the development of the Himalayas as a mountain range vast masses 

 of granite welled up from below." 



If this granite range had been standing alone, its vertical origin would 

 never have been questioned, but the sedimentary rocks on its southern 

 flank have been folded and elevated. On the other hand, if these folds 

 had been standing alone and had been unaccompanied by the granitic 

 outbursts, geologists would have felt little doubt that the mountains had 

 been raised by horizontal compression of the crust. The granite outbursts 

 and the horizontal folds are in juxtaposition. The problem now pre- 

 sented to us is: Has the vertical uplift of the granite been incidental to 

 the borizontal compression, or has the folding been incidental to the 

 vertical rise of the granite? 



Bowie holds that the folding has been incidental to the vertical uplift. 

 A strong argument in support of this contention is that the folded sedi- 

 mentaries are inferior in height and mass to the granite intrusions with 

 which thev are in contact. 



8 American Journal of Science, vol. ii. July. 1021. 



7 Sketch of Himalayan geography and geology, 1007, pp. 7 and S. 



