MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 77 



overthrusting " as an explanation of Alpine structure is steadily 

 gaining ground, it is none the less a fact that there is much criticism 

 of the mechanics of the processes which have been invoked, and 

 the writer believes that this criticism is well founded. Swiss 

 geologists profess to regard this phase of the subject as of slight 

 importance; but so long as it is necessary to reconstruct from 

 incomplete surface indications folds which are in part concealed 

 below the surface, the contention of these geologists can hardly 

 be admitted to be justified. No better illustration could be 

 offered than contrasting the "double fold" and the "blanketing 

 nappes" theories as applied to the classical district of the Glarus 

 (Fig. i). Both theories involve complex structures which have 

 never yet been found complete in any region, and discrimination 

 between them must of necessity depend in some measure upon the 

 possibility of accounting for their formation as a result of con- 

 ceivable stress-strain conditions during a profound deformation of 

 the region. In order to throw light upon their origin, all that is 

 known of the mechanics involved in the folding process should be 

 brought to bear, with due considergttion of the fact that the Alps 

 furnish a perfect illustration of the Asiatic type of mountain arc. 



THE FOLDING PROCESS STUDIED IN THE PLAN — ARCUATE 

 STRUCTURE 



Areal and morphological characteristics of Asiatic ranges.- — Study 

 of a modern map of Asia upon which the rehef has been indicated 

 brings out some quite remarkable facts of distribution of the 

 mountain ranges (Fig. 2). These facts might be stated in some- 

 what categorical form as follows: 



1. The ranges are arcs which present their convex sides to the 

 oceanic areas. 



2. The arcs taken together in part inclose a relatively rigid 

 mass of ancient rocks which from earliest geological times has been 

 a land area and has become known to geologists as the Angara 

 coign or shield. 



3. The inner arcs of the series are the older and simpler in 

 form and have the largest radius of curvature. 



