396 E. C. AXDREWS THE HYPOTHESIS OF MOUNTAIN FORMATION 



tites, whereas the granite, general!}' in the form of primary gneiss and 

 the basic rocks, had formed "pressure'' lenses and sills. In the region 

 nearer the surface the intrusives form tongues and dikes. In the land 

 crests near the continental nuclei, or in the more stable areas associated 

 with the folded region, floods of basic lava appeared at the surface. Dur- 

 ing the great revivals of mountain -making, volcanoes and earthquakes 

 were common accompaniments of the vertical uplifts and the warpings 

 of the surface. 



Relation of Heatinc4 of underlying folded Sediments to 



Mountain Formation 



The Archeozoic and lower Proterozoic rocks, where exposed, are in- 

 tensely altered. Sillimanite, chiastolite, andalusite, mica, and other 

 schists result commonly from the alteration of ancient silts and shales. 

 This suggests the action of long and continued heating under heavy load. 

 A study of the Australian exposures belonging to similar periods suggests 

 that the alteration is due not so much to the igneous rocks which have 

 intruded the sediments as to an action which has the same source as the 

 igneous intrusives themselves. This indicates that the sediments affected 

 must have locally increased considerably in volume. 



Eelation of Isostasy to Mountain Formation 



Bowie and Burrard^^ have insisted on the necessity for the considera- 

 tion of the doctrine of isostasy in discussions on mountain-making. 



Burrard considers that isostasy demands a vertical uplift for mountain 

 formations. He maintains that the uplift of the Himalayas is mainly 

 vertical, with the crushing of the Siwaliks over a zone which is only rela- 

 tively narrow. 



Bowie says in his discussion on mountains : 



"If this mountain mass is not an extra load, then it could not have been 

 brought from some other area to the one it now occupies. If it is not an extra 

 load, it could not be due to horizontal thrusts operating in the earth's isostatic 

 shell and extending far beyond the mountain 'area.* " 



Both authors appreciate the fact that horizontal movements do exist in 

 the formation of mountains, but they consider that the main agent is 

 vertical uplift and, moreover, that horizontal movements may be inci- 

 dental to a vertical uplift. 



' Bowie : Origin of mountain ranges. 

 Colonel Sir Sidney Burrard : Geog. Jour., vol. Iviii. no. 3. 



