Chap. VI.] general structure of hills. 173 



pre-existence here of granitoid rocks. This, however, does not interfere 

 with a later intrusion of granite. The altogether lateral position of this 

 ridge in the mountain system precludes the idea of its being in any 

 sense a central axis of Himalayan elevation ; yet its structure seems 

 to require its close connection with that system, and to preclude the 

 idea of its being a partially independent ridge of early origin, such as 

 the eastern ridge of the Andes is represented to be. 



On the whole, the interpretation that seems to accord best with the 

 little information we possess regarding the central mountain region 

 is an adaptation of the Babbage and Herschell theory, assimilating 

 the case to that of the Appalachians as explained by Mr. Hall ; it 

 will serve at least to fix our ideas. A great area of subsidence to the 

 north of the crystalline axis, connected with, if not caused by, great 

 deposition, and entailing compression by which flexures of contortion 

 were produced from the northwards : a line of weakness, perhaps in- 

 duced by denudation, along the present granitic axis, which would 

 thus have become a line of relative upheaval accompanied by intru- 

 sion. The granitic character of this intrusion may be largely due 

 to the nature of the rocks passed through, the same material may have 

 turned into trappean rocks among the highly basic strata to the north. 

 The subsequent general upheaval of the area would, under the same 

 theory, be explained by the slow thorough heating of the newly made 

 crust and of the mass beneath it. 



It is, however, difficult to reconcile the mode of explanation I have 

 just applied to the elevation and contortion of the older rocks with the 

 view I have taken of the similar phenomena in the Sub-Himalayan 

 regions. Elevation by heating is a mere swelling (not involving an 

 increased accumulation of matter) and the only provision the theory 

 makes for its reduction is loss of volume by loss of temperature conse- 

 quent upon denudation. Subsidence of this kind could hardly produce 

 the lateral pressure, of the occurrence of which, subsequent to the 



