R. D. Oldham — Essays in Theoretical Geology. 73 



little disturbance and contortion, while sucli compression as tbey 

 had been subject to did not belong to the Himalayan system of 

 distui'bance. From this time onwards the beds have been con- 

 tinuously subject to compression, contortion, disturbance and elevation, 

 and the Himalayas have been continuously an area of denudation. 



As soon as the Himalayas were defined as a distinct hill range, 

 a series of subaerial deposits of great thickness began to be deposited 

 in a region of subsidence along their outer edge, and the demarcation 

 between the areas of subsidence and deposition on the one hand, 

 and of elevation and denudation on the other, was abrupt, exhibiting 

 itself at the present day as a gigantic fault. As the Himalayas rose, . 

 the boundary between the two areas advanced step by step to the 

 southwards ; the beds which had been deposited along the foot of 

 the original hills were compressed, disturbed, elevated, and, con- 

 sequently, exposed to denudation; but the new limit between the 

 area of elevation and of depression was again an abrupt one, which, 

 on a subsequent further advance of the hill area, showed itself as a 

 fault with an upthrow towards the central range. Concurrently with 

 this southward march of the margin of the Himalayas, the depression 

 occupied by the Indo-Gangetic alluvium extended itself to the south- 

 wards by the gradual subsidence of the peninsular rock area. 



Such are the main features of the history of the Himalaya. As 

 I have shown, there have probably been minor elevations of the 

 range unaccomjjanied by disturbance ; but, in the main, its elevation 

 is the direct result of, and has been accompanied by, compression 

 and contortion of the beds of which it is composed. 



U.—The Theory. 



It is now generally admitted that the elevation of what may be 

 called true mountain ranges is but a secondary effect of that com- 

 pression which their structure shows they have undergone. There 

 is also a general concensus of opinion in favour of the Herschel- 

 Babbage doctrine that denudation and elevation, deposition and 

 subsidence, are closely connected with each other, at least in so far 

 that denudation acts in intensifying the effect of the causes which 

 lead to elevation, and deposition of those that lead to subsidence. 



The most i-ecent and complete adaptation of this doctrine to the 

 theory of mountain formation is contained in the Eev. 0. Fisher's 

 work on the " Physics of the Earth's Crust." Mr. Fisher's hypo- 

 thesis demands a solid crust resting on a denser magma, whose 

 condition is actullay or virtually that of a fluid. The crust, on being 

 subjected to compression, yields along certain lines and as a result 

 is thickened, both upwards and downwards, from a zone somewhere 

 in the thickness of the crust " above which the material will on 

 the average be sheared upwards, and below it downwards ; " this 

 zone is called the " neutral zone," and, for reasons which it is 

 needless to enter into here, is placed at three-fifths of the thickness 

 of the crust from its upper surface.^ 



As, for many reasons, it is highly improbable that the elevation 

 ' Physics of the Earth's Crust, second edition, pp. 183-184. 



