SUMMARY AND CONCLUSIONS. 121 



of the deduction that the outer edge of the hills marks the posi- 

 tion of a structure similar in character to the faults which traverse 

 the Siwalik area, and form its northern boundary for a large portion 

 of the length of the Himalayas. 1 



There remains only the question of whether the compression, 

 which the rocks of the Himalayas have unquestionably under- 

 gone, is the cause, or merely the accompaniment, of the elevation 

 of the range. The treatment of this question is impossible without 

 considering that of the origin of the Himalayas and a discussion, 

 which need not be detailed, of the explanations which have been 

 offered, of the origin of the Himalayas, and of the closely con- 

 nected problem of the origin of the (iangetic trough. 



It has already been shown that there is some suggestion of the 

 boundary faults, and with them of the tectonic processes which 

 have modified the underground form of the floor of the trough, 

 being phenomena of the upper part of the crust alone, and inde- 

 pendent of the more deep-seated changes in the distribution of 

 density on which the compensation depends. 1 This being so, it 

 is obviously possible that the same conclusion might be extended 

 to the whole of the trough, and its existence be regarded as due 

 to processes which were confined to the upper part of the crust 

 proper, with the result that there would be neither need nor reason 

 to look for any more deep-seated cause of origin. The magnitude 

 and extent of the trough seem to make any such localised cause 

 inappropriate, and the radical difference in the form and boundary 

 of the southern part, as compared with the northern fringe, makes 

 it probable that an entirely different set of processes have been at 

 work, and that the trough as a whole may be due to deep-seated 

 and widespread forces, involving the crust, as a whole, and the 

 material which underlies it. In this case we cannot ascribe the 

 trough to any deformation of a part of the crust, such as has pro- 

 foundly modified the form, and defined the boundary, on the north, 

 but rather to a general subsidence of the crust, increasing in amount 

 from south to north. 



In searching for a cause, which could have produced this depres- 

 sion, we must first of all reject the notion that it can be a direct 

 downward pressure due to the weight of the alluvium. The notion 



1 Supra p. lnO. 

 [ 209 ] 



