138 OLDHAM: THE STRUCTURE OF THE HIMALAYAS, ETC. 



as regards the region to the south of the range, now occupied by the 

 Gangetic trough, and if this trough is really due to a withdrawal of 

 material towards the hills, we have a process which is the converse 

 of that suggested by Mr. Fisher. In the one case the crust is supposed 

 to have been borne down, displacing a certain amount of denser 

 material from beneath it, in the other the underlying material is 

 supposed to have been withdrawn, leading to a settling down of 

 the lighter material above, and as the form of the resulting trough, 

 developed in Sir S. G. Burrard's latest exposition of his explanation, 1 

 is practically identical with that resulting from the present in- 

 vestigation, the geodetic effects would be identical in either case, 

 and we have no criterion for discrimination between the two inter- 

 pretations. 



Nor do we get any help from the geological evidence. There 

 is no indication that the region of the Gangetic trough is one of 

 tension, as suggested by Sir S. G. Burrard, but equally there is no 

 certainty that it is not ; within the region of the alluvium all evi- 

 dence, one way or the other, has been obliterated, and only by con- 

 sideration of the associated phenomena can a criterion be obtained. 

 It has been shown that the view which regards the origin of the 

 Gangetic trough as a consequence of the process of the elevation 

 of the range, and the disturbance produced in the equilibrium of a 

 floating crust, is in agreement with the geological and geodetic 

 observations along the border of the alluvium and in the count rv 

 beyond ; the same cannot be said of the alternative explanation. 

 On the southern side it is not incompatible with the facts, and 

 might give rise to the phenomenon of the Hidden Range of excess 

 of gravity ; on the northern, in the region of the Himalayas, there 

 are the same fundamental objections, which were pointed out in 

 dealing with Prof. Suess' explanation, that the hypothesis does 

 not admit of a sufficient range of movement to account for the 

 structure, and that it is inconsistent with the existence of com- 

 pensation, ,and more especially of the alternate excess and defect 

 of compensation, of the range. So far, however, as the explanation 

 refers the origin of the Himalayas to an invasion of the region of 

 the hills by the lower layers of the crust, independent of the de- 

 formation which has taken place in the upper layers, it is in accord 

 with the investigation which has been developed in this memoir ; 

 for it has been shown that the facts, as they are known, seem 



*Proc. Roy. Soc, Series A, XCI, 1915, p. 233. 

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