126 OLDHAM: THE STRICTURE OF THE HIMALAYAS, ETC. 



crust, of sufficient strength to enable it to be forced downwards into 

 the denser matter underlying it, in the manner which lias been 

 outlined, it is improbable that so large a depression would at once 

 die out into a condition of equilibrium on the further side from the 

 hills. The very strength of the crust which enabled the depres- 

 sion to be formed would be likely to uplift the crust, on the further 

 side, beyond the point of equilibrium, before it finally sank down 

 into a normal condition, unaffected by the exceptional circum- 

 stances connected with the Himalayan range. In this way the 

 depression of the Gangetic trough would be bordered on the south 

 by a tract where the crust was uplifted, as a whole, with the con- 

 sequence of the rise of the denser matter from below into the hollow 

 formed in the under surface of the crust, and so give rise to pre- 

 cisely the phenomenon which Sir S. G. Burrard found necessary 

 to invoke, in order to account for the observed deflections of the 

 plumb-line. 



The argument of the last paragraph may be made clearer by 

 reference to fig. 10, where a cross-section is depicted, from the 

 centre of the Himalayas to about the centre of the Peninsula, 

 covering about 10° of latitude or a distance of some 700 miles. 

 In this figure the actual relief of the surface is indicated on a some- 

 what exaggerated vertical scale, in order to make it recognisable ; 

 below is represented, on an equally reduced scale, the under surface 

 of the crust, adopting Mr. Fisher's constants of a thickness of 25 

 miles for the undisturbed crust, and a ratio of 9- 6 : 1 between the 

 prominences on the under and upper surface of the crust, respec- 

 tively. 1 In this part of the figure there are two lines, one firm and 

 the other dotted, of these the dotted line represents the under sur- 

 face of the crust as it would be if there was at every point a 

 complete compensation of the surface irregularity, the firm line 

 represents the form of the under surface of the crust as it would 

 have to be in accordance with the departures from exact com- 

 pensation which have been established or inferred. The treatment 

 is in fact the reverse of that adopted in fig. 8, in which the adjust- 

 ment was made by an alteration of the surface level, and the hills 

 supposed to be either held down or uplifted. 



* 



1 It is obvious that the clotted line, which represents what the under surface should be, 

 were the inequalities in the surface and the Gangetic trough completely compensated 

 under every point, may also be regarded as representing the proportionate amount of 

 the compensation, irrespective of any theory of how it is brought about. 



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