SUMMARY AND CONCLUSIONS. 123 



along the foot of the range, which would grow in depth, and in 



breadth, as the range increased in height. The reasoning is per- 

 fectly sound from a mechanical point of view ; given a crust of some 

 degree of strength and rigidity, supported by flotation, the processes 

 conceived will follow with logical necessity, and it is interesting to 

 note that the results of this purely mathematical investigation agree 

 remarkably with the deductions which result from geological 

 examination as to the character of the southern margin of the 

 alluvium, the history of its gradual extension to the south, and the 

 radical contrast in character between the southern and northern 

 margins of the trough. The only doubt is as to whether the cause 

 invoked by Mr. Fisher would be quantitatively sufficient to pro- 

 duce the results, and with regard to this it may be pointed out 

 that the action, which he conceived, would be reinforced by the 

 effect of an increase in the buoyancy under the range, such 

 as has been indicated in the preceding chapter, so that it is possible 

 for the combined efTect of the two causes, working in the same 

 direction, to have given rise to the depression of the Gangetic 

 trough, though neither of them would, independently, have been 

 sufficient. 



The only test which we can apply to this interpretation is to 

 be derived from the geodetic data. It is evident that a depres- 

 sion of the lower surface of the crust, witli the consequent displace- 

 ment of denser by less dense material, would produce an effect on 

 the plumb-line and the pendulum, it would cause a northerly deflec- 

 tion to the north of the trough, and a southerly deflection to the 

 south, and would give rise to a defect of gravity, greatest along the 

 line of maximum depression and decreasing on either side. These 

 effects, it will be noticed, are similar in kind to those produced by 

 the alluvial trough, but, being much smaller in amount, are so 

 effectively masked by those due to the alluvium itself that it is 

 difficult to disentangle them. An attempt was made, by a com- 

 parison of the results derived from the deflections and the gravity 

 observations, to separate out the efTect of a possible depression of 

 the crust as a whole from that of its upper surface, the attempt 

 led to an apparent confirmation of the hypothesis, but it involved 

 too many considerations of very doubtful validity to justify the 

 space necessary for its exposition. There are, however, within 

 the area of the alluvium some observations, otherwise difficult 

 to understand, which find an easy interpretation in this way, namely, 



[ 271 ] 



