122 OLDHAM: THE STRUCTURE OE THE HIMALAYAS, ETC. 



that the deposit of sediment on the surface of the earth must cause 

 a subsidence, in consequence of the additional load, is one which 

 has had some vogue ; it is unnecessary here to discuss the justifica- 

 tion of this idea, it is sufficient to point out that the cause is obvi- 

 ously inapplicable in the case of the Gangetic trough. Not only is the 

 surface of the alluvium at a lower level than that of the rock areas 

 to the north and the south, but the density of the material is very 

 considerably less than that of the rocks on either side ; consequently 

 the load borne by the crust in the region of the Gangetic trough 

 must be less than in the Himalayas to the north, or in the peninsular 

 rock area to the south, as is proved by the result of gravity observa- 

 tions in the alluvial plain. But though the weight of the sediment 

 cannot have been the originating cause of the depression of the 

 Gano-etic trough, it may well have had considerable influence in 

 determining the magnitude of its dimensions, for if there had been 

 some other cause capable of forcing down the level of the crust 

 to a given depth before the resistance to further movement became 

 equal to the force, then the addition of a load of alluvium would 

 enable the same force to lower the level to a greater extent than 

 if the hollow had been left empty or only filled with water. The 

 amount of this extra depression would depend on the balance 

 between the force and the resistance ; if both remained appreciably 

 constant, within the limits of movement involved, the weight of 

 the alluvium would enable this to be carried about five times 

 further than would otherwise be the case, so that the Gangetic 

 trough, taken as l~),0i;0 feet deep, would only have had a depth 

 of about 3,000 feet had it not been filled with alluvium as fast as 

 it was formed. 



One such possible cause has , been indicated by Mr. Fisher. He 

 pointed out that if material is removed by denudation from the 

 surface of a range, and deposited by its side, the centre of gravity 

 of that portion of the crust comprising the two regions would be 

 shifted laterally, and, on the assumption of a crust supported by 

 flotation, there would be a disturbance of the condition of equili- 

 brium, so that the centres of gravity and of buoyancy would no 

 longer lie on the same vertical line. As a result, a couple would 

 be set up, tending to raise the range and depress the crust along- 

 side it, till the loss of buoyancy under the range, and the gain under 

 the plain, led to a re-establishment of a condition of equilibrium 

 and, as a further result, a depression of the surface would be formed 



[ 270 ] 



