338 Status of the Theory of Isostasy. 



which would result from the hypothesis of a floating crust, 

 supported on a denser, plastic, but not necessarily liquid, sub- 

 stratum, is in close accordance with the larger features of the 

 structure of the country south of the Himalayas. It provides 

 for the trough, for the elevation of part of the earlier deposits 

 formed from the waste of the hills on the north of this trough, 

 and for a gradual extension, by progressive regular subsidence, 

 to the southwards, as the range itself grew in magnitude ; it 

 provides also for that belt of positive anomaly of gravity, travers- 

 ing the Peninsula, with its concomitant effect on the plumb-line ; 

 and it may be added that the strength of the crust, required to 

 produce these effects, is much the same as that deduced by Prof. 

 Barrell from the geodetic work in North America. This agree- 

 ment, between the results of conclusions drawn from observation 

 and those obtained by deduction, lends considerable support to 

 the hypothesis on which the deductions were based, but it must 

 be confessed that the Himalayas are the only range where any- 

 thing like this agreement has been found, yet even this may rather 

 strengthen than weaken the support, for it may well result from 

 the magnitude of the range, which is not attained by any other 

 mountains of the world. It is conceivable that only in the moun- 

 tain system, of which the Himalayas form the culminating 

 member, do the gravitational stresses set up by the processes of 

 mountain formation reach a magnitude which enables them to 

 dominate all other influences, and to produce a simplicity and 

 magnitude of structure, obscured in other cases by the action of 

 other influences and resistances, which become more prominent 

 with the decrease in the magnitude of the gravitational 

 stresses. " 3S 



The outer Himalayas thus possess a partly uncom- 

 pensated excess of mass and tend to press down the 

 crust. The Gangetic trough, with a depth on the north 

 of 15,000 to 20,000 feet, has an uncompensated deficiency 

 of mass. It is depressed by the weight of the overthrust 

 mountains to the north. The Hidden Range to the south 

 of the trough has likewise an excess of mass. These 

 relations account for the large residuals and the change 

 in sign of the deflections on the two sides of the alluvial 

 basin. 



In this great modern geosyncline there is thus geodetic 

 evidence that the weight of sediment is far from being 

 the cause of depression. The depression is due to the 

 weight of the adjacent mountains and, so far as the 

 alluvial filling is concerned, its deficiency of mass tends 

 toward uplift and erosion, not depression and further 

 sedimentation. 



Yale University, 

 New Haven, Conn. 



B B. D. Oldham, op. cit. pp. 114-115, 128-129. 



