1916.] The Third Indian Science Congress. Ixxxv 



brium, not perfect equilibrium like the condition of flotation. 

 Isostasy is a condition of compensation in a solid crust ; it 

 does not necessarily imply hydrostatic support, as flotation 

 does. I therefore hesitate to apply the word Isostasy to the 

 Flotation theory ; for Isostasy can exist without flotation. 1 



Mountains Originate at Great Depths. 



A very important work has been that of Mr. Hayford 

 who has recently discussed the results of the plumb-line at a 

 large number of stations in America. He has confirmed Pratt. 

 Hayford has investigated the depth to which the deficiency of 

 density underlying mountains goes down, and he has found 

 that that depth is between 60 and 90 miles. That is to say, he 

 has shown that the depth of subterranean compensation is very 

 great compared with the height of mountains. The discovery 

 that mountains originate from the great depth of 60 to 90 miles 

 is the second important contribution of geodesy to this study; 

 the first was compensation, the second is great depth. 



Most books are written on the assumption that mountains 

 are surface wrinkles and that their structure can be determined 

 by examining surface rocks. 



The Satpura range runs east and west south of the Nar- 

 bada ; the plateaux of Hazaribagh and Chota Nagpore are the 

 eastward continuation of the Satpura range. A high authority 

 has stated that the Hazaribagh and Chota Nagpore plateaux 

 can have no real connection with the Satpura range, because 

 they are formed of different rocks. But if we regard this 

 range as rising from a depth of 75 miles, its elevation will be 

 seen to be due to a deep-seated cause that has nothing to do 

 with the surface rocks. One deep-seated cause has lifted up 

 this range from the Narbada to Hazaribagh irrespective of the 

 kind of rocks lying on the surface. 



The Gangetic Trough. 



I have now discussed the two principal theories of Hima- 

 layan elevation, the Contraction theory and the Flotation 

 theory. Let us consider for one moment how this deep Gange- 

 tic trough is explained by these two theories. For a great 

 number of years the Contraction theory ignored this trough ; 

 it was, I think. Professor Suess who first recognised that the 

 trough had to be fitted into the Contraction theory. His ex- 

 planation of it was this : as the Earth's interior contracts, the 

 surface of Asia is wrinkled, the wrinkles get pushed south- 



1 The idea of flotation has arisen because the question of mountain- 

 support has been given precedence of the question of mountain-elevation. 

 Questions of support and maintenance should be subsidiary to questions 

 of formation and origin. If mountains are due to the vertical expansion 

 of rock, a theory of flotation is superfluous. 





