1916.] The Third Indian Science Congress, xcv 



surface rocks, and yet an expansion of onlv two per cent 

 would account for both the elevation and the compensation of 

 the Ghats. Geodetic observations show that the compensation 

 is not perfect, and that the Gbats contain an amount of rock 

 in slight excess of the normal crust : the vertical expansion of 

 rock must thus have been accompanied by a slight horizontal 

 compression insufficient to fold the surface strata, but suffi- 

 cient to account for the imperfection of the compensation. 



The heterogeneous rocks composing the Earth's crust are 

 continually undergoing changes of structure, known to geolo- 

 gists as metamorphism. At a depth of 30 miles the tempera- 

 ture is sufficiently high to melt all known rocks; but increas* 

 of pressure raises the melting point, and the increase of pres- 

 sure underground may be sufficiently great to counteract the 

 effects of the increase of temperature. So that at a depth of 

 even 60 miles rocks may still be solid and rigid, as geodesy 

 leads us to believe they are. 



We have to imagine how deep-seated rocks, that have 

 been buried for millions of years under high temperatures and 

 enormous pressures, how they would behave, if a crack pene- 

 trating downwards from the Earth's surface reached and dis- 

 turbed them. I suggest for your consideration that two cracks, 

 opening one on the West Coast and one on the East Coast of 

 India, have compressed the Indian Peninsula between them. 

 This lateral pressure was insufficient to crumple the table-land ; 

 but may it not have been the exciting cause that led deep and 

 ancient rocks to expand vertically and elevate the Deccan ? 



Penologists will be better able to discuss this question than 

 I am. 



The main ranges of the Himalaya are composed of granite ; 

 this granite has protruded upwards from below. I suggest 

 that the protrusion of granite is due to expansion of rocks in 

 the sub-crust. The great Himalayan range is 5 miles high ; 

 and the compensation of this range, that is, its underlying 

 deficiency of density, is estimated to extend downwards to a 

 depth of perhaps 75 miles. An underground expansion of 7 

 per cent would be sufficient to account for the elevation of the 

 Himalaya. J 



Many of the faults which intersect the Himalaya may, I 

 think, be ascribed to the shearing, which must have ensued 

 when certain areas of the crust were forced vertically upwards 

 by the metamorphism of sub-crustal rock. Many distortions of 



J If underlying deficiency of mass is greater than the excess of mass 

 m a mountain, the plumb-line will be deflected away from the mountain. 

 Ver " compensation would therefore account for deflections away from 

 mountains. But it would not account for tension or subsidence in the 

 tore-deep. Pendulum observations in the outer Himalaya and at Ootaca- 

 "lund indicate not over-compensation but imperfect compensation. 



