1916.] The Third Indian Science Congress. Ixxxvii 



was found to hang decisively away from the mountains ; at 

 Fyzabad, Cawnpore, Benares, the plumb-line is deflected south- 

 wards : here at Lucknow it is deflected 9" to the south. If 

 the Himalaya were simply compensated, this plumb-line 

 should be hanging at Lucknow exactly vertical ; if the moun- 

 tains were not compensated, it should be deflected here about 

 53" towards the north. But it is deflected 9" towards the 

 south. The observers were astonished to find that at places in 

 sight of Himalayan peaks the plumb-line turned away from 

 the mountain mass ; that at Amritsar in sight of the Dhaula- 

 dhar snows it was deflected towards the low Punjab plains ; 

 that at Multan in sight of the Takht-i-Suleiman mountains it 

 was deflected towards the desert ; at Bombay it was deflected 

 seawards away from the Western Ghats ; on the east coast of 



India it was deflected seawards away from the Eastern Ghats 

 (Plate A). 



The new lesson to be learnt from the piumb-line is this : a 

 hidden subterranean channel of deficient density must be skirt- 

 ing the mountains of India. Here in North India is a wide 

 zone of deficient density, of crustal attenuation ; it is the pre- 

 sence of this zone of deficiency that accounts for the southerly 

 deflection of the plumb-line. What is the meaning of this 

 zone ? How has it come into existence ? 



If you look at this section (Plate B) the Earth's crust in 

 these outer Himalaya has been compressed laterally : of this 

 there is no doubt. The area between the snowy range and the 

 foothills is a zone of crustal compression. And I suggest 

 for your consideration that the Gangetic trough, this zone of 

 deficiency, is a zone of tension in the crust. The crust has 

 been stretched here and attenuated. Here you have a compres- 

 sion, and alongside is the tension. The tension is the comple- 

 ment of the compression. I have pointed out that the Himalaya 

 mountains are largely , but not completely compensated by their 

 underlying deficiencies of density : their compensation is how- 

 ever rendered complete by the presence of the Ganges trough ; 

 if the Himalayan compression and the Gangetic tension are con- 

 sidered together, it will be found that there is no extra mass. 



Geodesy thus teaches that the Gangetic trough and the 

 Himalaya Mountains are parts of one whole. The Contraction 

 theory and the Flotation theory both treat the Gangetic 

 trough as though it were a secondary effect of Himalayan 

 elevation. But this Gangetic trough may have been the first 

 and the decisive event; the Himalaya Mountains may have 



been a secondarv effect, a sequel to the opening of the 

 trough . 



Hypothesis of a Rift. 



I showed you on the evidence of the plumb-line that the 

 Gangetic trough was a zone of crustal attenuation, a zone in 



