1916.] The Third Indian Science Congress. xciii 



The Depth of the Gangetic Rift. 



I have been describing zones of deficiency and have sug- 

 gested that they are cracks in the sub-crust. I have now the 

 task of discussing the possible depths of these cracks. 



By the depth of the Gangetic rift I do not mean merely 

 the depth of the loose sediment : I do not mean the depth at 

 which solid rock is first met with. If a rift has extended to a 

 considerable depth, it may in its lower portion have become 

 filled by solid rock that has fallen in from the sides, or by vol- 

 canic eruptions. Even if the Ganges sediment continues down 

 to a depth of some miles, it may itself become consolidated by 

 pressure and heat. 



I define the depth of the rift as follows : it is that depth 

 at which the rocks under the Ganges plains are similar to 

 rocks at the same depth under the table-land There may be 

 a solid floor to the Gangetic trough at a depth of 6 miles 

 under Gorakhpur, but if the rocks which are deeper than 6 

 miles under Gorakhpur are different from and lighter than the 

 rocks of the same depth under the Vindhyan plateau, the solid 

 floor is not the bottom of the rift. When a crack occurs, 

 volcanic eruptions are to be expected, and although there are 

 no volcanic cones rising now from the trough of the Ganges, 

 there probably were at one time. Dr. Pilgrim has discovered 

 that there was great volcanic activity in the Persian Gulf at 

 one time and that the islands now existing in the Gulf are 

 isolated volcanic peaks. There exists also an old volcanic 

 region in the Syrian desert between Baghdad and Damascus. 



In considering the depth of the Gangetic rift we must 

 appeal firstly to geodesy, and then to seismology. Now 

 geodesy tells us that the compensation of the Himalaya {i.e., 

 the root of the Himalaya) extends downwards to a great 

 depth : Mr. Hayford estimates 75 miles. We do not contend, 

 and Mr. Hayford does not contend, that this value of depth 

 is proved. The depth may be 60 miles : it is I think of that 

 order. Geodesy says that the depth is great. I regard the 

 Gangetic Plains and the Himalayan range to be the two parts 

 of one whole ; I believe that they have originated together, and 

 if the depth of Himalayan compensation extends down to 60 

 miles, then I think that the Gangetic rift may extend down to 

 that depth also. 



Now let us turn to seismology : seismologists are able to 

 form rough estimates of the depths of earthquakes. The 

 earthquakes that visit Northern India seem generally to be 

 niost violent at places in the outer hills, such as Dharmsala, 

 Katmandu, Shillong. But the line of fracture that occurs in 

 the sub-crust at an earthquake may not be vertically under the 

 place which suffers most. If for example a fracture in the 

 sub-crust occurred at 60 miles depth under Gorakhpur. the 



