THE IMAGINARY RANGE AND TROUGH. 53 



of the Gangetic plain is about 18, but the deeper layers have cer- 

 tainly a greater density than this ; at the same time they can hardly 

 attain a greater density than that of the Siwaliks, which are com- 

 posed of the same materials and have been subjected to the pressure 

 of superincumbent deposits, as well as to the induration due to age 

 and the compression to which they have been subjected in the 

 course of the upheaval of the Sub-Himalayas. This fixes the upper 

 limit of density at 2'2 and the probable mean density must lie 

 somewhere between the two, though nearer the higher than the 

 lower limit. In my earlier investigations a density of 2'1 was 

 accepted, or a deficiency of two- ninths of the mean density of the 

 rock forming the floor and sides of the trough ; later a slightly higher 

 density was adopted, for convenience of calculation, and the defi- 

 ciency put at two-tenths of the mean density of the rocky floor of 

 the trough, representing a density of 2*16. 



Doubts have been expressed 1 as to the reality of so great a 

 difference in density between the material forming the Himalayas 

 and that which fills the Gangetic trough, and especially it has been 

 urged that the material in the lower layers of the trough would be 

 compacted, by the pressure of the superincumbent material and the 

 percolation of water holding carbonate of lime in solution, till the 

 difference in density between it and ordinary rock would be neg- 

 ligible. These objections might be valid where depths of many miles 

 are postulated, but, as will be seen further on, there is no need to 

 suppose that the Gangetic trough is anywhere more than 20,000 ft. 

 in depth, and as the Siwalik rocks, which have been subjected to 

 the pressure of superincumbent deposits of about the same thick- 

 ness, have an average density of only 2*2 or not much greater than 

 the mean density assumed for the whole of the deposits in the Gan- 

 getic trough, of which the Siwalik rocks are the most dense, it is 

 evident that the deficiency of two-tenths, corresponding to a mean 

 density of 2*16, does not err on the side of being too high. 



At first sight it might seem strange that there should be so great 

 a difference between the density of the rocks forming the Himalayas 

 and the material filling the Gangetic trough, seeing that the latter 

 is the debris of the former, but all the denser minerals of the former 

 have been decomposed, oxydised and hydrated, and the hard quartz- 

 ites of the Himalayas broken up, to form the soft sandstones 



1 S. G. Burrard, Prof. Paper, Sunt. Ind., No. 12, p. 4 and T. H. Holland. B.A., Report 

 1914, p. 355. 



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