24 M1DDLEMISS: PHYSICAL GEOLOGY OF SUB-HIMALAYA. 



on. The Siwalik conglomerate usually interbeds itself rapidly, but 

 still gradually, with the sand-rock below. 



The thickness of the Siwalik conglomerate is very variable, 

 according as the locality is near to or distant from a large river. A 

 calculation across the ridge south of the Pcitli dun, along the line of 

 the Gaujpcini Rau, gives a thickness of 2,970 feet ; or a little over half 

 a mile. Mr. Medlicott mentions 1 that in some localities, in the coun- 

 try further north-west, the conglomerates are at least 5,000 feet 

 thick. 



Although the previous formation passes down into the M. Siwalik, 



or sand-rock stage, the latter is sufficiently 

 Sand-rock stage. 



well-marked off from the former, through its 



main thickness, to warrant a separate description. Previous writers 

 on the Sub-Himalayan zone have been unanimous in naming its re- 

 presentative elsewhere a soft sandstone. That it passes down into 

 soft sandstone and shales, undistinguishable from the Nahans, I shall 

 show later on ; but, seeing that there is a large thickness having the 

 characteristics of a sand-rock, rather than a sandstone, besides pos- 

 sessing other peculiarities, I think it not unnecessary to give this 

 division of the Siwaliks the above name of the sand-rock stage. The 

 passage of it down into the Nahans was a moot question when Mr. 

 Medlicott wrote his book. It is entirely owing to the clearer nature 

 of the sections, which are easy to read in the districts covered by this 

 memoir, that I can make this statement so confidently and without 

 reserve. But, although the passage is undoubted, the gradations are 

 rapid ; so that there can only be a very small zone which is doubt- 

 fully the one or the other. Hence the mapping of the boundary, 

 though in a sense arbitrary, is not so to any great extent. A very little 

 way on each side of the boundary, the material is distinctly sand- 

 rock on one side, and sandstone on the other. A simple and prac- 

 tical distinction between the two is, that while the sand-rock crum- 

 bles under the hammer and refuses to make a coherent rock speci- 



1 Manual of the Geology of India, Pt. I, p. 536. 



( 82 ) 



