H. D. Oldham — Assays in Theoretical Geology. 15 



Siwalik groups, it will be well to briefly examine the nature and 

 distribution of the deposits now being formed along the foot of the 

 Himalayas. Along the outer margin of the hills there is every- 

 where a zone of gravel and boulder deposits ; near the debouchure 

 of the great rivers this consists of well-rounded fragments of 

 crystalline rocks, quartzite, and some limestone ; where there are 

 no lai-ge rivers, the fragments are less rounded, and consist of the 

 rocks exposed within the drainage-area of the streams flowing down 

 from the hills. The similarity of these gravels to those brought 

 down at the present day, and the fact that their diversity is matched 

 by that of the gravels of the existing streams, leaves no room for 

 doubt that they deposited the gravels even in those districts where 

 they have since cut deep channels through their old deposits. 



Outside the zone of boulders and gravel comes a region of sands, 

 of various degrees of fineness, and outside this again the clays and 

 fine silts which form the bulk of the Gangetic alluvium. 



It must not be understood that this division into three zones is 

 absolute. Sand and clay are occasionally found close up to the foot 

 of the hills, even near the debouchures of the great rivers, but none 

 the less it truly represents the general disposition of the recent 

 deposits. The width of the zones is not constant, and the boulder 

 and gravel zone in particular expands opposite the debouchures of 

 the great rivers. With all these local variations the main fact 

 remains, that on any line drawn transverse to the boundary of hill 

 and plain, the deposits, omitting minor variations and local excep- 

 tions, decrease in coarseness as we pass away from the hills, that the 

 coarse deposits of gravel and boulders are confined to the immediate 

 neighbourhood of the boundary, and that it is only near the debou- 

 chure of the principal rivers that these are composed of large and 

 well-rounded fragments of hard crystalline and metamorphic rocks. ^ 



Eeturning now to the Siwalik deposits, we find that the same 

 relation between the composition of the gravels and the existing 

 drainage system which is exhibited by the recent deposits of the 

 northern margin of the plains, has also been noticed by every 

 observer in the conglomerates of Upper Siwalik age. 



In the neighbourhood of the Jumna and Ganges rivers we find 

 a great development of conglomerates, composed of well-rounded 

 boulders of crystalline and metamorphic rocks. East of the Ganges, 

 there comes a ti'act where the watershed is only a few miles removed 

 from the edge of the hills, and coincident with this there is a narrow- 

 ing of the Siwalik zone, and a cessation of the thick deposits of 

 sandstone and conglomerate, till the Eamganga and Kosi rivers 

 are reached. Here the Siwalik beds again attain a great width and 

 thickness, and there is a large development of conglomerates. - 



In the country between the Ganges and the Jumna the con- 

 glomerates are largely developed and extend for some little distance 



' Boulder gravels composed of well-rounded blocks of hard rock are also found 

 in the beds of streams draining from the Upper Siwalik conglomerates ; but such 

 exceptions are obvious. 



* Middlemiss, Mem. Geol. Surv. India, vol. xxiv. pt. 2, 



