122 Report of the Magnetic Survey of India. [No. 2. 



The deposits consist in part of gravel and sand, in part of very 

 finely grained clayey and calcareous strata, of a light yellowish 

 colour. They are interstratified with each other; the shells are 

 chiefly found in the marls and clays, which are specially predo- 

 minant in the central parts of the basin ; and at Mangnang, Tosing* 

 &c. the fossil bones are found both in the marls and in some fine 

 grained sands which accompany them. 



The strata lay everywhere quite horizontal. The thickness of 

 these deposits is very variable, since the original rocky surface of 

 the valley is very undulating, as is shown in the deep valleys of 

 denudation along all the rivers. 



The average thickness may be assumed to be from 1,000 to 1,500 

 E. E. but the maximum exceeds 3,000 E. E. 



19th. The Sutlej and its numerous tributaries in Thibet, form 

 one of the finest examples of the mode in which the erosive power 

 of water acts upon loose deposits and upon solid rocks, under various 

 circumstances. The rivers have excavated valleys of denudation 

 2,000 and sometimes even 3,000 English feet in depth. These 

 valleys are not excavated in the lacustrine tertiary deposits only, 

 but very generally along the Sutlej, solid rocks are cut through to 

 p a depth of 1,000 and 1,500 E. E. This great depth of the valleys 

 of denudation is evidently due to the fact, that the Sutlej after- 

 wards enters the Himalayas where the fall of the river per mile is 

 enormous. The great acceleration experienced there, has been gra- 

 dually reaching upwards, and has affected the whole river system 

 of the Sutlej basin. 



When re-entering the Himalayas, after having examined these great 

 denudation valleys, we proposed to ourselves to investigate the effect 

 produced by the Himalayan rivers, which have such an enormous fall 

 per mile, upon the excavation of their valleys. We soon convinced 

 ourselves that, though the general direction of these valleys was 

 without doubt originally caused by faults, and by the general 

 arrangement of the mountain chains, their forms had afterwards 

 been altered to an immense extent by the action of the rivers, and 

 by the rain falling in enormous quantities during the rainy season. 

 "We have ample proofs by the existence of ancient river deposits 

 and water marks, at great heights above the present rivers, and from 



