456 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 28, 



river, which, while sloping in a direction coinciding with the general 

 course of the stream, are as regards any line at right angles to that 

 direction, level ; that is, the surface of them is a plain inclined with 

 the valley of the river, and not appreciably curving over to the sides 

 of it ; the word, with this meaning, agrees with one of the ordinary 

 uses of it. And while thus keeping it distinct from the more ter- 

 restrial deposits on the one hand, I wish to separate it on the other 

 from the lacustrine beds which, as we shall see, occur in some of the 

 same valleys. 



Alluvium, in this slightly restricted sense, is found in large quan- 

 tities in the country of the Upper Indus ; and the mode of occurrence 

 of it denotes .important changes of condition, which we may endea- 

 vour to analyze. It occurs at all levels ; it may begin as high np 

 in the heart of the mountains as the ravines in their ramifications 

 reach ; it is steep in the higher parts, when composed of frag- 

 ments yet undisintegrated, and makes a more gentle slope lower 

 down, when due to streams of greater volume and when consisting 

 of smaller material. I shall mention instances of alluvium in various 

 localities, beginning with the higher tributaries of the Indus. 



Near the sources of the Zanskar river is a flat alluvial plain about 

 two miles wide between the mountains, part of the uplands of 

 Rupshu. This is sometimes described as a tableland, but is really an 

 elevated but level valley ; the flat of it is 15,000 feet above the sea ; 

 the mountains around it are between 19,000 and 20,000 feet. 

 At one side of this flat, one comes to a sudden drop, a ravine 500 

 feet deep and some hundred yards across, with a flat at the bottom 

 through which flows a stream in many channels. The sides of the 

 ravine are cliff's, in part worn into pinnacles, in part sheer, and in 

 part weathered into shingly slopes ; the composition is beds of 

 rounded pebbles, mostly of limestone, regularly stratified : the whole 

 thickness of this deposit is the 500 feet of the cliff; for at the foot of it 

 the rock has been reached. The whole spread of flat ground men- 

 tioned, which is on the right bank, must be composed for a great 

 depth, probably a depth equal to this, of the same alluvial deposit ; 

 on the left bank is a similar high cliff of the same pebble-beds, 

 rising to the same height at the summit, at which level there is 

 quite a narrow plateau ; and behind that rise the mountains. 



Such deposits as this Capt. Henry Strachey has described in his 

 paper on Western Tibet ; and he has spoken of some on a still larger 

 scale of thickness in the higher parts of the Sutlej basin. He refers 

 them to a marine origin, and, remarking on the existence of the 

 strata up to a height of 16,500 feet, supposes that they must have 

 been laid out under a general sea while the framework of the moun- 

 tain was in its present form, and afterwards upheaved by the 

 equable rising of a whole continent*. I think that there is no 

 necessity for the supposition, and even that there are positive 

 objections to it. In the first place, above this very flat surface at 

 this particular valley (which on the marine theory must have 

 been formed by a deposition of material in a narrow strait) there 

 * II. Strachey on the Physical Geography of Western Tibet, p. 20. 



