470 PKOCEEDESrGS 01? THE GEOLOGICAL SOCIETY. [May 28, 



moved through b ; therefore the material moved through a is greater 

 than that moved through b, aud that moved through b is greater 

 than what is moved through c, and so on ; and, lastly, the material 

 moved through / is greater than that supplied from m. So that 

 here, as one goes up stream, the matter moved diminishes in quan- 

 tity, and there is less supplied from the rocks than what is carried 

 on in the alluvial district. 



Thus it appears that the change from accumulation to denuda- 

 tion or from denudation to accumulation in a river-bed corresponds 

 to a change in the relations between the carrying-power of the water, 

 and the disintegrating-power of the elements. That is to say : — 



When an accumulator becomes a denuder, the carrying-power of 

 the water has increased, or the disintegrating action has decreased, 

 or both ; and 



When a denuder becomes an accumulator, the carrying-power of 

 the water has decreased, or the disintegrating action has increased, 

 or both. 



Now the great agent for disjointing and disintegrating rocks, 

 more especially bard rocks, such as occupy by far the greater part 

 of the Indus valley, is well-known to be frost ; and as we know 

 of a period of more severe frost (the glacial period, evidenced in 

 this country too by marks of extension of the glaciers), it is fair for 

 us to connect such changes as the above with the increase and de- 

 crease of cold. 



Whether in that period there was any variation in the transporting- 

 power of the streams, I do not at present see a way to determine ; the 

 material is, as a whole, of the size that is even now brought down by 

 the streams, taking the spring and the occasional floods into 

 account ; nor is it clear to me whether, the amount of precipitation 

 remaining the same, snow and ice periodically melting, or rain 

 would have the greater transporting power. 



Leaving then out of our consideration, as neutral, the element of 

 transporting-power, we see at one time an increase of cold, at 

 another a diminishing cold to account for the changing ratio 

 between the amount of material supplied from the rocks, and that 

 carried down by the streams. 



The intense cold of the glacial period brought about a greater 

 disintegration of the rocks ; this caused the streams to be accumu- 

 lators. Prom the lessening of the frost-power at the close of that 

 period, the supply from above of disintegrated material so far 

 diminished as to allow of the streams both carrying down what 

 was then being disintegrated and eating into the alluvium that had 

 before been formed. 



The conclusion then is that the greater deposits of alluvium were 

 made at some part of the glacial period, and that the denudation of 

 them occurred, or began, at the close of that period, when the 

 lessening cold diminished the rate of waste of the rocks. 



To corroborate this I may recall how many instances occurred in 

 the description of the old alluvium where there were special pheno- 

 mena (as the waved strata, the lapping round moraines, and others) 



