404 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuUC 5, 



' High Tartary, Yarkand, and Kashgar,' p. 97, thus alludes to some 

 he noticed in the Karakash valley: — 



" Nov. 2 [1868] : Marched down the Karakash stream, which now 

 flows freely between ice-borders. It is fed by numerous warm 

 springs ; hence its freedom from ice. But these springs give the 

 whole water a brackish taste. 



" A couple of miles from last night's camp we crossed a little 

 plain dotted over with small craters, four or five yards across and 

 two or three feet deep in the middle. The bottom of these craters 

 is occupied by a deposit of common salt or saltpetre. The servants 

 took a supply for common use." 



Again, at p. 98 : — 



" Nov. 5 : A succession of fine meadow -plains full of salt-craters, 

 larger than the former ones (some six or seven yards across). Some 

 were fuU of concentrated brine (unfrozen in most), which on evapo- 

 rating will give the usual salt deposit, I suppose. In this valley, 

 wherever there is grass there is also saline efflorescence in the soil. 

 I fancy both depend on the presence of moisture, and hence occur 

 together." 



Mr. Shaw makes no mention of the pits near Tarl Dat. 



My theory of the formation of these pits is as follows. I suppose 

 that under the surface there is a layer of sand, and under the sand 

 a stratum of clay ; and that the water which sinks into the ground 

 at the head of the valley flows in sand or gravel under the latter. In 

 the Karakash valley, quicksands and quagmires are very common ; 

 and whilst exploring the pits there, my horse repeatedly sank up to 

 the girths of the saddle. In the upper part of its course the Kara- 

 kash river sometimes disappears for miles and flows under the sur- 

 face of the ground. Where the pits are formed I assume the exis- 

 tence of a layer of clay which keeps the water down until it issues 

 in a series of springs at Tarl Dat, where the ground slopes more 

 rapidly. I suppose that the water, flowing in very varying quantity 

 at different times, gradually eats away the clay in certain places, and 

 allows the sand to escape, and circular patches of the surface subside 

 and form the pits. Depressions formed in this way are very common 

 in the Punjab, particularly about Umballa and between Jhelum and 

 Rawil Pindi. Sometimes at Umballa many square yards of ground 

 thus subside and leave an enormous hole twenty or more feet in 

 depth, with vertical sides. This process has gone on to such 

 an extent in some localities between Jhelum and Pindi that more 

 than half the surface-area has been lowered thirty or forty feet, and 

 the whole country has been cut up into ravines with nearly perpen- 

 dicular sides. I account for the mud-discharges at Tarl Dat by sup- 

 posing that after a fall of rain or snow the air contained in the 

 water-bearing stratum would get churned up with the water and 

 mud, and be ejected as frothy mud at Tarl Dat. I believe the brine- 

 and salt-pits in the Karakash valley are formed in the same way. 

 This river rises and falls several feet every day ; at some seasons it 

 is almost dry, at others it overflows its banks. It is fed entirely by 



