94 W. T. Blanford — On the Physical Geography [No. 2, 



or bamboo, the only instrument likely to have been used in general for 

 sounding. Sir Bartle Frere says that he has been assured that the depth of 

 one has been measured and found to be 70 feet.* This shows of course 

 considerable depression below the level of the Indus alluvial plain, for the 

 Narra, which must of course be a little below the average level of the plain, 

 supplies, or used to supply, the water for the ' dhandhs' in its immediate 

 neighbourhood. 



There are, however, a large number of small lakes isolated amongst the 

 sand-hills and not in communication with the ' dhandhs' fed by the Narra, 

 and these isolated lakes are all salt ; those farthest from the Narra being 

 apparently the most saline, and some being so concentrated that salt 

 crystallizes on their margins. All these salt ' dhandhs' appeared to me to be 

 at a lower level than the freshwater lakes, and this view was confirmed 

 by my finding that small streams fed by springs amongst the sand-hills enter 

 in many cases at the western edge of the salt ' dhandhs', and that where, as 

 not unfrequently happens, there are more than one ' dhandh' in the same 

 hollow, a stream often flows from the western pool to that lying more to the 

 eastward. Now the water can only be derived by percolation through 

 the sand from the freshwater ' dhands' to the westward : it is true that 

 springs often rise from the margin of the latter, and that these springs are 

 sometimes above the surface of the lakes, but they are usually below, and if 

 higher they are not so far above as in the case of the salt lakes to the 

 eastward, on the edge of which I found springs issuing from the ground 

 15 or 20 feet above the water, t It is a natural conclusion that the origi- 

 nal surface of the ground at this spot was not higher than the bottom of 

 the ' dhandhs' are now, that it was much lower than the present alluvium of 

 the Indus, and that the Indus plain has been raised to its present height by 

 the accumulated silt deposited from the river since the ' dhandhs' have been 

 cut off and isolated by the sand-hills. 



§ 7. Marine MollusJc living in salt lakes. — One more observation gave 

 the clue to the original conditions of the ground. I found in some of the 

 salt lakes in which the water, although very salt, was rather less so than 

 that of the ocean, a living mollusk which has been identified by Mr. Nevill 

 with JPotamides (Pirenelhi) Layardi, H. Ad. This species inhabits the salt 

 water of back-waters or lagoons and harbours : it is not found to the best of 

 my belief on open coasts, nor yet in the brackish water inside the mouths 

 of rivers, and although, like most other forms of JPotamides, it is rather 



* J. R. GL S. XL, p. 189. 



f The springs on the edges of the freshwater dhandhs are doubtless due to the 

 water which percolates into the sand when the dhandhs are at their highest level from 

 floods brought down by the Narra. 



