20i Records of lie Geological Saroeij of India. [vol. xiii. 
east, drain a considerable portion of the range, and the waters remain sweet 
throughout. 
The wells sunk in the Vindhyan rooks, on the western side of the range, 
contain, without exception, sweet water, while those sunk in the alluTium a 
short distance off contain brackish water. 
As it seems improbable that the salt is derived from either the Arvali or the 
Vindhyan series, the only visible source remaining is the alluvium. It has been 
suggested that the alluvium is, in part at least, of marine origin, and that the 
salt lakes are parts of the old sea bottom in which the salt has accumulated. As 
no good sections of the alluvium west of the Arvali range are exposed, the beds 
of all rivers being shallow, seldom more than 20 feet below the level of the plain, 
and as large areas, more especially in the northern portion, are covered by blown 
sand, it is difficult to produce any dii’ect evidence on this point. 
Mr. Blanford found a mollusk (Potamkles laganli), an inhabitant of salt 
lagoons on the coast, in one of the salt pools near Umerkot, from which ho in¬ 
ferred that an arm of the sea extended as far as this in recent times. But 
Umerkot is more than 300 miles nearer the sea and several hundred feet lower than 
the level of the plain near the Sambhar lake. In the portion of the alluvium 
that I have examined, extending south to 25° north latitude, I have not seen a 
marine shell; but in several places, in the old banks of the rivers, 1 have found 
fresh-water shells of existing species. 
In my examination of the alluvium west of the Arvali range, I mot with 
several ridges, many miles in length, of water-worn boulders, often as much 
as 1 foot in diameter. Nearly all of them were formed of the hard quartzite 
of the Arvali series, and must have undergone much rolling to reduce thorn 
to their present shape. A short distance west of Khatu, and about 35 miles 
west of the Arvali range, there are two parallel iddgcs formed of these boulders, 
about a mile apart at their nearest points and about 6 miles in length, running 
nearly north and south. How far north they may extend I am iunable to say; 
but to the south they reach in a broken line to the Luni river, a distance 
of upwards of 70 miles. The boulders sometimes rest upon the Vindhyans, and 
are frequently isolated in the alluvium. The base of the ridges is never above 
the level of the plain. The boulders are clearly superficial to the Vindhyans and 
of comparatively recent origin; but whether they mark the course of a large 
river or of an ancient coast lino, it would be rash at present to decide. 
General condusiom .—If the question of origin from rocks underlying the allu¬ 
vium could be decided from all that is visible of these rocks above the alluvium, 
and if the whole nature of these covering deposits could be told from what we can 
see of them in shallow sections at the surface, the answer in both cases would bo 
in the nogativo, and the only source remaining for the salt would be its local 
production from natural causes still in operation. Theoretically the case i.s 
possible, as any modern schoolboy knows that lakes without an outflow, and 
to a great extent the ocean itself, become saline from the continual concentration 
by evaporation of what wo call sweet water. In the present case, however, the 
process with known data seems inadequate to the result. For a long time the 
