On the morning of the 25th of September, our day's journey commenced by rounding the north end of the lake, keeping at some distance from its margin to avoid swamp. For about two miles from the northern end, the ground continued almost level, and contained great masses of the lacustrine clay quite horizontally stratified, and very little higher than the surface of the water, but here quite without shells. A wide valley, rising gently towards the north, lay beyond this level plain; but our road, passing across the end of the lake, ascended another valley, which ran in a north-west direction from its north-west corner. The slope of this valley was very gentle. It was bounded by low undulating or rocky hills, on which, where the surface was suitable, the same remarkable water-mark could be traced continuously, and still, to all appearance, quite horizontal. The centre of the valley was occupied by clay, at first non-fossiliferous, but a little further on containing a great abundance of shells, the same as in the bed seen the day before. A few specimens of a very small bivalve, seemingly a species of _Cyclas_, were also met with; but they were so very rare, that they bore an infinitesimally small proportion to the _Lymnæa_.
[Sidenote: ANCIENT WATER-MARK. _September, 1847._]
[Sidenote: FORMER OUTLET OF LAKE. _September, 1847._]
For several miles the ancient water-mark could be traced along the sides of the hills, appearing to descend gradually, as the valley slightly rose in elevation. Beds of clay continued to occupy the middle of the valley nearly as long as the water-mark remained visible. At last it disappeared where a depression on the left, leading to the valley of Rukchin, seemed to indicate the former drainage of the lake, at a time when its waters occupied a much higher level, and contained in a living state the large mollusca of which the shelly coverings still remain in such vast abundance in the clay. As it was at the very edge of the lacustrine clay formation that the shells were so abundant, while the masses of clay in the vicinity of our encampment of the 25th, at the north-east extremity of the lake, were without any, it would appear that the species was quite littoral, while in the more central parts fine mud was deposited, without shells. The outlet was indicated to me by Major Cunningham, who in a previous journey had travelled along a part of the Rukchin valley in descending from the Lachalang pass towards the salt lake. As it may fairly be inferred that the lake was quite fresh at the time when it was inhabited by _Lymnææ_ and _Cyclades_, it is satisfactory to know that so very small an increase of the height of the surface of the water, as about 150 feet, would be sufficient to admit of its discharging its waters along the course of an open valley into one of the tributaries of the Zanskar river.