102 SCIENTIFIC MISSION TO THIBET. 
began falling in the evening, and by next morning the ground was 
covered to the depth of three mches. Beyond the cold we ex- 
perienced no difficulty in crossing the Pass, but of course 
botanizing was out of the question. The descent was rapid, and 
we quickly left the snow behind us, and are now at an elevation 
of 13,500 feet; and we expect to-morrow’s march will bring us 
down to 11,500, so that I trust soon to enter a region where 
vegetation is not thus injured. So long as the species are recog- 
nizable, I consider one great object to be gained. For a month 
the plants have been in a bad state, too far advanced to make 
good specimens, indeed, mostly in fruit; but only within the last 
few days have they been so much injured as not to be worth col- 
lecting. There is the less reason to regret the lateness of the 
season, because there are few indigenous plants, comparatively 
speaking, in the elevated regions we have been lately traversing ; 
and I quite believe hardly any have escaped me, unless it be a 
very few early spring species. And, where spring begins in June, 
the number of plants peculiar to that season, cannot be great. 
The most interesting object that I have seen during the last 
few days is a Salt Lake, at the elevation of 15,000 feet above the 
sea. It has no outlet, (and this, I believe, to be characteristic of 
all salt lakes,) and occupies the centre of a plain, bounded on every 
side by hills, which are marked, 200 feet above the present surface 
of the lake, with a most remarkably distinct ancient water-mark, 
traceable all round the lake, and which is seen at one point, towards 
the south, to be connected with a valley, running in that direc- 
tion, and which must have been the former outlet of the lake. - 
All round the lake, and in some places up to within a few feet of 
this water-mark, there is an alluvial deposit of fine clay, containing 
in many parts, an immense quantity of fossil shells, all of which, 
except a very few specimens of a minute bivalve, belong to one, 
or possibly two species (for they vary considerably) of Zymnaa, a 
fresh-water shell, clearly proving that the lake was originally 
fresh, and that its present saline state is due to the shutting up — — 
. of its outlet. No other shells occur, at present, so far as Í was — — 
able to detect, at this height. I infer, therefore, that the whole : 
