ACADEMY OF SCIENCES. 39 
which I called Lake Valley Glacier. Taking its rise in snow fountains 
amongst the high peaks in the neighborhood of Silver Mountain, this great 
glacier flowed northwards down Lake Valley, and gathering tributaries from 
the summit ridges on either side of the valley, but especially from the higher 
western summits, it filled the basin of Lake Tahoe, forming a great mer de 
glace, 50 miles long, 15 miles wide, and at least 2,000 feet deep, and finally 
escaped northeastward to the plains. The outlets of this great mer de glace 
are yet imperfectly known. A part of the ice certainly escaped by Truckee 
Caion, (the present outlet of the lake); a part probably went over the north- 
eastern margin of the basin. My studies during the summer were confined 
to some of the larger tributaries of this great glacier. 
Truckee Caron and Donner Lake Glaciers.—I have said that one of the out- 
lets of the great mer de glace was by the Truckee River Cafion. The stage 
road to Lake Tahoe runs in this canon for fifteen miles. In most parts of 
the canon the rocks are voleanic and crumbling, and therefore ill adapted to 
retain glacial marks; yet in some places where the rock is harder these marks 
are unmistakable. On my way to and from Lake Tahoe, I observed that the 
Truckee Caton glacier was joined at the town of Truckee by a short but 
powerful tributary, which, taking its rise in an immense rocky amphitheater 
surrounding the head of Donner Lake, flowed eastward. Donner Lake, 
which occupies the lower portion of this amphitheater, was evidently formed 
by the down-flowing of the ice from the steep slopes of the upper portion 
near the summit. The stage road from Truckee to the summit runs along the 
base of a moraine close by the margin of the lake on one side, while on the 
other side, along the apparently almost perpendicular rocky face of the am- 
phitheater, 1,000 feet above the surface of the lake, the Central Pacific Rail- 
road winds its fearful way to the same place. In the upper portion of this 
amphitheater large patches of snow still remain unmelted during the summer. 
My examination of these two glaciers, however, was very cursory. I hasten 
on, therefore, to others which I traced more carefully. 
As already stated in my former paper, Lake Tahoe lies countersunk on the 
very top of the Sierra. This great range is here divided into two summit 
ridges, between which lies a trough 50 miles long, 20 nriles wide, and 3,000- 
3,500 feet deep. This trough is Lake Valley. Its lower half is filled with the 
waters of Lake Tahoe. The area of this lake is about 250 square miles, its 
depth 1,640 feet, and its altitude 6,200 feet. It is certain that during the 
fullness of glacial times this trough was a great mer de glace, receiving trib- 
utaries from all directions except the north. But as the glacial epoch waned— 
as the great mer de glace dwindled and melted away, and the lake basin be- 
came occupied by water instead, the tributaries still remained as separate 
glaciers flowing into the lake. The tracks of these lingering smaller glaciers 
are far more easily traced, and their records far more easily read, than are 
those of the greater but more ancient glacier of which they were. but once 
the tributaries. 
Of the two summit ridges mentioned above, the western is the higher. It 
bears the most snow now, and in glacial times gave origin to the grandest 
