J. LeConte—Ancient Glaciers of the Sierra Nevada. 127 
tributary, which, taking its rise in an immense rocky amphi- 
theatre surrounding the head of Donner Lake, flowed east- 
ward. Donner Lake, which occupies the lower portion of this 
amphitheatre, 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 moruine close by the margin of the lake on 
one side, while on the other side, along the apparently almost 
perpendicular rocky face of the amphitheatre, 1,000 feet above 
the surface of the lake, the Central Pacific Railroad winds its 
fearful way to the same place. In the upper portion of this 
amphitheatre large patches of snow still remain unmelte 
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 miles 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 2 
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 tributaries from all direc- 
e great “mer de glace” dwindled and melted away, and the 
luke basin became occupied by water instead, the tributaries 
traced and their records more easily read, than are those of 
the greater but more ancient ‘glacier of which they were once 
ut the tributaries, 
f _the two summit ridges mentioned above the western is 
the higher, It bears the most snow now, and in glacial times 
a 
