the rocky masses over which they move. Hence it will
be seen that the grooving and marking, the fluting and beveling, the
planing and smoothing processes of the ice are materially aided and
abetted by the very hardness and weight of the granite and other rocks
it carries with it.

Now let Joseph LeConte take up the theme and give us of the rich
treasure-store of his knowledge and observation. In the _American
Journal of Science and Arts_, Third Series, for 1875, he discussed
the very field we are now interested in, and his fascinating and
illuminating explanations render the subject perfectly clear. Said he:

Last summer I had again an opportunity of examining the
pathways of some of the ancient glaciers of the Sierra. One
of the grandest of these is what I call the _Lake Valley
Glacier_.[1] Taking its rise in snow fountains among the
high peaks in the neighborhood of Silver Mountain, this great
glacier flowed northward 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 2000 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 Canyon (the present outlet of the
Lake); a part probably went over the northeastern margin of
the basin. My studies during the summer were confined to some
of the larger tributaries of this great glacier.

[Footnote 1: This is the name given by Dr. LeConte to the Basin in
which Lake Tahoe rests and including the meadow lands above Tallac.]

[Illustration: Pyramid Peak and Lake of the Woods, near Lake Tahoe,
Calif.]

[Illustration: Snow Bank, Desolation Valley, near Lake Tahoe]

[Illustration: Grass Lake, near Glen Alpine Springs]

_Truckee Canyon and Donner Lake Glaciers_. I have said
that one of the outlets of the great "mer de glace" was by
the Truckee River Canyon. The stage road to Lake Tahoe runs in
this canyon for fifteen miles. In most parts of the canyon the
rocks are volcanic 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 th