which
extend between the Truckee River Canyon and the Washoe Valley. In the
region extending northward from Lake Tahoe to Sierra Valley enormous
andesitic eruptions took place, and the products of these volcanoes
are now piled up as high mountains, among which Mount Pluto nearly
attains 9000 feet."

These are the volcanic lavas which united the two crests forming the
eastern and western borders of the Tahoe basin or depression, and
through which the Truckee River had in some way to find passage ere it
could discharge its waters into Pyramid Lake, resting in the bosom of
the Great Basin.

Here, then, we have the crude Tahoe basin ready for the reception
of water. This came from the snow and rainfall on its large and
mountainous drainage area, a hundred greater and lesser streams
directly and indirectly discharging their flow into its tremendous
gulf.

Its later topography has been materially modified by glacial action,
and this is fully discussed by Professor Joseph Le Conte in the
following chapter.

It should not be forgotten, however, that while Mt. Pluto was being
formed, other vast volcanic outpourings were taking place. Well back
to the west of the Tahoe region great volcanoes poured out rhyolite,
a massive rock of light gray to pink color and of fine grain, which
shows small crystals of quartz and sanidine in a streaky and glossy
ground mass. On the summits nearer to Tahoe the volcanic outflows were
of andesite, a rough and porous rock of dark gray to dark brown color.
Lindgren says: "By far the greater part of the andesite occurs in the
form of a tuffaceous breccia in numerous superimposed flows. These
breccias must have issued from fissures near the summit of the range
and were, either before their eruption or at the time of issue, mixed
with enormous quantities of water, forming mud flows sufficiently
fluid to spread down the slope for distances of fifty or sixty
miles. The derivation of the water and the exact mode of eruption are
difficult to determine.... Towards the summits the breccias gradually
lose their stratified character and become more firmly cemented. Over
large areas in the Truckee quadrangle the andesite masses consist of
breccias containing numerous dykes and necks of massive andesite....

"The andesite volcanoes were mainly located along the crest of the
Sierra, in fact, almost continuously from Thompson Peak, west of Honey
Lake, down to latitude 38 deg. degrees 10'. Farther south the e