At the north of Lake Tahoe, "southwest of Reno, a large andesitic volcano poured forth lavas 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.