Here, then, were two crest lines—the old Cretaceous line of which the Crystal Range immediately overlooking Desolation Valley on the west, with Pyramid and Agassiz Peaks as its salient points,—and the new Tertiary crest line, reaching somewhat irregularly from Honey Lake in the north to Mono Lake in the south. 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. 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....