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° degrees 10'. Farther south the eruptions diminished greatly in intensity.... Along the first summit of the range west of Tahoe the greatest number of vents are found. Beginning at Webber Lake on the north, they include Mount Lola, Castle Peak, Mount Lincoln, Tinker Knob, Mount Mildred and Twin Peak. The andesite masses here in places attain a thickness of 2000 feet. An interval followed in the northern part of the Pyramid Peak quadrangle where no important volcanoes were located, but they appear again in full force in Alpine County. Round Top, attaining an elevation of 10,430 feet, and the adjacent peaks, were the sources of the enormous flows which covered a large part of Eldorado County. Still another volcanic complex with many eruptive vents is that situated in the western part of Alpine County, near Markleeville, which culminates in Highland Peak and Raymond Peak, the former almost reaching 11,000 feet. The total thickness of the volcanic flows in this locality is as much as 4000 feet."
It is to these breccias we owe the volcanic appearances in the Truckee River Canyon, a few miles before reaching the Lake. There are several layers of the andesites breccias at the head of Bear Creek Canyon, above Deer Park Springs.
"None of the craters," says Lindgren, "of these volcanoes are preserved, and at the time of their greatest activity they may have reached a height of several thousand feet above the present summits."