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.