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 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.
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