312 The American Geologist. May.iv.it 
from the area about two miles west of Eureka peak contains 61.47J1 ol 
silica as determined by George Steiger. 
Anai,vm> of Andesites. 
No. Hi'. No. 198, No. 12, 
Plumas county. Plumas county. Plumas county. 
Silica 57.32 59.9*3 60. 
Lime 7.VS 6.09 6.04 
Potassa 1.39 1.35 1.44 
Soda 4.14 3.98 3.35 
Older Basalt. This older basalt is a dense black rock showing 
macroscopically no porphyritic constituents except occasional 
olivines. It is frequently roughly columnar, the prisms being 
small, seldom more than three or four inches in diameter. 
This rock lias nowhere been seen by the writer except in the 
massive form. 
Microscopically the rock ib composed of lath-like plagio- 
clase, with more or less olivine and sonic augite in a ground 
mass, which is rendered dark by the abundant magnetite. The 
groundmass sometimes contains considerable glass and fre- 
quently grains and prisms of augite. Much of the olivine is 
altered to serpentine. The plagioclases are usually small and 
of nearly even size. One slide from the basalt of the Moore- 
ville ridge at the Hudson hydraulic mine shows comparatively 
little magnetite, and is nearly holocrystalline, with a few por- 
phyritic plagioclases and olivines. The rock at this locality 
is interesting, from containing abundant crystals of chabazite 
in cavities. Although differing in microscopic character at 
different points, the rock i- never coarse grained and always 
dark in color. 
So far as known to the writer this older basalt only occurs in Plumas 
and Butte counties, li is the lava of the Walker plains in Plumas 
county and also occurs to the north and south of Onion valley creek, 
and in small areas at numerous points. In Butte county it caps the 
Mooreville ridge and several ridges to the north of thai ridge, also Oro- 
villc tabic mountain and Kanaka peak: and at all of these places it rests 
upon Tertiary deposits, chiefly auriferous river gravels, or upon the pre- 
Tertiary bed rock. It forms the lava of Iron canyon of Chico creek, ap- 
parently rest i in: there directly on Cretaceous sandstone of the < Jhico for- 
mation. Tlie basalt above described is older than the hornblende-pyrox- 
ene-andesite-breccia. 
The black lava capping the Millerton table mountain in Fresno 
county resembles macroscopically the older hasalis of Butte and Plumas 
counties. 
The basalt of the Tuolumne table mountain is also a dark, heavy rock. 
