Geological Notes on the Sierra Nevada. — Turner. 311 
as mud Hows finds confirmation in the canyon of Canyon creek al Poker 
Plat, Sierra county. Here the canyon has cui into the old peneplain to 
the depth of fifteen hundred feet, righl across one of these old fissures, 
exposing the dike-like mass of fragmental andesite mixed up with frag- 
ments of the pre-Cretaceous wall rock, which is largely serpentine. 
'This fragmental material forms also the bed of the creek, where the 
stream crosses it. Some years ago a shaft was sunk near the creek in 
this breccia to the depth of about one hundred feet without a change of 
rock, according to a. miner who lived thereat the time. Portions of 
this dike-like mass are stratified, the layers dipping al an angle of from 
10° to 15°. 
The composition of the andesite forming the breccias of the Sierra 
probably varies more or less. The partial analysis given (No. 69, 
Plumas county) is from a boulder in the breccia, two miles northeast 
of Laporte. 
4. Pyroxene-andesite. This rock is represented on the Dow- 
nieville sheet by two areas on the ridge sooth of Grizzly peak. 
It is microscopically a light gray, medium grained rock (No. 
308, Plumas county). Under the microscope it is seen to be 
made of plagioclase, augite, and hypersthene phenocrysts in an 
almost crypto-erystalline groundmass of feldspar and magne- 
tite. It thus differs very little in mineralogical composition 
from such of the dolerite as contains no olivine, but differs in 
structure in that it has a definite groundmass. It should con- 
tain more silica than the dolerite, but no analysis has been 
made of this rock. It greatly resembles the hornblende- 
pyroxene-andesite, andmay be considered merely n variety of 
that type. 
There is also a peculiar eruptive that appears to be later than the 
other andesites. It is macroscopicallj a light gray, fine grained rock, 
with frequently a slaty fracture. Microscopically ii is nearly or quite 
holocrystalline and composed of slender laths of plagioclase, of augite, 
and of a slightly pleochroic rhombic pyroxene with magnetite grains. 
There is a dike-like occurrence of this rock (No. 332 Plumas county) 
on the east side of Mohawk valley and a little east of the bridge over 
the middle fork of the Feather river at Denten's. In contact with this 
dike the Pleistocene lake beds of Mohawk \alle\ dipal an angle of 25 
from horizontality. This lava occurs al various points in Plumas and 
Sierra counties, bul has not I u noted bj the writer elsewhere in the 
Sierra Nevada. It forms the high poinl one and a halt miles northeast 
of Goodyear's bar, the ragged cresl one mile northeast of St. Charles 
ranch, and occurs on I he soul h wesl slope of Saddle Pack, and also one 
mile south of Dead wood; all of these points being on the Downieville 
sheet. 
A. specimen (No. 38*3 Plumas county) of this late pyroxene-andesite 
