IGNEOUS ROC, OF DHE SHB RRA WNPAVAI A), 399 
fibrous hornblende. On the southwest slope of Mt. Elwell, the 
augitic tuffs that overlie the quartz-porphyry series are cut by a 
dike of a white rock (No. 108 Plumas) of a fine-grained granular 
structure, and composed of quartz and feldspar with smaller 
amounts of secondary epidote and fibrous hornblende. The 
rock is thought to be an aplite. The same augitic tuff series at 
a point about one and three-fourth miles southeast of Mt. Elwell 
is cut by a dike of hornblende-porphyrite (No. 206 Sierra 
County) Lommine  themeasiy base lol iuneka: Peale 1isiaudar, 
coarse, granular rock which is seen under the microscope to be 
a gabbro. The contact of this rock with the quartz-porphyry is 
sharp, and as there are angular fragments of the gabbro enclosed 
in the porphyry, it appears that the gabbro is the older rock. 
Ata point eight-tenths of a mile (No. 133 Plumas), and at 
another point one and three-fourths miles southeast of Eureka 
Peak, there are in this gabbro white dikes that microscopically 
appear to be aplites. Cutting No. 135 is a darker colored dike 
(No. 136) which contains outlines of squarish crystals, one 
showing truncated corners, presumably originally augite (but now 
replaced by calcite, epidote, quartz and chlorite), in a microlitic 
feldspar groundmass. In a ravine on the northeast slope of 
Eureka Peak enclosed in the quartz-porphyry there are frag- 
ments of a rock weathering reddish (No. 383 Plumas). On 
microscopic examination these proved to be apparently quartz- 
diabase, identical in structure with the rock (No. 550 Calaveras) 
that occurs as a dike in the granitoid quartz-porphyry southeast 
of Milton near Rock Creek (Jackson atlas sheet). The rock is 
ophitic in structure, the divergent plagioclases penetrating the 
quartz grains as well as the chlorite and epidote which seem to 
represent original augite. In No. 550, the metasilicate is chiefly 
hornblende, which is presumed also to have been augite. In 
addition there are numerous grains of iron ore. This peculiar 
ophitic rock has thus far been noted by the writer only at the two 
points above mentioned. As the rock needs more investigation, 
it will not be further considered here. 
Six and seven-tenths miles southeast of the Sierra Buttes in 
