400 LAL LLE J OOKINATE OLN GHA OE OG 
he granite (granodiorite) is a dike of augite-porphyrite (No. 
226 S. N.) in which the augite is largely altered to fibrous 
green hornblende which is grouped in radiating brushes. Some 
of the augites are unaltered at the center with a rim of the 
coarsely fibrous hornblende. If there was olivine in the rock, 
itis now completely gone. The rock is dark green in color, 
strongly resembling the rock that forms a small area on the 
summit of the highest of the Sierra Buttes, and this latter rock 
appears to have contained olivine and would then be a melaphyre. 
Along the augite-porphyrite dike (No. 226 S. N.) is a white 
dike (No. 227 S. N.) about two feet wide which thus occurs 
between the augite-porphyrite and the granite. This rock 
appears to be a micropegmatitic form of aplite. The time rela- 
tion of the augite-porphyrite and the aplite was not ascertained. 
Chemical analyses of the dikes have not been made, but if 
the writer is correct in his determination of these altered rocks, 
we have the following succession in the Eureka Peak—Sierra 
Buttes region. 
MEDIUM BASIC - Gabbro— Boss. 
AcID - - - - Quartz-porphyry — effusive, large sheet. 
MEDIUM BASIC - Augite porphyrite — effusive, mostly tuff, large sheet. 
AcIbD - - - - Granite (granodiorite). ) 
ACID =e ev Atplite: | 
Basic - - - - Augite-porphyrite (?) or melaphyre (?) ( belay: 
MEDIUM BASIC(?) Hornblende-porphyrite. | 
The relative age of the aplite, late augite-porphyrite, and 
hornblende-porphyrite yet remains in doubt, the only evidence 
being that the greenish dike (No. 136), apparently an augite- 
porphyrite, cuts the aplitic dike (No. 133). 
In the area of the Jackson sheet are a number of bodies of 
rock that are designated on the geologic map of that area as 
quartz-porphyrite. Analyses* of these rocks, however, show 
them to have a composition very similar to the quartz-porphyry 
of the Sierra Buttes. The quartz-porphyrites should perhaps 
be restricted to altered andesitic rocks with free quartz, and with 
less than 70 per cent. of silica, corresponding to modern dacites. 
*Fourteenth An. Rep. U.S. Geol. Survey, p. 484. 
