Geological Notes on the Sierra Nevada. — Turner. 301 
HoKN BL KNDE-PoK I'M Y KITE. 
Hornblende-porphyi'ite occurs abundantly in the southern 
part of Eldorado county, and to a limited extent in Amador 
county as fragmental areas and dikes in the western belt of 
the Calaveras formation (see Placerville atlas sheet). Big 
Sugar Loaf, in Eldorado county, is a massif of hornblende- 
porphyrite. As may be noted in the text of the Placerville 
atlas sheet, the hornblende-porphyrite in Eldorado county 
grades over into rocks of the diabase series. It is probable 
that these hornblende-porphyrites are of the age of the en- 
closing slates, that is. Carboniferous. Some of these porphy- 
rites were surface lavas, corresponding to the Tertiary horn- 
blende-andesites. In a thin section from the hornblende- 
porphyrite area one mile west of Logtown in Eldorado county 
the devitrified glass shows flow structure very plainly. 
In Plumas county on the ridge south of the middle fork of 
the Feather river dikes of hornblende-porphyrite cut the ser- 
pentine. A dike of the same rock cuts the diabase one mile 
northwest of Gold lake. 
It is noted under diorite that there are dikes of a rock al- 
lied to camptonite which cut the granite, the schists and the 
quartz-porphyrite, and in which there are developed poryhy- 
ritic hornblendes. These rocks are sometimes designated 
hornblende-porphyrite. but they may he observed to pass over 
into undoubted diorites of the camptonite series. 
Hornblende-mica-porphyrite. At Indian valley in Siena 
county is a small biotite-granite area, and in the surrounding 
mica and greenstone schists and clay slates, are very abundant 
dikes of hornblende-mica-porphyrite. At first glance these 
would seem to he offshoots of the granite mass, hut attheeasl 
end of the valley on the north side of the Yuba river are sev- 
eral of these dikes from three inches to five feet in width 
cutting the granite. While, therefore, the dikes may have 
originated from the same igneous reservoir as the granite, it 
would appear that they are later in age. 
On the south hank of the Yuba river west of Indian valley 
a dark fine-grained dike six Inches wide was noted cutting a 
dike of hornblende-mica-porphyrite. A microscopic exami- 
nation shows this fine-grained dike to be a diorite of the 
camptonite series. 
