304 The American Geologist, May, 189* 
On the Jackson atlas sheet pyroxenite was found altering into mas- 
sive amphibolite. Some of the hornblende grains contain cores of py- 
roxene undergoing alteration to hornblende. 
Diorite. 
At numerous points in the range are dikes and small areas 
of fine to medium grained rocks, usually showing to the naked 
eye abundant hornblende needles. Under the microscope these 
rocks are seen to be composed of a fine grained groundmass 
of feldspar and minute brown hornblende needles, through 
which are scattered larger brown hornblende needles. Occa- 
sionally the rock is more nearly granular, the hornblende 
being in short prisms. In two instances noted the rock con- 
tains augite phenocrysts in addition to the hornblende. One 
of these (No. 155 Amador county) is from a dike-like exten- 
sion of a hornblende-gabbro area, and is presumably a modifi- 
cation of the same magma as the gabbro (No. 181 Amador 
county ). 
Seine df these line grained diorites are nearh or quite identical with 
n diorite described by Hawes, which is now called camptonite. 
Thej appear to be among the latest of the older intrusive rocks. 
Near Wesl Point in Calaveras county, at Spanish peak in Plumas 
county, near Merrimac and Swede's Flat in Butte county, the campton- 
ite occurs as dikes in the granodiorite; near Railroad Flat in Calaveras 
county dikes of the same rock cut the schists of the Calaveras forma- 
tion: in the bed of the Cosumnes river to the west of Lamb's bridge 
(see Placerville sheet ) tlie quartz-porph\ rite is cut In dikes of this rock. 
Some of the camptonites have sometimes been designated hornblende- 
porphyrite. 
Granodiorite. 
This name was introduced by Mr. Becker for the potash- 
poor granite of the Sierra Nevada in the general text descrip- 
tive of the Gold Belt geological atlas sheets. The rock thor- 
oughly resembles and is usually called granite. It is a non- 
crystalline rock of hypidiomorphic structure and is composed 
of plagioclase. quartz, hornblende and brown mica, with a 
varying amount of potash-feldspar, which is in almost all 
cases exceeded in amount by the soda-lime feldspar. There 
are also various accessory minerals. Where the potash feld- 
spar is in large amount the rock might be called a biotite- 
hornblende granite or a hornblende-granitite ; other speci- 
mens with very little potash-feldspar are practically quartz- 
mica-diorite. 
