302 The American Geologist. May, 1894 
Gabbro and Norite, 
Hocks of the gabbro series occur not infrequently in the 
Sierra Nevada, but seldom in large areas. In general they 
seem genetically related to the granite series on the one hand. 
and to the peridotite- pyroxenite series on the other. 
A mica-gabbro forms a considerable hill, more than a mile in greatesi 
diameter, in Amador county, about two miles wesl of Plymouth. At 
Hi her points gabbro occurs associated wit h serpent ine dikes. 
The area of gabbro about Whitmore's, in the oortheasl corner of the 
Jackson sheet, is a very variable one. While the largerparl of the area 
is a gabbro, portions contain olivine, forming an olivine-gabbro; other 
portions contain (inly pyroxene, forming pyroxenite, some of which is 
entirely altered to hornblende, forming a massive amphibolite: brown 
mica occurs locally, forming a mica-gabbro; at one point a specimen 
contains primary quartz, brown mica, plagioclase and pyroxene, with 
secondary hornblende, forming kersantite; and on the south slope of 
Mount Crossman is a mass of rock composed chiefly of primary brown 
hornblende that seemed to be geneticallj a part of the gabbro area. 
In Plumas county gabbro forms the east slope of Eureka peak below 
the 6,800 foot contour. It there may be seen in sharp contact with the 
quartz-porphyry that forms the crest of the peak, and as fragments of 
the gabbro are enclosed in the quartz-porphyry, it would appear that 
the gabbrO is here the older TOCk. 
The area lying four miles soul h wesi of Grizzly peak in Plumas county 
is in part a typical gabbro (No. 341 Plumas county) of plagioclase and 
diallage, and in part is composed of hypersthene and plagioclase forming 
a norite. At one point in the area are developed large black hornblende 
crystals, some of them more than an inch long. There are also curious 
concretionary structures in the norite (No. 343 Plumas county). These 
concretions consist of a central portion that appears to be largely the 
same as the general mass of the norite. that is. hypersthene and plagio- 
clase, surrounded in the particular one examined by three or more con- 
centric layers, which in thin section are seen to he composed of olivine 
and magnetite, with intervening layers of plagioclase. 
Peridotite and Pyroxenite. 
Fresh peridotite has been found, so far as known to the 
writer, tit very few points in the areas studied, it having, as 
a rule, been altered to serpentine. Still, traces of the original 
(divine, together with pyroxene, have been found in many of 
the serpentine areas, and at a number of points beautifully 
fresh pyroxenite. 
The largest single dike of serpentine, which is known to 
have been originally a peridotite or pyroxenite, occurs in the 
northern end of the range in Sierra and Plumas counties. It 
