136 THE AMERICAN NATURALIST. [VoL. XXXII. 
PETROGRAPHY. 
California Eruptive Rocks.— The “ bed-rock” series of the 
Sierra Nevadas underlying the sands, gravels, and volcanic rocks in 
the vicinity of Nevada City and Grass Valley, Cal., consists of 
highly altered sedimentary rocks, crystalline schists, and igneous 
rocks, much resembling pre-Cambrian complexes elsewhere, but 
which here are known to be much younger than Cambrian. 
Lindgren ! describes the old igneous rocks as comprising grano- 
diorite, a type of rock intermediate between granite and quartz, — 
mica-diorite, aplite, granite-porphyry, diorite-porphyrite, diorite, gab- 
bro, serpentine, diabase, porphyrite, augite-syenite, and amphibolite. 
The limits of variation of the granodiorite are shown by the follow- 
ing figures : 
310; ALO, F&O, FeO CaO MgO 5,0  Na,O 
59—68.5 14—17 1.5-2.25 1.5-4.5 3-6.5 I-2.5 1-3.5 2.5-4.5 
Its predominant feldspar is a plagioclase, though orthoclase is 
present in small quantities, often intergrown with albite forming 
micro-perthite. 
The gabbros are distinguished from the diorites by the character 
of their feldspathic component. This is a mediumly acid variety in 
the diorites and a basic variety in the gabbro. The ferromag- 
nesian constituent in the latter rock may be either pyroxene, horn- 
blende, or mica; though, as a matter of fact, all the gabbros described 
by the author contain some form of pyroxene or its alteration prod- 
uct. The serpentine is derived from pyroxenite and peridotite. 
The diabases and porphyrites probably represent the cores of old 
volcanoes. ‘These rocks grade into each other through so many 
different types that the author finds it difficult to classify them. 
The principal distinction made use of in defining them appears to 
be coarseness of grain, “since the diabase may readily become por- 
phyritic, the resulting rock being referred to as diabase-porphyrite. 
A more pronounced porphyritic structure with finer-grained holo- 
crystalline groundmass gradually leads over into the porphyrites, 
referred to as augite-porphyrites or hornblende-porphyrites.” A 
majority of the porphyrites might be classed as apo-andesites, 
though the rocks are very different from the andesites of the dis- 
trict. 
1 Seventeenth Annual Report of the U. S. Geol. Survey, vol. ii, p. 2. Wash- 
ington, 1896. 
