135

to stelliform groups, each of them surrounded by a corona of colorless homogeneous glass.

Annexed to these pitchstones there is a light gray glassy rock with somewhat pumiceous appearance, presenting very few phenocrysts of feldspar and quartz. Under the microscope it is seen to consist of a colorless glass, alternating in patches with yellow microfelsitic parts. Trichites are scattered through the whole mass.

ROCKS OF THE VAN HORN MOUNTAINS.

Three specimens of a coarse-grained olivine diabase very similar to the above mentioned olivine diabase from Black gulch, Diablo mountains. According to the tabular habitus of the feldspars, and the inclination to idiomorphic shape of the pyroxene, the structure of the rock is more gabbroid than ophitique. Olivine in grains and crystals is changing to serpentine and frequently enclosed in the pyroxene. The latter, without the lamination of diallage, presents a well-marked zonal structure, the borders being seen as darker colored than the core. The pleocroism is:

• a. Light greenish yellow.
• b. Dark grayish violet.
• c. Dark gray.

Apatite in large needles can be seen in the rock by means of a lens.

One of the collected specimens consists more than half of opaque iron ores, probably magnetite (the rock shows an evident influence upon a compass needle); the ore grains frequently bordered by a small rim of a reddish brown mica. Pyroxene is wanting. Plagioclase and olivine show the mentioned properties; the latter is somewhat more abundant than in the common olivine diabase. This rock represents a basic concretion, as they are found associated generally with rocks of the gabbro series, and partly in large extension (Taberg, etc.).

Some specimens of hypersthene andesite with hyalopilitic ground mass and the common properties.

From the "96 canyon" there are some black basaltic rocks forming dikes, according to von Streeruwitz. They show some large phenocrysts of a basaltic hornblende (up to 3 cm. in size) and large plates of mica. One of the specimens studied under the microscope comes under the head of nepheline basanite. There are phenocrysts of olivine and pyroxene in a coarse-grained ground mass made up of pyroxene, lath-shaped plagioclase, some reddish brown mica leaves, and an allotriomorphous cement of nepheline, changing to a fibrous mineral.

From "Mica Tanks" some specimens of pegmatite and graphic granite, the latter showing red and white microcline intergrown with albite and quartz. Large plates of muscovite and biotite probably come out of this pegmatite.