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ROCKS OF THE SIERRA BLANCA.

A fine grained granitic rock with some larger phenocrysts of orthoclase, resembling macroscopically the augite granitite from Laveline, (Vogesen), described by Rosenbusch. Under the microscope the rock is seen to bear biotite and a pale green pyroxene (diopside) about in the same amount. Quartz is scarce. The feldspars show a tendency to regular shape and thereby an approach to panidiomorphic structure. The specimen is probably a basic concretion of augite granitite.

A white rock jointed in tabular plates of about 2 cm. in thickness. It shows in a fine crystalline ground mass, phenocrysts of orthoclase (sanidine) quartz and biotite. Under the microscope the ground mass is seen to be an allotriomorphus aggregate of quartz and orthoclase. The rock belongs under the head of a microgranite or a holocrystalline rhyolite.

ROCKS OF THE WILEY MOUNTAINS.

From the Wiley mountains there are some specimens of graphic granite, a hypersthene andesite (the rhombic pyroxene altering to bastite, the ground mass being exceedingly fine-grained and of hyalopilitic structure) and a feldspar basalt (from Fremont). The latter contains phenocrysts of olivine, pyroxene, and little plagioclase in a ground mass of plagioclase laths and pyroxene crystals. A colorless glass occurs only in traces and has no influence at all on the structure of the rock. According to that the rock belongs to the so-called "Siebengebirgs" typus of basalt.

ROCKS FROM THE DIABLO MOUNTAINS.

Some specimens of a very fresh olivine diabase collected at Black Gulch, Diablo mountains. It is a gray coarse-grained rock, showing grayish white lath-shaped plagioclase, black grains of a pyroxene, and yellow brown grains of olivine surrounded by some brown iron hydroxyd. Long colorless needles of apatite are scattered through the rock.

Under the microscope the olivine in irregular grains is very fresh, showing only on the rough cleavage cracks some iron hydroxyd as the first symptom of decomposition, a proof that the mineral contains a considerable amount of iron. Pyroxene also forms irregular grains; it has the pleochroism of pyroxenes containing titanic acid. The pleochroism is a light brownish or greenish yellow and dark brown violet and grayish brown. Absorption, c > b > a. The angle of extinction is large: c: c=52-54º on (010), circa 40º on (110).

A strongly pleochroitic mica is but scarce; it is always bordering the iron ore grains. Some crystals of titanite. The feldspar has an angle of