IGNEOUS ROCKS FROM SAN CARLOS FIELD.
The specimens submitted represent three distinct rock types, which may be classified as follows :
(1) Rhyolite breccia, composed chiefly of fragmental rhyolitic mater i :il.
(2) Quartz-pantellerite, containing quartz, anorthoelase, and mono clinic pyroxene, as principal mineral constituents.
(3) Basalt, having olivine, augite, and plagioclase as essential minerals.
Under this heading are considered a series of yellowish, or purplish grey, very fine-grained, vesicular breccias, and a series of brownish red more massive rhyolites (5, 23), 1 containing little fragmental material. The specimens are considerably weathered. They adhere to the tongue, and have a strong clayey odor when breathed upon.
(With the aid of the microscope considerable variation in the minera logical composition of these pyroclastics is discernible. Angular parti cles of quartz, orthoclase, plagioclase, biotite, muscovite, hornblende, magnetite, and occasionally titanite, augite, and zircon, together with fragments of rhyolite, sandstone, limestone, and basalt, are seen cemented together by rhyolite glass decomposing into argillaceous material. ~7This groundmass, although greatly altered to quartz and chalcedony and impregnated with calcite, chlorite, and limonite, showed, with two exceptions (Nos. 12 and 13), 2 fluidal phenomena.
Microscopic cavities are very abundant in these rocks. They are in many cases elongated, approximately in the direction of flow, and are more or less completely filled with silica, either in the form of radi ating quartz (17), chalcedony (5), or opal (18). This secondary silica is stained reddish or yellowish brown by infiltrated iron ore.
In one instance (No. 13) the siliceous cement of the rock was almost completely replaced by calcite.
On comparing the relative amount of fragmental material occurring in the different members of the series, it was found that Nos. 10, 12, 13, 17, and 18 contain all the clastic constituents enumerated above, whereas Nos. 5 and 23, representing the more massive flows, were characterized by the scarcity of foreign material and by the abun dance of corroded quartz, orthoclase, and biotite phenocrysts. These latter specimens are much decomposed, and owe their peculiar brown ish-red color to the presence of limonite and iron oxide in the ground mass of the rock. The feldspar phenocrysts are, in many cases, found completely altered to an aggregate of cryptocrystalline quartz. In some instances the rock from flow No. 5 is much fractured, and near
i The numbers used in referring to the rock specimens are the same as those used by Mr. Vaughan in describing the section 6 miles west of south of Chispa, p. 76. 2These specimens were not vesicular, and may perhaps be more properly designated tuffs.