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RIO GRANDE COAL FIELDS OF TEXAS.
[BULL. 164.
surface very vesicular. The fissures and amygdaloidal cavities lied with quartz and chalcedony.
No,-;. 10 and 12 differ from the other specimens described in the predominance of augite, plagioclase, magnetite, and fragments of basalt over the debris of more acid rocks.
Quartz-Pantellerite.
This rock type is characterized macroscopically by the presence of large feldspar phenocrysts embedded in a dense, aphanitic ground mass. The prevailing color of the rock is grayish green, with a slight yellowish tint.
The altered specimens are either spotted brown or have a brownish red color similar to much-altered varieties of the fragmental rhyolites Nos. 5 and 23. On exposed surfaces the pantellerite weathers yellow ish brown and the feldspars are completely decomposed and leached out. Under the microscope fye porphyritic character of the lava is more pronounced by the identification, besides feldspar, of porphy ritic quartz, augite, magnetite, and apatite. These minerals are strewn in a line-grained, holocrystalline groundmass composed of a^girine augite, brown hornblende, feldspar, quartz, and magnetite.
The feldspar phenocrysts are thick tabular, formed after M (010). The planes M (010), P (001), y(201), T(110), and 1 (110) were identified. The crystals are large, measuring about one-third cm. in thickness, one-half cm. in width, and two-thirds cm. in length, and cleave readily || M and P.
On cleavage plates parallel to the latter face a small extinction angle of not over 3° was observed, whereas on the plates parallel to the brachypinacoid the angle from ato a was circa 10°. These triclinic feldspar crystals are apparently single individuals, but occasionally show a wavy extinction, owing, presumably, to polysynthetic twinning. The specific gravity of the feldspar was determined with great care from selected material, by means of the Thoulet solution, and found to be 2.59, or that characteristic of anorthoclase.
These crystals contain inclusions of magnetite, augite, and apatite, and are frequently considerably altered, either to calcite or natrolite and kaolin.
The augite of intratelluric origin is of a light-green color, and slightly pleochroic: || jC pale olive green, || a and || ft yellowish green. It has an extinction angle of about 42°, and occurs in long prismatic form, with imperfect terminal faces. On basal sections the crystals are seen bounded by oo P (110), oo P^6 (010), and oo PO6 (100) in about equal development.
Considerable variation in the relative amount of augite was noticed in the different specimens of pantellerite. In the slides containing * least quartz the pyroxene was found to be most abundant.
