90 RIO GRANDE COAL FIELDS OF TEXAS. [bull. 164. 
the surface very vesicular. The fissures and amygdaloidal cavities 
are filled with quartz and chalcedony. 
Nos. 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 f ragmental 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 the 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 fine-grained, noncrystalline groundmass composed of aagirine- 
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(ll0), 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 
I 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 a to a was circa 10°. These triclinic 
feldspar crystals are apparently single individuals, but occasionally 
show a wavy extinction, owing, presumably, to poly synthetic 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: || C pale olive green, || a and || h 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^o (010), and oo P oo (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. 
