lord.] IGNEOUS ROCKS FROM SAN CARLOS FIELD. 91 
The quartz phenocrysts are small and much rounded by magmatic 
corrosion. They are quite free from inclusions and reasonably abun- 
dant in rocks poor in augite. 
Magnetite and apatite occur in the form usually met with in rhyo- 
litic rocks. Phenocrysts of the former mineral are frequently covered 
with a coating of limonite, which discolors the surrounding rock for 
some distance and produces the brown spots referred to above. 
In the groundmass of the pantellerite the presence of segirine-augite 
and hornblende is of special interest. The former mineral occurs in 
the form of minute, irregularly bounded crystals of a rude prismatic 
habit. 
The microlites are of a grass-green color, and in polarized light show 
characteristic pleochroism: || a grass green, || b sap green, and || JCpale 
yellowish green. The absorption is a>ft>C. On prismatic sections 
the axis of greatest elasticity (a) was found to lie nearest c, the 
extinction angle not exceeding 25°. Twins after oo P ^ (100) are occa- 
sionally observed. 
The hornblende is not so abundant in the groundmass of this rock 
type as is the pyroxene. It occurs in thin sections, in irregular 
wedge-shaped patches, showing distinct prismatic cleavage, and is 
often intimately associated, although never intergrown, with the 
Fegirine-augite. It is dark brown and exceedingly pleochroic: | C dark 
brown, || h purplish brown, and || a brownish yellow. Absorption is 
strong: jC>fr>a. 
The extinction is considerable, jC forming on sections in the prism 
zone a maximum angle of 15° with the trace of the prismatic cleavage. 
From the optical properties mentioned it is probable that this 
amphibole is closely related to barkevicite. 1 
Both the brown hornblende and the aagirine-augite are in many cases 
altered to chlorite and limonite. The fibrous mineral is regularly dis- 
tributed through the rock; it occasionally occurs associated with 
serpentine as the probable alteration product of preexisting olivine 
crystals. 
Feldspar and quartz of the effusive period are of microgram tic 
development and constitute the greater part of the pantellerite ground- 
mass. The former mineral is of prismatic form, and occasionally quite 
idiomorphic. The microlites are elongated parallel h, but in no instance 
could twinning be identified. As prismatic sections they show extinc- 
tion angles not exceeding 10°, and may properly be regarded as a 
second generation of anorthoclase. The interstices of the rock are 
filled with allotriomorphous quartz. 
The following analysis of the quartz- pantellerite from Presidio 
County, Texas, was made by Mr. Geo. Steiger, of the United States 
Geological Survey. Before making the analysis the calcium carbo- 
! H. Rosenbusch: Mikros. Physiographie, Vol. I, 3d edition, Stuttgart, 1892, p. 561. 
