lokd.] IGNEOUS ROOKS FROM SAN CARLOS FIELD. 89 
The specimens submitted represent throe distinct rock types, which 
may be classified as follows: 
(1) Rhyolite breccia, composed chiefly of fragmental rhyolitic 
material. 
(2) Quartz-pantellerite, containing quartz, anorthoclase, and mono- 
clinic pyroxene, as principal mineral constituents. 
(3) Basalt, having olivine, augite, and plagioclase as essential 
minerals. 
RHYOLITE BRECCIA. 
Under this heading are considered a series of yellowish, or purplish- 
grey, very line-grained, vesicular breccias, and a series of brownish- 
red more massive rhyolites (5, 23), x 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. This 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 cryptoorystalline quartz. In 
some instances the rock from flow No. 5 is much fractured, and near 
1 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. 
2 These specimens were not vesicular, and may perhaps be more properly designated tuft's. 
