lord.] IGNEOUS ROCKS FROM SAN CARLOS FIELD. 95 
(2) Dacitic rhyolite from Iceland, in Oexnadalr. 1 
..(3) Trachyte from Sierra Madre, on southwest flank, near the town 
of Tulancingo, Mexico. 2 
(5) Volcanite from the island of Volcano, of the Liparian group. 3 
BASALT. 
This rock varies in color from grayish black to dark brown, and 
on macroscopical examination presents a decided porphyritic charac- 
ter, owing to the development of large olivine and plagioclase pheno- 
crysts. 
It appears in the form of an intruded sheet between rhyolite brec- 
cias Nos. 10 and 12. Near its upper surface the rock is a typical 
amygdaloid. 
With the aid of the microscope the basalt was found to pass from a 
dense holocrystalline porphyritic rock to a vesicular, hypocrystalline 
porphyritic variety. 
The first-mentioned type (11) is composed of large, tabular-formed 
labradorites, irregularly bounded olivine and magnetite crystals of 
intratelluric origin, and light-brown augite, plagioclase, and magnetite 
of the effusive period. The augite is rarely of idiomorphic form, and 
contains abundant plagioclase inclusions. In many cases this pyrox- 
ene was the last mineral constituent of the groundmass to crystallize, 
and in taking the part of a mesostasis produced a characteristic ophitic 
structure common to many types of diabase. In some cases the olivine 
is replaced by the questionable mineral iddingsite. 
The vitreous and porous variety of basalt (11') differs from the holo- 
crystalline rock in the development of a dark-brown granulated glass 
base in the groundmass. This rock type is, however, very much decom- 
posed, olivine and augite being often completely altered to limonite, 
chlorite, and serpentine. These secondary minerals, together with 
either calcite, natrolite, or opal, are concentrated in the amygdaloidal 
cavities of the rock. 
!P. Schirlitz, Islandische Gesteine: Min. u. Petrographische Mitth., Vol. IV, 1882, p. 423. 
2 C. A. Tenne, Ueber Gesteine des Cerro de las Vajos (Messerberg) in Mexico; Zeitschr. Deutsch. 
?eol. Gesell., Vol. XXXII, 1885, pp. 612-613. 
3 W. H. Hobbs, Ueber den Volcanit, ein Anorthoklas-Augit-Gestein von dor chemischen Zusammen- 
ietzung der Dacite: Zeitschr. Deutsch. geol. Gesell., J893, p. 578. 
