99'95
A
very fine oligoclase occurs at Dixon's Quarry, New Castle County, Del.;
and at West Chester, Delaware County, Pa., a striated variety which
admits of a handsome polish.
Obsidian,
a peculiar, glasslike stone of volcanic origin, is found along Pitt
River, Cal., where handsome specimens of the streaked marekanite or "
mountain mahogany" are found, also in Owen Valley, in the same State,
where it occurs in red fragments, and also banded with alternate layers
of black and brown. Near Sante Fe, N. M., it is found in rounded
pebbles over an inch across, resembling moldavite, as the variety from
Moravia is called, only not quite so green. A porphyrite and sperolite
obsidian occurs under the trachyte on Gunnison River, and a heavy vein
of porphyrite obsidian is found under the Grande pyramid, continuing
from thence southward through the trachytic bed. Nodules are found in
the lower members of the trachytic veins. There is a dyke of light-gray
and clear obsidian, with concentric structure, near the Colorado
Central lode, north of Saguache Creek, near Georgetown, Col. Obsidian
in fine pieces is very abundant ten miles southeast of Silver Peak,
Nev., and at Obsidian Cliff in the Yellowstone Park, Wyo. This locality
is described by Joseph P. Iddings2 who says : "The cliff
presents the partial sections of a floor of obsidian, the dense glass
constituting the lower portion, which is from 75 to 100 feet thick. One
of its remarkable features is a prismatic column, forming its southern
extremity, which rises 50 or 60 feet, and is only 2 to 4 feet in
diameter. The
1
E. L. Sperry's Analysis in Mineralogical Notes by S. L. Penfield and E.
A. Sperry, Am. J. Sci. III., Vol. 36., p. 325, Nov., 1888.
J Seventh Annual Report of the United States Geological Survey, p. 254 et seq.
