for
cutting into gems but the most of it is worth more as specimens,
because of the perfection of the crystals. They range from colorless to
pale bluish green, and some are yellow from iron stains on the surface.
Phenacite was observed in small transparent colorless crystals only,
which were attached to or partly imbedded in feldspar, quartz, or
topaz. Crystals of phenacite measuring over hah an men across are
reported to have been found. The phenacite crystals from Baldface
Mountain have been described by O. C. Farrington1 and W. T. Schaller.2
At
the lower prospect on the northeast shoulder of Baldface Mountain,
topaz occurs in a thin pegmatite vein cutting the granite with a strike
of N. 35° W. and a dip of 45° SW. This vein has been exposed for about
75 feet by the dropping away of the northeast side attached to a huge
block of granite. Crystals have also been obtained from the vein
attached to the loose block about 50 feet lower down the mountain side.
The vein contains fine graphic granite in places and small miarolitic
cavities with mineral associations similar to those at the first
prospect described.
TEXAS.
The
occurrence of topaz in Mason County, Tex., has been men-' tioned in
these reports for previous years and some of the deposits have been
described by H. Conrad Meyer.3 Several of the localities
were visited in July, 1913, when the notes for the following
description were obtained. The discovery of topaz was made in 1904 by
R. L. Parker. Other prospects were found and all were developed on a
small scale between 1908 and 1910. The owners of the different
prospects failed to realize on the topaz found on their property and
therefore forbade further work. Since that time a limited 'amount of
prospecting has been done by the property owners, in some places with
promising results. Meyer first recorded the occurrence of cassiterite
or stream tin associated with the topaz of Mason County.
Mason
County is in the broken plateau country of central Texas. Much of the
county is a basin-like area with included ridges and hills. The county
is drained chiefly by Llano River, on which the lowest elevation is
less than 1,200 feet above sea level. The topaz localities examined are
at elevations of from 1,600 to 1,800 feet above sea level. Water is
scarce and only the larger streams flow the whole year.
The basin-like portion of Mason County is part of the pre-Cambrian basin described t>y Sidney Paige.4
The surrounding rim rock or scarp is part of a dissected plateau
composed chiefly of Paleozoic rocks, among which limestone is
prominent. The pre-Cambrian rocks in the topaz areas consist largely of
granite, but a belt of rock composed of mica schist and mica gneiss was
observed between the two groups of deposits. The principal granite of
the topaz region is a coarse red granite, with porpnritic texture
locally developed. The feldspar phenocrysts of this granite measure
over an inch long in some places. A finer grained granite, also
reddish, was observed in the region but not associated with the topaz
deposits. Pegmatite is
1 Notes on various minerals in the Museum collection: Field Columbian Mus. Pub., Geol. Ser., vol. 8, pp. 157-158,1908.
2 Notes on crystallography of phenacite: U. S. Geol. Survey Bull. 490, p. 53,1911.
3 Topaz and stream tin in Mason County, Tex.: Eng. and Min. Jour., Mar. 8,1913, pp. 511-512. * TJ. 8. Geol. Survey Geol. Atlas, Llano-Burnet folio (No. 183), 1912.
