have
been found in only one of them. The basic rocks are olivine gabbro and
hornblende hypersthenite, with transitional phases between them.
Diorite lies to the west of these, and all nave been surrounded and
intruded by biotite granite.
The
emerald-bearing vein has an irregular strike approximating east and
west with a high dip north. It is medium to coarse grained, and is
composed of quartz and feldspar, part of which, at least, is albitc,
with some black tourmaline sprinkled through it and an occasional
emerald or green beryl crystal. The crystallization of the minerals of
the pegmatite is not good, but a few partly developed crystals are
found in small, irregular miarolitic cavities. Crystals found in these
cavities are quartz (colorless and smoky), albite, feldspar, black
tourmaline, and a little beryl. The cavities in the pegmatite are
partly filled with reddish-brown, greasy-feeling clay, and the same
material along with limonite stains has permeated joints and seams
through the pegmatite. The feldspar of the pegmatite has partly
decomposed m places, so that the rock breaks down rather easily. Some
of the emerald crystals are firmly attached to other minerals, and some
are loose and may be obtained by washing the semidecom-posed pegmatite.
At first many small fragments and crystals of emeralds were found in
this way, but later gem emerald was also found in place in the rock.
The
crystallization of the quartz and feldspar so far found in the
pegmatite vein is not so perfect as that in the veins once worked for
beryl and hiddenite at Hiddenite, N. C. The albite assumes the form of
rough crystals and of aggregations of stout crystals, though not of the
clevelandite type common in many gem-bearing pegmatites. The quartz
occurs in crystals of average perfection, and in many of the specimens
exhibits trapezohedral faces indicating a right-hand character. Some of
the quartz is nearly colorless and other is smoky. One crystal of
quartz examined is penetrated by numerous fine light-colored needles of
actinolite. The emerald crystals are simple hexagonal crystals of beryl
with the prism faces and base. Many of them are deeply striated and
etched, especially on the prism faces. Other crystals have internal
striations or irregularly shaped tubes extending through their length.
Some of these tubes are of considerable size compared with the crystal
inclosing them and have been filled with clay or iron stains. The finer
tubes appear as silky striations in the crystals.
Veins
containing quartz crystals occur in the same formation as the
emerald-bearing pegmatite. Some of the quartz crystals from these veins
are fairly well developed and clear; others contain inclusions of fine
needles of actinolite and cut into very pretty gems. Veins of quartz
crystals occur over a large area in this region and yield some fine
specimens. Large crystals containing liquid with bubbles have been
round about a mile northwest of the emerald mine in a similar rock
formation. Black tourmaline is plentiful both at the emerald mine and
in other places.
Determinations
of chromium oxide were made in four rocks from the emerald regiom by R.
C. Wells, of the United States Geological Survey. Hornblende
hypersthenite wall rock for the emerald vein carried 0.16 per cent Cr303; olivine gabbro, also close to the emerald vein, contained 0.17 per cent Cr303; hornblende-hypersthene-augite
