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PRECIOUS STONES.
801
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in
a plastic condition. Veinlets of quartz in the original rock have been
folded and crumpled during this compression into forms resembling
folded ribbon. The country rock in the neighborhood of the veins has
been highly silicified by the addition of much quartz. This quartz
along with other minerals, as muscovite, rutile, pyrite, etc., has
replaced the biotite and feldspars and other minerals of the country
rock. This replacement was later than the compression of the rock and
about contemporaneous with the deposition of the vein matter. These
phases are beautifully illustrated in hand specimens which show typical
biotite gneiss with folded quartz veins, several feet from a vein, and
in similar rock, highly silicified, with a portion of the vein-filling
adhering. In the latter specimen the vein filling a fissure is glassy
quartz with calcite, rutile, and pyrite inclusions. The wall next to
the vein consists largely of granular quartz and an emerald-green
(chrome) muscovite, with a little rutile and pyrite. At about 1 inch
from the vein the replacement of the country rock is not so complete,
and biotite becomes gradually prominent in the rock. At 2 inches from
the vein the rock is nearly black biotite gneiss, rich in quartz. A
folded quartz veinlet cuts the gneiss to the vein wall. It is more
prominent in the black gneiss than in the highly replaced gneiss,
though it can readily be traced through the latter, since the quartz of
which it is composed was not so easily replaced as certain constituents
of the gneiss. Some of the rock cut by the veins contains much
chlorite and has a yellowish-green color.
The
material filling the gem-bearing veins consists of quartz, calcite,
dolomite, muscovite, rutile, black tourmaline, aquamarine and emerald
beryl, hiddenite, pyrite, chalcopyrite, and monazite. All of the veins
in the neighborhood do not contain all of these minerals, but each of
the numerous veins exposed in the workings contain some or all of them.
In the cavities all of these minerals occur in crystals; in solid-vein
matter certain ones only have crystal form. The calcite was introduced
after the other minerals had been deposited, in many places filling up
previously existing cavities. Crystals of the other minerals are
embedded in solid calcite veins, and calcite has been deposited between
broken fragments of beryl and other crystals. Rutile suitable for
cutting is plentiful in brilliant crystals, some long and slender,
others short and thick. The crystals are commonly twinned, several
crystals often joined or crossing each other at angles of 60°, forming
beautiful cabinet specimens of rosettes or reticulated masses of
needles. The gem minerals—emerald, aqamarine, and hiddenite—occur in
distinct crystals in the veins, and when lining the walls of cavities
and of good color they make a beautiful contrast with the associated
gangue minerals.
The
vein at the Ellis emerald mine is pegmatite, with cavities and pockets
included in it. The pegmatite strikes N. 50° E., with a high northerly
to vertical dip. There are stringers or arms of pegmatite along the
walls, and at one place the pegmatite is composed along one side
largely of small mica blocks. The country rock is biotite gneiss, small
dikes of quartz diorite being included. The latter weathers out in
rounded bowlders or "nigger heads," which are scattered over the
surface near the mine. The quartz in portions of the pegmatite is a
fairly dark rose color. So far, however, none suitable for gem purposes
has been found.
51506—m r 1907, pt 2------51
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