taining
pockets with beautiful stones in them. The deposit was worked in an
intermittent way by mining companies and mineral collectors until 1890,
when Mr. Loren B. Merrill obtained control; he has since operated it
successfully. The present dimensions of the quarry are about 150 feet
long by 100 feet wide and 20 feet deep in the deepest part. Work is
facilitated by a derrick operated by a horse windlass to remove the
waste. Drilling is done by hand, and black powder is used in order not
to shatter gem material more than necessary. The pegmatite is in a
general way conformable with the schistose country rock, which strikes
N. 50° to 60° E., with a dip of 20° to 30° SE., and is overlain by
schists on the southern side of the quarry to a depth of about 15 feet.
It is probable that tunneling will soon be necessary to avoid the
expense of removing a heavy overburden of schist. The thickness of the
pegmatite exposed in the quarry is about 20 feet, though the total is
probably somewhat greater. The principal constituent minerals of the
pegmatite are quartz, orthoclase, and micro-cline, muscovite, bjotite,
and black- tourmaline. The mode of association of these minerals and
the texture of the rock are very irregular. The feldspar crystals
attain dimensions of several feet in places, and mica of merchantable
size is sometimes found. Other interesting minerals beside tourmaline
are granular lepidolite, beryl, and spodumene. The beryl occurs both in
pockets and in the solid pegmatite. That in the pockets is the
pale-pinkish caesium gem beryl, and that in the solid rock is generally
opaque and pale green, though small clear portions are sometimes found
which yield aquamarines.
Gem
tourmaline is found almost entirely in pockets in the pegmatite. These
pockets seem to be confined to a zone from 6 inches to 7 feet wide,
which is not readily distinguished in appearance from the pegmatite
above it but is underlain by a narrow garnetiferous layer, beneath
which the rock is finer grained and apparently barren of gem minerals.
The pockets are sometimes very irregular in shape, and range in size
from about a pint in capacity to dimensions of several feet. In all,
about 430 pockets have been opened. Out of 350 opened by Mr. Merrill,
only 50 were of much value. The walls of these cavities are usually
lined with lepidolite, clevelandite, amblygonite, and quartz crystals;
and the bottoms of the cavities are generally covered with a sandy or
clayey mass, consisting of the decomposition products and fragments of
the minerals forming the walls. The tourmalines are embedded in this
decomposed matter. Some, whose form and color as they lie thus embedded
in the pockets seem to be perfect, crumble away when handled, often
leaving a rounded nodule of perfectly fresh mineral, which is
generally beautifully transparent. These nodules often yield the finest
and most perfect gems.
Most
of the gem tourmalines range in color from olivine green through
emerald green to blue green and nearly colorless. Beautiful pink tints
are also found. One shade usually predominates in a given cavity,
though this is not always the case. Single crystals in some cases shade
from white at one termination to emerald green, then light green, pink,
and finally colorless at the other termination. Green crystals tipped
with pink are especially common. Generally these transitions of color
are very gradual, but in other specimens the colors are not mingled in
the least and the crystals seem to be composed of several distinct
sections, though crystallographically they are continuous throughout.
The total value of gems and cabinet specimens obtained from Mount Mica to date is estimated at over $50,000.
