By L.P. GRATACAP.
In the county of Litchfield, Conn., in the midst of some of the most
attractive hill country of that region, a very striking mineral
fissure has been opened by Mr. S.L. Wilson, which, in both its
scientific and commercial aspects, is equally important and
interesting. It is a broad crevice, widened at the point of excavation
into something like a pocket and filled, between its inclosing walls
of gneiss, with a granitic mass whose elements have crystallized
separately, so that an almost complete mineralogical separation has
been effected of quartz, mica, and feldspar, while associated
aggregates, as beryl and garnet, have formed under conditions that
make them valuable gem fabrics.
The vein has a strike south of west and north of east and a distinct
dip northwest, by which it is brought below the gneiss rock, which
forms an overhanging wall, on the northerly side of the granitic mass,
while on the southerly edge the same gneiss rock makes an almost
vertical foot wall, and exhibits a sharp surface of demarkation and
contact. The rock has been worked as an open cut through short lateral
"plunges," or tunnels have been used for purposes of exploration in
the upper part of its extent. Its greatest width appears to be
fifty-one feet, and the present exposure of its length three hundred.
It undergoes compression at its upper end, and its complete extinction
upon the surface of the country at that point seems probable. At its
lower end at the foot of the slope wherein the whole mass appears, it
reveals considerable development, and affords further opportunities
for examination, and, possibly, profitable investment. It has been
formed by a powerful thrust coincident with the crumpling of the
entire region, whereby deeply seated beds have become liquefied, and
the magma either forced outward through a longitudinal vent or brought
to the surface by a process of progressive fusion as the heated
complex rose through superincumbent strata dissipated by its entrance
and contributing their substance to its contents. The present exposure
of the vein has been produced by denudation, as the coarsely
crystalline and dismembered condition of the granite, with its large
individuals of garnet and beryl, and the dense, glassy texture of the
latter, indicate a process of slow cooling and complete separation,
and for this result the congealing magma must necessarily have been
sealed in by strata through which its heat was disseminated slowly.
For upon the most cursory inspection of the vein, the eye is arrested
at once by the large masses of crystalline orthoclase, the heavy beds
of a gray, brecciated quartz and the zones and columns of large leaved
mica. It was to secure the latter that Mr. Wilson first exploited this
locality, and only latterly have the more precious contents of the
vein imparted to it a new and more significant character. The mica,
called by Mr. Atwood, the superintendent of the work, "book mica,"
occurs in thick crystals, ranged heterogeneously together in stringers
and "chimneys," and brilliantly reflecting the sunlight from their
diversely commingled laminæ. This mica yields stove sheets of about
two to three by four or five inches, and is of an excellent,
transparent quality. It seems to be a true muscovite, and is seldom
marred by magnetic markings or crystalline inclusions that would
interfere with its industrial use. Seams of decomposition occur, and a
yellowish scaly product, composed of hydrated mica flakes, fills them.
The mica does not everywhere present this coarsely crystalline
appearance, but in flexures and lines of union with the quartz and
orthoclase is degraded to a mica schist upon whose surfaces appear
uranates of lime and copper (autunite and torbernite), and in which
are inclosed garnet crystals of considerable size and beauty. The
enormous masses of clean feldspar made partially "graphic" by quartz
inclosures are a conspicuous feature of the mine. In one part of the
mine, wooden props support an overhanging ledge almost entirely
composed of feldspar, which underneath passes into the gray brecciated
quartz, which again grades into a white, more compact quartz rock. It
is in this gray brecciated quartz that the beryls are found. These
beautiful stones vary extremely in quality and color. Many of the
large crystals are opaque, extensively fractured, and irregular in
grain, but are found to inclose, especially at their centers, cores of
gem-making material.
The colors of the beryls grade from an almost colorless mineral
(goshenite) though faintly green, with blue reflections, yellowish
green of a peculiar oily liquidity (davidsonite), to honey yellows
which form the so-called "golden beryls" of the trade, and which have
a considerable value. These stones have a hardness of 8, and when cut
display much brilliancy. Many assume the true aquamarine tints, and
others seem to be almost identical with the "Diamond of the Rhine,"
which as early as the end of the fifteenth century was used as a
"fraudulent substitute for the true diamond" (King). Few, very few,
belong to the blue grades, and the best of these cannot compare with
those from Royalston, Mass. Those of amber and honey shades are
beautiful objects, and under artificial light have a fascination far
exceeding the olivine or chrysoberyl. These are not as frequent as the
paler varieties, but when found excite the admiration of visitor and
expert. It seems hardly probable that any true emeralds will be
uncovered and the yellow beryls may not increase in number. Their use
in the arts will be improved by combining them with other stones and
by preparing the larger specimens for single stone rings.
Very effective combinations of the aquamarine and blue species with
the yellow may be recommended. Tourmaline appears in some quantity,
forming almost a schist at some points, but no specimens of any value
have been extracted, the color being uniformly black. The garnets are
large trapezohedral-faced crystals of an intense color, but penetrated
with rifts and flaws. Many, no doubt, will afford serviceable gem
material, but their resources have not yet been tested by the
lapidary.
While granite considered as a building stone presents a complex of
quartz, mica, and feldspar so confusedly intercrystallized as to make
a homogeneous composite, in the present mass, like the larger and
similar developments in North Carolina, these elements have excluded
each other in their crystallization, and are found as three separate
groups only sparingly intermingled. The proportions of the constituent
minerals which form granite, according to Prof. Phillips, are twenty
parts of potash feldspar (orthoclase), five parts of quartz, and two
parts of potash mica (muscovite), and a survey of Mr. Wilson's quarry
exhibits these approximate relations with surprising force.
There can be but little doubt that this vein is a capital example of
hydrothermal fusion, whereby in original gneissic strata, at a
moderate temperature and considerable depth, through the action of
contained water, with the physical accompaniment of plication, a
solution of the country rock has been accomplished. And the cooling
and recrystallization has gone on so slowly that the elements of
granite have preserved a physical isolation, while the associated
silicates formed in the midst of this magma have attained a supremely
close and compact texture, owing to the favorable conditions of slow
growth giving them gem consistencies. The further development of the
vein may reveal interesting facts, and especially the following
downward of the rock mass, which we suspect will contract into a
narrower vein. At present the order of crystallization and separation
of the mineralogical units seems to have been feldspar, mica, garnet,
beryl, quartz.
In the artificial preparation of crystals it is invariably found that
perfect and symmetrical crystals, and crystals of large size, are
produced by slow, undisturbed cooling of solutions; the quiet
accretion permits complete molecular freedom and the crystal is built
up with precision. Nor is this all. In mixtures of chemical compounds
it is presumable that the separate factors will disengage themselves
from each other more and more completely, and form in purer masses as
the congelation is slowly carried on. A sort of concretionary affinity
comes into play, and the different chemical units congregate together.
At least such has been the case in the granitic magma of which Mr.
Wilson now possesses the solidified results. The feldspar, the quartz,
the mica, have approximately excluded each other, and appear side by
side in unmixed purity. And does it not seem probable that this
deliberate process of solidification has produced in the beryls, found
in the center of the vein at the points of slowest radiation, the
glassy gem texture which now makes them available for the purposes of
art and decoration?
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