A GEM-BEARING GRANITE VEIN IN WESTERN CONNECTICUT.
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?