The chief difference between this and the adularia is, that one contains fourteen potash and the other nine soda. Between this and the saussurite, or tenacious feldspar, the one contains eleven of lime, and the other only a trace.
The silicious feldspar, which I suspect to be the basis of the granite, crystallizes in thin rhomboidal tables. They are very frangible, and have one clivage perpendicular to the faces of the tables. Sometimes the tables have one lateral edge or more truncated. In one fragment of a crystal I observed a very obtuse acumination on the table, which appeared to be diedral, the sides being placed on the obtuse lateral edges of the tables. On account of the extreme frangibility of the crystals, it is certainly extremely difficult to seize their characters. Specific gravity only 2.333, probably owing to interstices between the tables. The colour is white, translucid, passing to semi-transparent; lustre sometimes dull, at others shining. The tables are sometimes so aggregated that their edges being exposed, offer wedge-shaped and stelliform figures. The tourmalines are chiefly contained in this vein. They are red, or green, rarely blue or black.
The green tourmalines vary from one-eighth of an inch to one inch in diameter; they are sometimes four inches in length, and are entirely confined to the inner vein of quartz. They are triedral prisms, with convex faces, striated longitudinally, and generally traversed perpendicularly to the axis, with very small fissures filled by some silicious substance, probably feldspar. These green crystals are opaque. The red tourmaline is frequently enclosed in the green. In certain parts of the vein almost every green crystal encloses a red one, which always corresponds by its sides and angles with the exterior crystal. Sometimes a thin layer of talc intervenes between the outer and inner crystal. In one specimen I found three crystals of the red aggregated together, and enclosed in one of the green. In another crystal I found pyrites in the place of the red tourmaline. The largest crystal of the red was one quarter of an inch in diameter, and four inches long. The red tourmalines vary in intensity of colour, and frequently (particularly in the interior) pass into violet. They pass from translucid to semi-transparent. I have found some that were terminated by triedral pyramids. The crystals are generally perpendicular to the sides of the vein. Small crystals of the red often run from the vein of quartz into the adjoining feldspar. The granite also contains minute crystals of dark and light blue tourmaline, and pale green emerald, with a very few garnets and pyrites. In the lower part of the vein, five to six feet from its interruption by the mica slate, the red tourmaline scarcely appears, and the vein contains chiefly bluish amorphous quartz and green tourmaline. It is therefore probable that this vein will not afford henceforward a great supply of this beautiful mineral.
About six miles from Chesterfield, in Goshen, is found the rose mica, with tourmalines and emeralds interspersed in the granite. Unfortunately the bed of granite has not been discovered, and the specimens we possess are taken from loose rocks, scattered over a small extent of ground in a valley, in the neighbourhood of mica slate. The rose mica is found in a large grained granite with amorphous quartz and silicious feldspar, crystallized and amorphous. The mica is generally of a rose red, sometimes yellowish green. It crystallizes in rhomboidal tables, rarely truncated on the acute angles, passing into the hexaedral table. The tourmalines are light and dark green and blue, of various shades of intensity, frequently acicular and stellated. The black, the red, and the violet tourmalines also occur, but more rarely. Sometimes the green prisms enclose others of blue and black. Specific gravity of these varieties from 3. to 3.1. The green and blue crystals in this locality are translucid or semi-transparent. The feldspar is generally white, rarely light blue. There are some emeralds in the granite. Among some specimens which Mr. Weeks of New-York, who discovered this locality, was so good as to give me, I found a beautiful rose emerald in its matrix. It is a hexaedral prism, about one and a quarter inch in diameter, the summit a plane, one of the lateral edges has a truncature. About half of the diameter of the prism is free from the matrix, and half an inch of the prism. The colour is a pale rose, rather more transparent than the emerald.
The colour of the mica of the Goshen granite calls to mind the lepidolite or lilalite, which (formerly considered as a distinct species) has now been united to mica. The lepidolite of Rosena is also accompanied by the tourmaline apyre, now the red tourmaline.
Art. V. Observations on the Minerals connected with the Gneiss range of Litchfield county.
Art. V. Observations on the Minerals connected with the Gneiss range of Litchfield county, by Mr. John P. Brace, of Litchfield, Conn.
The gneiss formation is the most extensive of any in Litchfield county, and embraces a number of very interesting minerals. It extends east into Hartford county. On the north it runs into Massachusetts, though frequently interrupted by the limestone formation, which rests upon it. It forms the principal, and in many cases the only rock of the eastern and northeastern sections of the county, and of the towns of Litchfield, Goshen, Warren, Cornwall, and Norfolk. In Washington and Canaan, it constitutes the rock of the high mountains, and is a part of the same range in the other towns, while the valleys and the more moderate elevations are covered with limestone.