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.
The river Housatonic appears to have made its way through this range, for the same rock continues on the western side of the river parallel to it in the mountains of Kent, Sharon, and Salisbury. In Litchfield commences a range of porphyritic granite, or porphyritic gneiss, which alternates with the common gneiss, and in some instances rests upon it. This rock begins at Mount Prospect, between Litchfield and Warren, and runs through South Farms, Bethlem, and Watertown. The crystals of feldspar in it, are often very perfect.
The primitive granite, as a rock, is not found, though it lies scattered on the surface in great quantities, and large masses. The graphic granite in this region is often remarkably fine. Mica slate constitutes a considerable part of those rocks that rest on the gneiss, though never found in such elevated situations. The mica slate rocks are always inclined at a great angle with the horizon, and follow the direction of the other range. Litchfield village, Chesnut hill, and great part of Harwington, are entirely composed of this rock. The Bantum and the Waterbury rivers have their bottoms of it. Some of the brooks entering the Waterbury, have cut their passage through the mica slate, leaving walls of 40 or 50 feet on each side, traversed by veins of a very coarse-grained granite, and often much mixed with sulphuret of iron. The slate near Harwington meeting-house contains a great quantity of sulphuret of iron. Mica slate likewise lies on the sides of the gneiss range in Canaan and Salisbury, where it dips under the limestone. Sienite is scattered on the surface in large masses, especially where the porphyritic gneiss is found. Sometimes, however, the masses are so large as to form mountains. Mount Tom, between Litchfield and Washington, is of this nature, being entirely composed of sienite, resting on gneiss. Slaty sienite is frequently found, having a very large proportion of hornblende.
The minerals that are found in this region, are much more interesting than its geology. In describing them, I shall confine myself to the district east of the limestone range, intending at some future time to investigate and describe the limestone country.
Carbonate of lime, the granular limestone, is scattered over the whole of this region. It often is found in the cavities of decayed quartz rocks, and contains tremolite and augite.
Cyanite or Sappar, is found in great quantities, especially in Harwington and Litchfield. A crystalline mass of this was found a few years ago, weighing probably 15 cwt.; it lay on a mica slate ridge, and undoubtedly had been formerly imbedded in the slate. Beautiful white talc, and small crystals of sulphuret of iron, are disseminated in the mass. Specimens of this mass are in almost all the cabinets in America. Smaller masses have been found associated with feldspar. Small crystals of this mineral are very common in mica slate, with staurotide and garnet. Two of these crystals are often arranged at right angles with each other. In Cornwall it is found in small crystals in the gneiss containing graphite.
Staurotide is very common and very beautiful. It is found principally in mica slate, and exhibits often the cross. It most generally is crystallized in four-sided prisms.
Quartz, of course, is common. Cornwall particularly is distinguished for the smoky variety. Ferruginous quartz is found in rolled masses in the whole of this range.
Petro silex, in rolled masses with ferruginous quartz, containing veins of chalcedony and hornstone, and geodes of quartz crystals, are common in Litchfield and Goshen. Sometimes these masses in the interior assume the appearance of Burrstone.
Common opal has been found in Litchfield, though rarely. It was part of a mass of ferruginous quartz, with indelible dendritic impression. It is very hard, and its fracture is conchoidal.
Mica is very common. It is found green, white, and perfectly black. It generally occurs in blocks of granite.
Schorl, in rounded crystals, is found in all the granite in this range; in radiating crystals on quartz; and in acicular crystals on mica slate. The large crystals are so brittle, that few of them can be obtained perfect. I once found it in Litchfield, near Plymouth, in prismatic crystals on earthy graphite.
Feldspar is very common and beautiful in all the towns. It is usually found in rhomboidal fragments, and has a fine lustre. It is blue, white, and red. Some of the granite of Torringford is very beautiful, being composed of white and smoky quartz, red feldspar, and green mica. In the porphyritic gneiss, feldspar is in six-sided prisms. One small crystal of adularia, well defined, has been found by E. Wilkins, Esq.
Beryl, both crystallized and massive, is often found in Litchfield in granite. Its colours are green, greenish yellow, pale yellow, and brown. Its crystals are often very perfect.
Garnets are common in all the towns of this range.
Epidote. Very beautiful crystals of this mineral have been found in Washington, associated with feldspar. They are so rounded as to render it very difficult to discover their form. They have a very fine lustre, and are of an olive green; in Litchfield, in crystals with hornblende, and graphic granite, and in veins in sienite.
Perhaps no region can be found containing more beautiful tremolite. All its varieties occur; the fibrous of Litchfield and Bethlem is very distinguished. In Canaan, it is found containing crystals of sulphuret of iron. I do not speak here of the tremolite found in the limestone range.
Common asbestus exists in Washington and New Milford.
The white augite is a mineral found in this range; in Litchfield, in six-sided prisms very much flattened, on quartz, and carbonate of lime with tremolite. They sometimes occur several inches long.
The lamellar and slaty varieties of common hornblende are very common.
Radiated actynolite of a beautiful bluish green in Litchfield; in Canton of a brownish green.
Steatite is common, and is quarried in Litchfield. The varieties of talc are very common, connected with steatite, cyanite, and chlorite.
Chlorite in Litchfield, is found on quartz, with talc.
Porcelain clay in Litchfield in small quantities, and in Washington.
Graphite is found in Cornwall in great quantities. Its gangue is gneiss and sienite. It is lamellar, and has a metallic lustre; is easily obtained, and might be made useful. Epidote and cyanite are found with it.
Ores are not common. Oxides of iron, and sulphuret of iron are scattered over the whole range. Near Mount Prospect in Litchfield, sulphuret of iron in mass is in great quantities; and sulphate of iron on the surface of the ground near it. A stone containing a few grains of native copper was found in Litchfield.
The red oxyde of titanium occurs in Litchfield sparingly. A very handsome specimen of the reticulated oxyde of titanium, was picked up. It was on mica, and the mica had an evident tendency towards the same form.
BOTANY.
