42 PHYSIOGRAPHY. 



Mica. 



4. Mica forms one of the constituents of various rocks, as granite, 

 mica-slate, gneiss, porphyry, and some kinds of sandstone. They form, 

 sometimes, more or less considerable concretions in these rocks, and 

 contain imbedded crystals of Topaz, Tourmaline, and other species. As 

 single crystals, they often appear imbedded in granular limestone, in ba- 

 salt and wacke, and as implanted crystals, they are {pund upon the spe- 

 cimens ejected from Vesuvius. Several varieties of Mica accompany 

 ores of tin and tungsten, in metalliferous beds. 



5. Mica, in extraordinarily large individuals, is found in Siberia; at 

 Zinnwald, in Saxony, it occurs in crystals, possessing two axes of double 

 refraction. It is also found in the Horlberg in Bavaria, in imbedded 

 globules in Moravia, at Mount St. Gothard in Switzerland, at Finbo in 

 Sweden, Pargas in Finland, at Wisenthal in Saxony, and Joachimsthal 

 in Bohemia, imbedded in basalt and wacke. At Mount Vesuvius, crys- 

 tals of Mica, with one axis, often of considerable size and transparency, 

 occur in the drusy cavities of the ejected specimens. Lepidolite occurs 

 near Rozena in Moravia, and at Uto in Sweden. Finite is found, in gran- 

 ite, at Schneeberg in Saxony, in Auvergne and Cornwall. 



The localities of Mica in the United States, are numerous and inter- 

 esting. Distinct crystals, often remarkable for their size and perfection, 

 occur at Acworth, (N. H.) in granite, implanted upon Feldspar: here 

 also occur the twin crystals, and massive variety, in large individuals, 

 aggregated also after the manner of the twin crystals. Very handsome 

 crystals are found at Greenfield, near Saratoga, in the granite vein that 

 contains Chrysoberyl, Tourmaline and Feldspar crystals: the Miea from 

 this place is of a rich oil-green color when viewed, across the axis, and 

 reddish-brown perpendicular to it. Small but distinct crystals exist in 

 the limestone of Orange county, (N. Y.) associated with Spinel and Bru- 

 cite. Rose-colored crystals are found in the tourmaline granite of Go- 

 shen and Chesterfield, (Mass,) and in general, nearly all the deposits of 

 Beryl throughout New England contain, occasionally, crystals of Mica. 

 Highly perfect cleavage-crystals, of great size and of dark greenish 

 black color, are contained in a vein about one foot in width, near the iron 

 mines of Munroe, at Greenwood, (N. Y.) A similar variety, but partly 

 decomposed, is found in the soil at Mendham, (N. J.) Individuals nearly 

 a foot across, occur at Paris in Maine : these are of a rich brown color, 

 and often penetrated by Tourmaline crystals. A yellow, somewhat cop- 

 per-colored variety, massive and in six-sided tables of large size, is found 

 at Henderson, Jefferson county, (N. Y.) A dark brown colored Mica, 



