PHYSIOGRAPHY. CLASS n. 



3. Mica forms one of the constituent parts of various 

 rocks, as granite, gneiss, mica-slate, and of several varie- 

 ties of porphyry. They form sometimes more or less 

 considerable nodules and concretions in these rocks, and 

 then contain imbedded crystals of prismatic Topaz, rhom- 

 bohedral Tourmaline, and other species. As single crystals, 

 they appear not unfrequently imbedded in granular lime- 

 stone, in basalt and wacke, in implanted crystals upon the 

 specimens ejected by Mount Vesuvius. Several varieties 

 of mica accompany in metalliferous beds, the ores of tin 

 and scheelium, and they occur likewise in ancient veins, 

 which consist of those species which are contained in the 

 rocks which they traverse. 



4. Remarkable varieties of Mica are found in Siberia, 

 particularly cleavable ones in large individuals, crystallised 

 ones at .Zinnwald in Saxony, possessing two axes of double 

 refraction. It is also found in the Hb'rlberg in Bavaria, in 

 imbedded globules in Moravia, in Mount St Gothard in 

 Switzerland, at Finbo in Sweden, in Pargas in Finland, 

 here with curved faces of cleavage ; at "Wiesenthal in Sax- 

 ony, and Joachimsthal in Bohemia, imbedded in basalt 

 and wacke, &c. At Mount Vesuvius, crystals of Mica 

 with one axis, often of considerable size and transparency, 

 occur in the drusy cavities of the ejected specimens. It 

 occurs besides in great variety in many other countries. 



5. Perfectly cleavable varieties, which are transparent at 

 least if reduced to a considerable tenuity, are used in Si- 

 beria, in Mexico and Peru, instead of window glass ; and 

 from the first of these countries, they have on that account 

 received the name of Muscovy glass. In Siberia, the ex- 

 traction of the large crystalline masses imbedded in granite, 

 forms an object of mining. It is sometimes used for vari- 

 ous optical purposes, and enters the composition of the 

 artificial avanturine. 



6. Lepidolite^ a substance included by some authors 

 within the species of mica, possesses two axes of double 

 refraction, and its forms therefore are likely to belong to 



