372 



DESCRIPTIVE MINERALOGY. 



Fig. 393. 



Fig. 394. 



observed at this locality, with the direction 

 of the cleavage. Figs. 393 and 394 repre- 

 sent compound crystals with five and six 

 sides, two of them forming a reentering 

 angle. All these crystals are usually very 

 perfect, and have their sides smooth and 

 polished. 



This mica has been analyzed by Prof. Von Kobell of Munich. The following are the 

 results :* 



Silica, 40 . 00 



Alumina, ^ 16.16 



O.xide of iron, 7.50 



Magnesia, 21 . 54 



Potassa, 10.83 



Oxide of titanium, . 20 



Fluoric acid, 0.53 



Water, 3.00 



According to Dr. Horton, the mica from this locality has long been used as a substitute for 

 coloured glass, for spectacles. Almost any shade of colour can be obtained by using a larger 

 or smaller number of plates. 



Near West-Point, mica is of frequent occurrence in granite. It is sometimes black ; at 

 others, bronze-coloured. Occasionally it is met with in six-sided prisms. 



Fig. 395. Six miles southeast of the village of Warwick, in the town of the 



same name, black crystallized mica is found in a vein of feldspar in 

 granite. It occurs in crystals and plates, which are sometimes a foot 

 in diameter. Fig. 395 represents one of the forms observed. M on Z 

 150° ; M on it 120°. 



Brown mica is found in white limestone near Edenville, while the silvery variety occurs 

 near the village of Amity. 



A mile northwest of Edenville it occurs in white limestone, in perfect hexagonal prisms 

 with the angles 120°, and in rhombic prisms. 



In the same vicinity, there are specimens of a silvery white colour, resembling mica, but 

 rather more brittle than is usual with that mineral. It melts before the blowpipe into a white 

 enamel, and agrees with the characters of Lithion-mica, as given by Dr. Turner. It seems 

 to pass into talc. 



Putnam County. Black mica, in rhomboidal and si.x-sided crystals, is found at Coldspring. 



Brcwster^s Edinburgh Journal of tScience. IX. 175. 



