MINERALOGY. 7g 



of the finest mica quarries on the continent, and much mica of the best 

 quaUty is extracted from them. Mica is, moreover, a most common and 

 interesting rock constituent. 



We have four species of mica, — biotite and lepidomelane (black micas), 

 which are uniaxial, and muscovite and lepidolite (white micas), which 

 are biaxial. These micas often occur together; but when their crystals 

 are in contact, there is usually some simple relationship between the 

 axes of the two crystals. It may be stated in general, before describing 

 the species, that all these micas have the same prismatic angle. Biotite 

 and lepidomelane are hexagonal and uniaxial, and have a prismatic angle 

 of 120°; and muscovite and lepidolite are biaxial and orthorhombic, but 

 have the same prismatic angle of 120°. The color of our granites, as well 

 as of many of our schists, is largely due to the kind of mica they contain. 

 Granites that contain the white biaxial micas are light colored, while the 

 black micas make the granite dark colored, and the darkness is propor- 

 tional to the quantity. Some granites are nearly black, on account of 

 the large amount of black mica they possess. 



54. Biotite [K^ (Fe, MgY AV Si' 0,i\. 



Black magnesia iron mica. 



This mica varies very much in color, according to the proportion of 

 iron that it contains. It is usually nearly black. It is of no economic 

 value ; but as has been already stated, it makes dark-colored granites, 

 which, by some, are more admired than are the white granites. The 

 mica in granite is generally in very small scales; but in the great gran- 

 itic veins that occur at Grafton, Alstead, Acworth, etc., all the ordinary 

 constituents of granite are found in very large crystals, and among them 

 are interesting specimens of biotite. The biotite of these veins is very 

 rich in iron, and is black by reflected light, though thin flakes of it are 

 brown by transmitted light. I analyzed a specimen of it at one time 

 from a granite vein in Middletown, Conn.,* and found it to be an uni- 

 silicate, containing 35.61 per cent, of silica, 20.03 of alumina, 21.85 of 

 iron protoxide, 5.23 of magnesia, and 9.69 of potash, with quite a Hst of 

 accessory elements, among which were lithia, fluorine, and titanic acid ; 

 and, as the physical appearance of the micas in all our granite veins is 



* American yottrnal of Science, \\\,\o\ xi, p. 432. 



