PHYSIOGRAPHY. 39 



Mica. 



different colors when viewed in these directions ; for in- 

 stance, oil-green in the first, and liver-brown in the second. 



Sectile. Thin laminae are elastic. Hardness =2-0 . . . 

 2-5. The acute edges of the laminae, however, will some- 

 times scratch Fluor. Sp. gr. =2-949, a greenish black va- 

 riety, in large individuals. =2*832 of Lepidolite. 



Compound Varieties. Twin-crystals ; axis of revolu- 

 tion perpendicular, face of composition parallel to one of 

 the faces of M. The composition is often repealed paral- 

 lel to both the faces of M. The individuals rarely project 

 beyond the face of composition. When they do not, the 

 composition is only obvious from the intersecting striae upon 

 P, and from the cleavages produced. In this way the star- 

 like crystals, of a large size, found at Acworth, (N. H.) 

 are produced. The same composition is frequent in mas- 

 sive varieties, in large individuals, particularly in the large 

 cleavage forms found at Munroe, (N.Y.) and at Mendham, 

 in New Jersey, whose numerous cleavages are easily 

 explained upon this supposition. Globular forms, both 

 imbedded and implanted : surface of the latter rough ; 

 composition columnar, sometimes joining in a second curved 

 lamellar composition. Massive : composition granular, of 

 various sizes of individuals; or also imperfectly columnar, 

 faces of composition irregularly streaked and rough. 



1. In the present species must be included the old species Finite, ori- 

 ginally described from France. It appears to be only an impure, crys- 

 tallized Mica, contaminated by chloride Talc, or in some cases pure 

 Mica, which is partially decomposed. The annexed figures represent 

 the bases of very distinct crystals, (some of which are at least an inch in 

 diameter,) from Lancaster, (Mass.) and from Haddam, (Conn.) 



