of several American Micas. 343 



other oblique and binaxial micas which belong to different 

 species. The optic axes in this species lay in the direction 

 of the longer diagonal of the prism. It is much the most 

 abundant variety, and is commonly found in granitic rocks. 



2. LepidoUte. — This species embraces all the lithia micas, 

 a group presenting, however, varied chemical characters 

 which will probably be subdivided by future research. They 

 are all binaxial, and as far as observed they yield a higher 

 angle than any other of the species of this family, being 

 75°-76°. The blowpipe reaction for lithia, as well as its high 

 polarization angle, enable this species to be very readily dis- 

 tinguished. Many of the varieties are easily recognised by 

 their rose or peach-blossom colour. 



3. Phlogopite. — This name was first proposed by Breithaupt 

 for the yellowish-brown mica associated with serpentine, 

 which is found at Natural Bridge, near Diana, in Jefferson 

 county, New York. This species is distinguished by a po- 

 larization angle between 7° and 18°, the angle most com- 

 monly observed being 13°-16° ; it rarely falls below 10° ; in 

 all cases the two axes are so near that both can be distinctly 

 seen in the field at one view, and if examined in thin plates, 

 and by a casual observer, it would be esteemed a uniaxial 

 mica. The crystalline form is trimetric, and it occurs often 

 in elongated and tapering hexagonal prisms, sometimes of 

 enormous size, as in the well-known individuals from Pope's 

 Mills, — specimens of which in the writer's possession are 5 

 by 8 inches in thickness, and perfect in form. The coloiu* is 

 usually yellowish-brown, bronze-yellow, and deep copper-red, 

 sometimes greenish-yellow, and rarely white. Its cleavage 

 resembles that of muscovite, but the laminae are not gene- 

 rally so elastic. In chemical constitution it is a distinct com- 

 pound, although but few analyses have yet been made of this 

 species. Like the biotite, it is remarkable for the amount of 

 protoxide bases which it contains, and the small quantity of 

 alumina, — ^giving for the ratio of the oxygen its protoxides, 

 alumina and silica, as deduced by Rose, 18 : 12 : 30 = 1 : § : If 

 (more exactly 7 : 4 : 11, according to Craw), while in the 

 muscovites it is generally 1 : 12 : 16. Its localities are much 

 more numerous than was at first supposed ; they abound 



