oe 
Optical Examination of several American Micas. 375 
York. This species is distinguished by a polarization angle be- 
13°-16°; it rarely falls below 10°; in all cases the two axes are so 
teemed a uniaxial mica. lhe crystalline form is trimetric, and it 
occurs often in elongated and tapering hexagonal prisms, some- 
: r red, some- 
times greenish yellow and rarely white. Its cleavage resembles — 
__ that of muscovite, but the lamin are not generally so elastic. In 
chemica! constitution it is a distinct compound although but few 
analyses have yet been made of this species. Like the biotite it 
is remarkable for the amount of protoxyd bases which it contains 
and the small quantity of alumina—giving for the ratio of the 
oxygen of its protoxyds, alumina and silica, as deduced by Rose, 
18:12:30 =1: 3:13, (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 particularly in northern New: York, in Canada, and 
in Morris and Sussex counties in New Jersey. One of the 
Most noted localities of this species is Edwards in St. Lawrence 
county, N. Y., where it is found both colorless, of an eminently 
Silvery luster, and also of a rich brownish yellow color. : 
4. Biotite.—This species includes the uniaxial or hexagonal mi- 
cas. Most of the vagieties of this species are of a dark color—often 
black or greenish black and transparent only in very thin lamine. 
Owing to this prevaléht dark color it is often difficult or quite impos- 
sible to obtain satisfactory evidence of the optical character, and 
there is little doubt that some localities quoted in this article as fur- 
nishing uniaxial micas, should be in fact.elassed among the phlo- 
gopites. Only one American variety of this species has yet been 
analyzed—viz., that from Monroe, N. Y., by von Kobell. | "hey 
are generally magnesian micas and have for the oxygen ratio of 
their protoxyds, alumina, and silica, the ratio 1: 1:2 = + 
#Si. This species and those anomalous specimens which are 
classed under it in the present article, but which probably belong 
elsewhere, offer interesting subjects for chemical examination. 
Beside the phlogopites and biotites, properly so called, there are 
Several micas which have fallen under my observation in this re- 
Search which are anomalous in character. These present under 
the influence of polarized light an elliptical colored image, in which 
however it is not possible to bring out clearly the two poles of a 
