a ae 
382 Optical Examination of several American Micas. 
cular. It is distinguished by its bronzy lustre and dark greenish 
olive color inclining to gray or black. It is imperfectly transpar- 
ent, having by tansmitted light a dusky or hazy appearance. It 
occurs in plates of immense size, which are marked on the cleav- 
age surfaces with rhombic, and triangular figures, (no distinct lat- 
eral planes have however been fo tind, s well as with transverse 
cleavage lines. It is slightly elastit bat very tough. One speci- 
men which the author has seen in the cabinet of Mr. C. M. 
Wheatley in New York is nearly two feet in diameter: Mr. 
_ Horton of Monroe has also furnished the writer with numerous 
very large specimens. No analysis of it has been published, but 
_. the author hopes to present one at a future time. 
The white mica fot Easton, Pa., which is very silvery ed 
slightly elastic and opake in thick — is probably a biotite, 
and, excepting the very similar white mica of Amity, N. Y., 1s 
the only white uniaxial variety yet sth in this country. 
Black micas are almost universally referable to the species 
3. he 
2 biotite, although many micas usually called black in collections 
are : 
reality dark brown and olive green and are frequently refer- 
phlogopite. Unfortunately very few of the: localities of 
ariety of color found in cabinets are labelled... I have one 
ym. Moors Slide on the Ottawa in Canada, furnished me by Mr. 
Hint of the Canada Geological Commission. Another black mica 
is found in St. Lawrence Co., N. Y., of which specimens were ob- 
tained by the author from. the cabinets of Mr. Wheatley and of 
the N. Y. Lyceum in New York. Two black micas from the 
Cambridge cabinet are unaxial, locality not know 
Geological relations.—It is worthy of notice thot the species 
muscovite is found almost entirely in granitic rocks ; in no instance 
as far as I have seen, has a specimen of this species been found in 
alime rock. On the other hand, the phlogopites, with a single 
exception, so far as has been ascertained, are found in limestone 
and often in dolomitic beds. _ The biotite is less well pare 
chondrodite ~~ fibrolite, an interesting confirmation of the sug- 
gestion here put forth. Can this distribution be unco rhe 
with ed chen il composition of the several compounds? 
é 
character of the phlogopites would seem to 
: the doloweiee position of the species, while the absence < this 
element i in Be muscovites is a negative fact of equal signmiiean 
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