sit, Optical Examination of several American Micas. 
-. in the same manner the other axis; the amount or angle of this 
4 ~~ revolution is. the angle between the axes. With this arrange- 
‘ment there is no difficulty after a little practice in obtaining a 
series of measurements on the same specimen, varying from 
each other but a few minutes at most,:without having recourse 
to lenses or other means of more accurately defining, the field of 
observation or reducing the area of the colored circles. Such 
modes of greater accuracy are important for the more delicate 
physical questions previously suggested; but for the purpose.of 
mineralogical determination, the means just described are quite 
sufficient, since it is shown that in a series of specimens from the 
“same locali ity there is generally a difference of angle greater than 
any error of. eee rerion arising from the imperfection of the in- 
= ApeEM name 
mo A 
corresponding differences in optical characters. For this reason 
we briefly recapitulate the divisions which are adopted by Prof. 
Dana in the late edition of his system, and which are also given 
Ser eeniietion of the chemical formulas on p. 118 0 this vol- 
cies of mica now recognized are muscovite, mar- 
arodite, emerylite, euphyllite, margarite, lepidolite, agit 
and biotite. Of these, all but the last are binaxial. Our obser- 
vations will be confined ainly to muscovite, lepidolite, hice 
pite, and biotite. 
1, Muscovite. —Thts name has been proposed by Dana to em- 
brace those binaxial. micas whose angle of polarization is between 
55° and pail excepting however the ‘lithia micas which, having @ 
liar composition and a very high angle, are included under 
He yapecies celia. The terms “ oblique ica” “ common mi- 
“ binaxial mica” formerly applied tothis species now fail 
e Ms distinctive, since we have 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 gran- 
itic rocks. 
2. elite. —This species embraces all the lithia micas, 
group peeseiting however varied chemical characters which vil 
probably be subdivided by future research. ‘They are all bin- 
axial and as far as observed they yield a higher angle than any 
other of the species of this family, being 75°-76°. The blow- 
pipe reaction for lithia as well as its high polarization angle, en- 
able this species to be very readily distinguished. Many of the 
0 gm are easily recognized by their rosy or peach-blossom 
color. 
3, Phlogopite—-This name was first proposed by Breithaupt 
for the — brown mica associated with serpentine which 
ee ee a.) 
