~ 
J. P. Cooke, Jr., on Cryophylilite. 221 
as 
investigations of De Senarmont on the optical relations of the 
micas 
presented. The magnesian micas phlogopite and biotite are suf- 
ficiently distinguished by the character of their bases, and con- 
form very closely to the general formula 2RO, SiO, ; bnt there 
does not seem to be any good ground for distinguishing between 
Ax. Jour. Sct.—Srconp Seures, Vou. XLIII, No: 128.—Maren, 1867. 
29 
