Wey: ; 
ree 
-a@ uniaxial cr 
376 Optical Examination of several American Micas. 
binaxial mica, nor, on the other hand, the symmetrical cross of 
1 he divergence is too constant and too 
regular to allow the supposition that the ellipticity is due toa 
mal-position of the lamine or to a separation between the thin 
plates (remarked on as a cause of irregularity in certain crystals 
very large rhombic’erystals oblique from an obtuse edge. P:M= 
112° — 1154°, M: M= 122°~ 125°, the angle of the basal edges Is 
119° 30’. Plane angle of P 119°. It hasa cleavage parallel to the 
longer axis. The obliquity of the optic axes appears to be nearly 
as great as that seen in some phlogopites of equal thickness, but 
the dark color of the mineral prevents a satisfactory examina- 
tion. Should the character ofthis mica be confirmed by a set © 
good analyses, it must in all probability form a distinct species a8 
suggested by Dana.* This variety is not to be confounded with 
the well crystallized: mica of Greenwood furnace which, as seet 
in ordinary specimenis, is oblique from the acute edge (sections of 
distorted acute rhombohedrons) and which is regarded as a unl 
axial mica. 
Enphyllite, margarodite and emerylite have hitherto been 
found in quantities too ineonsiderable and in specimens generally 
too poorly crystallized to furnish many measurements. 
* Mineralogy, 
SY, Pp. 690. eer 
+ For the soe Son of these species, see Dana’s Mineralogy, and also this v0 
4-118. se 
ume, pages 114-118. 
- 
