178 
Per Cent. Atomic Quotients. 
Giles ea AOS. OO RR. . CSE ee 
Aloming, cn.) 3018) 25, «00587 . ; 
Peroxide ofiron, 6°35 . . 0:079 } SSS. (Ca Sane 
PINE, ? riser O00). oe JOMOO 
Magnesia, . . O72 . . 0036\ 
Paragh. «0, Sag: I aoe (9298: ee 
sodas, it 4% teirace vt +), 0°000 
Loss byignition, S32... . OS91 ... Ite we 
99°61 
«Tf we take the mean of these analyses we find— 
Average Mica of Dublin, Wicklow, and Carlow Granite. 
Per Cent. Atomic Quotients. 
Sidi wa eS AAT 2200. SRD «2... Ga 
Alomina,) Sis:.4.. << S0SL S72), ..".2-O60L 
: z 0°667. 2 
Peroxide ofiron,. . 527 . . . . 0-066 } 7 
Tana pee oe = DBS OIPSP So 
Marnena, =. OOS See, 3. Oa | : 
Pou... se « LIRR)... 4 Se eda ! 
Sata Pt 6 SSO GEM 2) CO 
Loss by ignition,. . 566 . . .. 0629 .... 2 
99°76 
«“ There can be little doubt, from the foregoing analyses, 
that the transparent gray Mica ofthe district under considera- 
tion is a Margarodite, having the rational formula 
ROSiO, + 2(R.0,, SiO;) + 2HO. 
The mica which deviates most from this formula is the mica of 
the Three Rock Mountain, and this deviation may possibly be 
due to the presence of grains of black mica, which also occurs 
in the mass of the granite, and of which I was unable to obtain 
a sufficient quantity for chemical or optical examination ; the 
quantity of protoxides in the Three Rock mica is somewhat 
in excess of that required by the formula. 
‘«‘ The angles between the optic axes of these micas were 
carefully determined, and found to be as follows :— 
