178 
Per Cent. Atomic Quotients. 
Dulce. si ABE OBO. Shakes yi OES a ees 
Alumina, . . 3018 . . 0587 
0:66 2° ate ee 
Peroxide ofiron, 6°35 . . 0079 i ace 
Lime, ... 000 . . 0-000 
Magnesia, ... .\.. 0772) saan O26 
2 ; sues 
Potash, . . . 1240 . . 0262 puede nm : 
Soda,. . . . Trace, . . 0-000 
Loss by.ignition, 5°32... @S91 =. 27). Ise, ee 
99°61 
‘Tf we take the mean of these analyses we find— 
Average Mica of Dublin, Wicklow, and Carlow Granite. 
Per Cent. Atomic Quotients. 
Silica, 6 / yo a AAT Bs 5 O62» | hn eee Ss 
Alumina, 2: 1.) 3091 S20) 55" 0;601 
0667.4. 2 
Peroxide ofiron,. . 527 . . . . 0-066 7 
Times.) 27. 6a.0. 3. 20182 We} 5S 0;029 
Mapnesia, 2775. 5) OO2 ors: <, car Oto ae 
Potash..." s+ ° TOR. sy 03234 Meg ae 
Sodas-2o es Sk? SO GO mes 959 0,029 
Loss by ignition; 3° 2° S86, .. 2. sO620- ° . Steere 
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 
ROSi0, + 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 :— 
a 
