Chemistry and Phystes. 423 
from the blending of the two hyperbulas, pa to the thinness of 
the plate. The er of the axes is but tens and only per- 
— in the thicker lamin It is pbvioks, there- 
e ee are gonerly, if not invariably, twinned, and the 
twinning is the cause of this most remarkable variation in the 
optical angle, a will explained at len connection — 
ur description of Spent On this last salaidal the same 
Biotite micas, we give below: Ist. The results of the aa of 
Jefferisite by Prof. Brush. 2d. The same results, calculated for 
e anhydrous mineral. 3d. The = of an analysis of a Biotite 
mica, from Pargas, Finland, by Svanberg. In each case I have 
added the amounts of oxygen in the patos oxides, to show the 
atomic ratio 
Si Al #e Fe Mg “a H 
(1) 37°10 17:57 10°54 1°26 19°65 0°56 0°43 13°76=100°87 
19°78 S18. $16 “206 TRG AG. ae ee 
— $$$ 
19°78 11°35 8°37 13°23 
5: 3° 2: 3° 
Si Al #e Fe © Mg oe 
(2) 42°94 90:33 12°20 1:46 22°75 ‘66 ‘50==100°84 
22-90 9°47 3°66 32 910 19 ° 
‘a etnoaeninien seasons 
22°90 13°13 9°69 
5 3° 2° 
Si Al fo Mn Mg Co kK 2H 
(3) 42:58 21-68 10°39 -75 10°27 1°04 8°45 3°35 %51= 99°01 
22°71 10-10 3:12 ‘17 4°11 ‘30 1°48 2°98 
eS : iat 
22°71 13°22 8°99 
5* 3° 2° 
The general symbol of Jefferisite deduced from (1) would be,— 
a R, cd Si, . 6H, 0. 
ence is to be found in the fact that the mica contains cabin 
and basic hydrogen, in oan’ of more than one-half of the magne- 
ie he =e the Biotites resent a very wide Boric k Pg Sen 
rati n the amounts of the protoxide and ae e 
ate Sot —Tua Ss, Vou. VII, No. 40.—ApPri, 187: 
