388 SILICATES. Analyses of biotite a from Miask, b from Monroe, c from Karosulik, d from Bodenmais, all by v. Kobell, e from Pargas by Svanberg,/from the Baikal by H. Rose Silica . . . . Alumina . . . Red oxide of iron . Magnesia . . . Protoxide of iron . Potash . . . . Protox. man. . . Hydrofl. ac. . . Tit. acid . . . Water . . . . a b C 4,2-12 40-00 41-00 12-83 16-16 16-88 10-38 7-50 4-60 16-15 21-64 18-86 9-36 — 6-05 8-58 10-83 876 —■ — — — 0-53 — — 0-20 — ro7 3'00 4-30 d e / 40-86 42-69 42-01 15-13 21-68 16-06 13-00 10-39 493 22-00 10-27 25-97 (lime 0-26 -) 8'83 8-45 7-55 — 0-75 — (p 0’5l) 0-68 Ca 056 — 0-44 335 — Of the following varieties of mica, having nearly the same chemical constitution as biotite, the forms and optical pro perties are unknown. Mica g from Sahla and h from Rosendal near Stockholm by Svanberg, i from Yesuvius by Bromeis, k from the Zillerthal by Yarrentrapp, I from Miask by H. Rose■ 9 h i k 1 42-65 44-41 39-79 39-85 40’00 12-86 16-86 16-01 16'07 12-67 o-io 8-30 13-21 19-03 25-39 11-26 24-51 16-60 16-70 036 — — — — 7-11 20-71 — T1 1-63 1-06 0-46 — — 063 — 0-90 0-87 0-42 — — 0-43 — — — 6-03 4-05 8'79 13-68 5-61 062 0-41 0-51 FII 210 3-17 113 0-75 — — Silica . . Alumina . Aluminium Magnesia . Magnesium Protoxide of iron Protox. mangan. Lime Calcium Potash Pluorine Water . In k potash 13-68 includes soda and loss. In imbedded and attached crystals, usually very thin in a direction perpendicular to o ; in scaly and granular aggrega tions. Is much less widely diffused than the biaxal mica. Is found in the miascite, and in the veins of granite which traverse it, in the Ilmen mountains, in several parts of the Ural in chlorite slate, in large crystalline plates with inalacolite on the Baikal, at Monroe in New Jersey, Karosulik in Green land, in the older masses ejected by Yesuvius. The combina tion oaswvz was observed by Marignac in mica from Vesuvius. Some varieties of mica exhibit two optic axes, making with
