Mineralogy and Petrography. 733 
by the dark zones so characteristic of the mineral in rocks of this 
class; when treated with hydroehloric acid, the dark color of the 
external rim was destroyed, leaving an isotropic substance, which 
the author supposes to be amorphous silica. He explains the ori- 
gin of the zinc by supposing the biotite to have been acted upon 
by the remainder of the liquid magma, from which it had sepa- 
rated, and thus to have lost a portion of its alkaline and alkaline- 
earth constituents, which helped to form the feldspar, forming with 
small plates of biotite the entire groundmass of the rock. The 
iron left by this decomposition separated out as magnetite in the 
dark zine——In a late number of the American Journal of Science 
Mr. Kemp’ describes a dyke of camptonite cutting the rocks in the 
Forest of Dean magnetite mine in Orange county, N. Y. The 
rock differs from the typical? camptonite in containing a larger 
proportion of feldspar and smaller crystals of hornblende. The 
feldspar is an oligoclase with the composition :— 
Si 
1810 Te’ IS Ges 1 LU 89 2a 
— While engaged in studying a peridotite from Little Deer Isle, in 
Maine, Mr. Merrill? noticed the enlargement of its augitic consti- 
tuents by the growth around it of a secondary augite with the same 
optical orientation, but a different color. 
MINERALOGICAL News.—In a very interesting paper on the 
chemical constitution of the different colored micas occurring in a 
pegmatite at Schüttenhofen, Bohemia, Scharizer* records the analy- 
ses of several members of the mica group of minerals, and draws 
some general conclusions in regard to them. The pegmatite is sur- 
rounded on all sides by limestone. Its constituents are pairs 2 in 
three zones, in the first of which lepidomelane and white and brown 
muscovite occur. In the second a yellowish white muscovite, and 
in the third lithium micas. Analyses of these are given as follows:— 
Lepido- Brow Yell.-white Lithium- 
Mus. mica. § 
melane. Mus. 
SiO, 35.31 43.67 44.08 49.26 
ae mer 06 
Al,O, 22.62 36.69 36.83 25.27 
F e0, 5.68 2,10 .48 
e 18.04 .55 74 84 
a 1.19 tr 25 85 
3.69 
CaO 1.99 
` Amer. Jour. Sci., April, 1888, p. 331. 
Woah pir 
‘one , 1888, p. 488. 
. f. Krystallographie, 1888, xiii., p. 449. 
* Cf, Am. Jour, Sci., xxxii., p. 358. dd 
