1022 General Notes. - [Nov. ; 
The relations existing between them may be best shown by 
supposing two of the univalent groups (AIO) to replace one 
(R'ZO,). Assuming this, the formulas may be written: 
R R” (SiO ) = Rockport mica. 
“u 2 45 
R (A10) R” 
12 2 2 
R’ (A10) R” (SiO ) = Litchfield mica. 
10 Se 45 
(SiO ) = Baltimore mica. 
46 
In the same paper there are also given analyses of iron-micas 
from Auburn, Me., and Pike’s Peak, Colorado, and one of a 
muscovite from Alexander County, N. C., containing 1.10 per 
cent. of Ti —Howlite—This mineral was first identified by 
Professor H. How,’ of Windsor, Nova Scotia, who named it silico- 
borocalcite. Messrs. Penfield and Sperry,? having recently come 
into the possession of a comparatively pure specimen of the min- 
eral, have re-examined it. They find it to be composed as 
follows: 
; SiO, B,O, CaO Na,O K,O H,0 
15.33 44.52 27.94 0.53 org 11.55 
They regard it as a distinct species with the formula H,Ca,B,Si0w 
—Cassiterite —The tin-stone of Mexico is divided by Professor 
Genth 3 into two varieties, a red and a yellow variety. Both are 
supposed to have been formed by precipitation from solution. 
ie red variety is found in very small crystals, with a hexage 
habit. Analysis shows it to consist of dioxide of tin, containing — 
ies of arsenic — 
* Philos. Mag., iv. xxxv 2 2 Amer. Jour. Sci., 1887 P. Pais - 
pe Pin qa from the Choma! Laboratory of the University ee? : 
XXIX., 1687, P. 4, ; 
