Jrom the Tilly Foster Iron Mine. 453 
11. As to the original mineral, the only additional remark I 
now venture to make is that probably it was not any known 
mineral species. Chrysolite is orthorhombic; and has cleay- 
age in one direction rather distinct, becoming more so and open- 
ing as alteration advances; and also it is green and contains 
protoxide of i a ge See characteristics in favor of its being the 
original mineral. But it has no ingredient that could i 
produced the air-cells its escape; and no observed facts 
authorize the opinion that its crystals and crystalline masses 
could have been resolved into tables like those in the pseudo- 
morph here described. 
Anhydrite is orthorhombic and has a perfect cleavage in one 
direction, but it has good cleavage in other me also; an 
it contains no iron and no ingredient to escape as a gas. 
B. PSEUDOMORPHS CONSISTING OF BRUCITE OR HYDRATE OF MAGNESIA. 
12. Brucite Pseudomorphs (?) after Dolomite. 
Brucite is sometimes found making a thin covering over the 
exterior of the masses of chondroditic rock, like the serpentine, 
onda product, resulting from the alteration of some magne- 
sian mineral in the ore- -bed, for it is not among the constituents 
of the rock-masses; and also that it was formed cotempora- 
neously with the serpentine. 
It is also found, as has been stated, constituting with dolo- 
mite the filling of veins in the ore-bed, the filling being some- 
times part dolomite and part foliated brucite, one interpene- 
trating the other. In such cases both minerals cover splendent 
crystallizations of chondrodite, and each contains isolated crys- 
s of the same mineral equally splendent. The brucite effer- 
ee more or less, owing to the presence in it of some dolo- 
brucite; moreover, as stated on the page referred to, both the 
Seen and brucite are altered, over the exterior of the mass, 
serpentine. The fibers of the fibrous surfaces cross at angles 
of 60° and 120°, and minute stellar forms occur in some parts 
of the dolomite. The fact that the fibrous mineral is essen- 
pally brucite, as suggested by its Sieenon 3 is sustained 
Ly a partial chemical analysis made by Mr. G. W. Hawes. 
e found it to contain a trace of silica, Sag probably, 
Am. Jour, eee Vou. VIII, No. 48.— Dec., 
