56 Fietp CoLtumBIAN MuseumM—GeEo.oey, VoL. III. | 
of the lamellae is such as to bisect the above-noted cleavage angle 
of 53°. Single cleavage cracks || with the lamellae are also occa- 
sionally seen. 
The interpretation of this structure is difficult, but the following 
may be suggested: The cleavage cracks at angles of 53° are those of 
prisms of which the crystal groups are composed. These prisms have 
the symbol 540, corresponding to an angle of 52° 44°. Thisis a new 
form for zoisite. The twins of which these prisms are made up are 
formed on ¢ as the twinning axis and the twinning plane is some 
highly inclined brachy-dome such as ¢ (061). 
The lustre of fragments of the mineral is vitreous and the frac- 
ture sub-conchoidal. Hardness 6.5 and specific gravity, determined 
with a chemical balance, 3.32. The mineral fuses B. B. at 3 with in- 
tumescence, to a brownish enamel and is only slightly attacked by 
hydrochloric acid. Qualitative tests showed it to be essentially a 
hydrous calcium aluminum silicate, from which the water could be 
driven off only by strong ignition. Quantitative analysis by Mr. H. 
W. Nichols gave the following result: | 

Ratio 
9i0, 38.15 3.02 
ALO. 29.50 } 
Fe,O, 4.60 ( ass 
MuO 0.55 
CaO oy ty 
MgO 0.63 aie 
H,O enh 1.00 
K-07} ' 
Na,O f 3 
99.99 
These ratios lead to the formula H, Ca, Al, Si, O,,, which is 
that usually accepted for zoisite with the addition of one molecule 
of water. For the determination of the water both of Penfield’s 
methods* were employed. By the first method, that of heating in a 
blast lamp, 1.81% of water was obtained. The mineral did not fuse. 
By the second method, which consists in heating the tube containing the 
assay in an oven of fire-brick lined with charcoal, an additional percent- 
age of water amounting to 1.95% was obtained. Under this treatment 
the mineral fused completely. The close similarity between the per- 
centages of water obtained by the two methods, each corresponding to 
one molecule, suggests that the molecules may be differently com- 
bined. Thus one may be united with aluminum and the other with 
* Amer. Jour. Sci. 1894, 3rd ser. Vol. XLVIII, pp. 30-37 
