574 The American Naturalist. [July, 
that with increasing percentage of iron the index of refraction increases, 
while the plane of the optical axes is changed from the base (001) to 
the macropinacoid (100). A specimen containing 26.58% FeO shows 
an optical angle of 21° 53’ in the basal plane for thallium light, is uni- 
axial for sodium light, and has an angle of 15° 3’ in the macropinacoid 
With 35.05% FeO the crystals are found to be negative, whereas those 
with less iron are optically positive. It is suggested that in the pure 
manganese molecule, the change may be.found so great that the brachy- 
pinacoid is the plane of the optical axes. 
Native Sulphur in Michigan.—Scherzer® reports an occurrence 
of sulphur a mile west of Scofield, Monroe Co., Michigan. It is found 
in a stratum of impure cavernous limestone about one to three feet in 
thickness. The pockets, varying from a fraction of an inch up to three 
feet in diameter, are often lined with calcite and celestite crystals with 
bright lustrous masses of sulphur toward the center. The removal of 
about an acre of this bed has yielded 100 barrels of pure sulphur. The 
sulphur seems to have originated from hydrogen sulphide which is 
abundant in the waters of the neighborhood. The hydrogen sulphide, 
in turn, may be a product of decomposing organic matter. 
Leadhillite Pseudomorphs at Granby, Mo.—The occurrence 
of leadhillite at Granby in the form of pseudomorphs after calcite and 
galena is made the subject of a note by Foote. Scalenohedrons in a 
chert calamine rock are composed usually of pure cerussite; more. 
rarely the substance is found to be leadhillite. Galena cubes replaced 
by leadhillite were also observed. In these cases the secondary min- 
eral is usually mixed with remnants of the original galena, producing 
a “gray amorphous mass.” In a few specimens the leadhillite is 
pure. 
fora hs oi on 
Celestite from Giershagen.—According to Arzruni and Thad- 
déef* the axial ratio of “normal” celestite is a: b:¢e— .78093:1: 
1.28324. The mineral from Giershagen, which appears to be chemically 
pure Sr SO,, has the ratio a: b: c = .77962 : 1: 1.28533. The mean of 
four determinations places the specific gravity at 3.9665. The optical 
angle of “ normal ” celestite is given as 2 V,x,==50° 34’. This inves- 
tigation adds another to the list of chemically pure compounds whose — 
EA 
D A a NN a EREN TEAN Sa SEUA aP EE e a EN A Saa T gr 
5 Am. Jour. Sci., L, pp. 246-248, Sept., 1895. 
€ Am. Jour. Sci., L, p. 99, August, 1895. 
t Zeitschr. f. Krd. XXY, pp. ~a 1895. 
