850 The American Naturalist. [October, 
stance whose six orthorhombic pyramids build up the perfect dodeca- 
hedral crystals. Its mean index of refraction is 1.74 and its optical 
angle 2 V = 56° 5. The author regards his observations as settling 
the question as to the cause of optical anomalies in garnet in favor of 
his own theory and in opposition to the theories of Klein and of 
Brauns, the former of whom regards them as due to the dimorphism 
of the garnet molecule and the tension resulting from its attempt to 
pass to a more stable form than that in which it crystallized, and the 
latter as due to isomorphous mixtures. 
Examination of thin sections of beryl crystals from the Ilmenge- 
birge in the Urals leads Karnojitzky" to the belief that the anomalies 
discoverable therein are dependent directly upon the limiting faces of 
the crystals. When these differ the character of the internal struc- 
ture differs, as is also often true in the case of garnet. An optically 
anomalous beryl crystal consists of several elemental individuals, the- 
number and position of which correspond closely with the number 
and positions of the limiting planes of the crystals. The positions of 
the individuals preclude the notion of twinning. The author thinks 
the anomalies due to the isomorphous mixture of the beryl substance 
with some other, probably tourmaline. In a section of dioptase cut 
parallel to the base the same investigator” found uniaxial areas distrib- 
uted among the normal biaxial areas in such a way as to convince him 
that the interior structure of the mineral is determined to some extent 
by its exterior form, as in the case of the beryl. 
Mineralogical News.—Genth and Penfield? have obtained 
hitbnerite crystals from two localities near Silverton, Col., from White 
Oaks, N. M., and from Nye Co., Nev. Those from the North Star 
Mine, near Silverton, were doubly terminated, so that by their meas- 
urement the axial ratio, 8362: 1 : .8668, was determined. Cleavage 
sections parallel to o Pæ extinguish at about 17° from ¢ in the obtuse 
#, which direction is the axis of least elasticity. Pleochroism is 
marked, being green parallel to C and yellowish brown parallel to B. 
Density is 6.713 and composition WO, = 74.75 ; FeO — 2.91; MnO 
= 21.93; CaO =.11; MgO = tr. Bismuthite from the phenacite 
locality of Mount Antero, Col., and hessite from the Refugio Mine, 
Jolisco, Mex., are also briefly described by the same authors. They 
were, however, too impure to yield good analytical results. A natrolite 
NZeits. f. Kryst., 1891, xix, p- 209. 
12Ib., 1891, xix, p. 593. 
“Amer. Jour. Sci., March, 1892, p. 184. 
