1885.] Mineralogy and Petrography. 599 
of regular crystals are in many cases due to chemical impurity, 
especially the crystallizing together of isomorphous compounds, 
lum, rocksalt, garnet and other substances are described. The 
same reason is assigned for uniaxial minerals exhibiting a biaxial 
character. 
W. Voigt,' of Gottingen, has developed mathematically a theory 
to account for the peculiar interference figures exhibited by cached 
pleochroic (idiocyclophanic) crystals recently described by 
tin (Zeitschr. f. Kryst., 11, p. 449). 
Tue FELDSPARS.—Professor Des Cloizeaux, of Paris, has just 
published at Tours a valuable memoir of ninety-two pages, entitled 
“Oligoclases et Andésine,” being an enlargement of his former 
paper, “ Nouvelles recherches sur les propriétés optiques des oligo- 
clases,” which appeared in 1880, and a direct continuation of his 
still more recent studies on albite? (vid. NATURALIST, 1884, p. 184). 
The latter paper describes thirty-four specimens from different 
localities, of which ten are American (Mineral Hill, Pa., Moriah, 
, Middletown, Ct., and Canada). In the present communi- 
cation the number of o oligoclase specimens examined has been in- 
creased from forty-four to sixty-six. These are divided into the 
following four classes : 
Ist. Anomalous oligoclase or oligoclase-albite. —The surface per- 
pendicular to the plane of the optical axes truncates the acute 
edge P:M., making with P an angle of 93°. The extinction on M 
makes an angle of 6°-12° with the edge P:M, in a positive sense 
according to Schuster. Oxygen ratios vary between I: 3: I 
3: 10.7., corresponding to the mixtures. Abs An ae 
nh. 
2d. Anomalous oligoclase——Plane of the optical axes is parallel 
to the basal pinacoid or perpendicular to the brachypinacoid. 
Extinction on M is positive, 6°-9° inclined to the edge P:M. 
xygen ratios vary from I: 3: = to I: 3: 9, corresponding 
to the mixtures Ab, An;—Ab, 
3d. Normal oligoclase. \ ee perpendicular to the opt. axial 
plane truncates the obtuse edge P:M, making with P an angle of 
g8°-104°. The extinction on M is positive, inclined 1°-6° (gen- 
erally 2°—4°) to the edge P:M. Oxygen ratio is 1: 3: 9:= Ab, 
a Andesine.—Surface perpendicular to the plane of the optical 
axes truncates the od/use edge P:M, making an angle of 110°-120° 
with P. The extinction on M is negative, varying from 1°-10°. 
Sp. Gr. = 2.67. Oxygen ratio 1: 3: 8, corresponding to Ab, An,. 
The first-class includes nine specimens examined, of which one 
is American (Colton, N. Y.). The second-class includes eleven 
1 Neues Jahrbuch fiir Min., etc., 1885, 1, 119-141. 
2 Bull. Soc, Min. de France, 1880, 111, 157. 
3 Ib., 1883, VI, 89-121. 
