Cross and Hillebrand—Cryolite from Colorado. 29% 
they are but different phases of alteration from a common 
source. The granular mineral occurs in sufficient purity to. 
afford material for chemical analysis, and its individuals are 
large enough to admit of the preparation of thin sections with 
detinite relations to the cleavage planes. The analysis first 
proved the identity of this mineral with prosopite, the optical 
properties shown by the thin section confirmed this determina- 
tion and quite recently a few minute crystals were found in one 
Specimen which agree with the published data on the Saxon 
mineral. As the identification of this rare species, particularly 
In its present association, is a matter of considerable interest, 
we will describe it somewhat in detail. 
PROSOPITE. 
Crystalline form and physical properties.—The minute crys- 
tals found in a single specimen from vein B as well as those 
_ therefore make no pretensions to crystal- 
lographical accuracy. The crystals are colorless and trans- 
parent, have uniformly a tabular form through the develop- 
tent of 7-4 (010) and show plainly the prism and two pyramids. 
which may be considered as 1 (111) and —2-2 (211) for 
extinction takes place nearly or quite parallel to the edge 
of —2°2, which is, according to DesCloizeaux* and Groth,f the 
Position of the bisectrix. : : 
Thin sections prepared as nearly as possible perpendicular to. 
the edge of the two cleavage faces in the irregular granular 
Individuals, show that the angle of the cleavage planes is very 
nearly 135°, and that extinction takes place parallel to the 
direction bisecting that angle. This behavior agrees perfectly 
with the statements concerning prosopite, according to which 
the chief cleavage is parallel to —2°2, the angle of which is 
about 134°. The present material does not allow of a definite 
settlement of the question of the crystalline form of prosopite, 
* Bull. Soc. Min. de Fr., y, 317. + 1. c., p. 290. 
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