4 
274 = Cross and Hillebrand—Cryolite from Colorado. 
CRYOLITE. 
Within the mass of fluorides in vein A there is a surpris- 
ingly large amount of fresh cryolite. It can be obtained in 
solid pieces several inches in diameter, and the process of alter- 
ation can be readily followed from the perfectly fresh material. 
It occurs in massive aggregates of crystalline individuals which, 
as shown by the continuous cleavage surfaces, are often two to 
three inches in diameter, and are never very small. The fresh- 
est substance has usually a delicate pink or even decidedly 
rose color; less frequently a faint greenish tinge, and none so 
far obtained has the snowy whiteness or the clearness of the 
s 
effected as is normal. The cause of this lies, undoubtedly, in 
the complicated polysynthetic twin structure revealed by the 
microscope. “ 
No crystals of cryolite have been found, and a thoroughly 
satisfactory study of the laws of twinning which appear in this 
massive material would require much more time than we have 
been able to devote to it. In some of the thin sections which 
and parallel to the diagonals of the prism as indicated by the 
cleavage fissures. This structure evidently indicates the law 
of twinning frequently noticed in the Greenland eryolite, 
matic cleavage surfaces. similar laminated structure 
appears, and extinction takes place at 30° to 33° from the 
twinning line, in opposed directions in alternate lamine, 31 
ically required in sections parallel to a prism face (Groth). 
Such a section usually shows two systems of lamin situated 
nearly at right angles to each other but seldom, if ever, cross 
ing. The second system seems probably to represent a twin- 
ning parallel to the base. Associated with these two systems 
