94 W. Oross—Sanidine and Topaz from Colorado. 
Art. XIII.-—Communications from the U. S. Geological Survey, 
Rocky Mountain division.—V. On Sanidine and Topaz, etc., in 
the Nevadite of Chalk Mowntain, Colorado; by WHITMAN 
Cross. 
CHALK MOUNTAIN is situated where Lake, Eagle and Sum- 
mit, Counties join, and is also upon the boundary of the maps 
of the Mosquito Range and of the Ten-Mile mining district, 
which are soon to be published with monographic reports by 
the U. S. Geological Survey. The description of the nevadite, 
which forms the mass of the mountain, will therefore be brief, 
and all references to manner of occurrence, etc., omitted. It is, 
however, thought desirable to describe at once the following 
interesting minerals occurring in the rock. 
The nevadite, while showing local variations in structure, 
of these notes. 
The lustrous surface is in the ortho-diagonal zone and in- 
clined a few degrees to the ortho-pinacoid, as is evident, in the 
Carlsbad twins, usually polysynthetic, the luster reaching its 
maximum of brightness simultaneously in alternate plates. 
Microscopic investigation shows a most perfect parting parallel 
to the surface of luster and with a knife blade flakes can be 
split off in this direction even more readily than paraliel to the 
basal cleavage plane. Thin plates parallel to the base (O) show 
a very fine striation at right angles to the line of 7-2 and + to 
the directions of extinction. Thin flakes split off parallel to 
the lustrous surface show, under the microscope, that the luster 
is due to interference of light in passing the films of air be- 
tween the extremely thin plates Bia a by the parting. The 
thinnest flakes, composed of a few plates, are transparent and 
exhibit delicate colors of interference, while those composed of 
more plates are dull translucent, or opaque, the light having 
been completely extinguished by the repeated interference. 
The luster is, then, due to reflected light from the air films near 
the surface, and to its interference. By examination with a 
ood hand lens a delicate play of colors may be seen upon the 
lastrous surface of the crystals. 
