a 
96 W. Cross—Sanidine and Topaz from Colorado. 
probably identical with that above described. Unfortunately, 
neither the original nor any references to it are accessible to 
me, hence I cannot draw any further parallel between the two 
cases. 
Accompanying the quartz and sanidine in the cavities are 
minute leaflets of biotite, a few ore grains, and in a few druses 
only, very perfect crystals of colorless, transparent topaz. Usu- 
ally but a single crystal of topaz is present in one of the druses 
and that is larger and more perfect in development than any 
other. The topaz is attached directly to the walls of the cavity 
and often bears small tablets of sanidine upon it. The crystals 
which can be recognized as topaz vary from 0:5™" to 3™ in 
length, but it seems quite a that there are some smaller 
ones indistinguishable from quar 
The determination as topaz foals upon the crystalline form, 
which is very distinct, a is that of common topaz. One 
erystal measuring gam j n length and 1™™ in thickness was 
removed from the rock and its angles measured with a Fuess 
reflection goniometer. This c se presents I (110), 2 (120) 
and 2-4 (021) as the Satie forms; O (001) is a narrow face 
and 4-4 (041) 2-7 (201), 2 (221) and 1 (111) are minute but very 
distinct. The angles measured are as follows: 
See § 124° 16’ 
é-2 Ai-2 over ¢% 938° 7 
O A 2% 136° 30’ 
ON1 134° 11’ 
OA2 115° 55’ 
2-7 appears as a very narrow face in the zone of 2 to2. This 
is the usual habitus with semanas addition of 7-4 and a more 
with 4-2 and 2 also recognizable and there are no signs 
hemimorphism. 
In some druses all crystals are say coated by a black in- 
<a which seems to be pyrolusi 
as I can ascertain, all paiyisoaly known and described 
acide on of topaz are in granite, gneiss or some other mem- 
ber of the series of metamorphic or crystalline schists. In the 
present case topaz is found in an eruptive rock, probably of 
early Tertiary age, while the appearance of the associated 
minerals certainly suggests that they may all be sublimation 
products, though it is not capable of direct proof. The sani- 
dine crystals from the druses, examined microscopically, con- 
tain gas inclusions, while neither fluid nor glass inclusions were 
seen. 
