6 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. 62. 
cite gouge, as proved by its well-developed crystal form. According to E. F. Blain 
superintendent of the property, the vein assays from $5 to $15 a ton in gold and 
$2 to $3. a toninsilver. No assays were made to determine whether the pyrite of 
the lower tunnel contains any gold and silver, but it is thought more probable that 
these metals are contained in the older sulphides. 
The samples examined were numbered SI 82 and the label accom- 
panying them reads ‘‘sericite from shear zone.” They consist in the 
main of the fine scaly sericite containing disseminated pyritohedral 
crystals of pyrite up to 1 cm. in diameter. One of the larger speci- 
mens contained a mass of the quartz containing sulphides of the 
first mineralization and this was washed free from sericite by scrub- 
bing with a brush under water. This was a nodule of white quartz 
which was rounded as if by solution, which contained pyrite, galena, 
tetrahedrite, and a colorless to light resin or greenish-yellow sphal- 
erite, the latter in well-formed crystals up to 1 cm. in diameter. 
There was also a little of a peculiar appearing mineral of grayish 
purple color with well-defined cleavage which agreed in optical prop- 
erties and blowpipe reactions with barite. 
Physical properties.—In the hand specimen the sericite is visibly 
micaceous with a very finely foliated structure and distinctly silky 
luster. The color varies from cream or dirty white to pale greenish. 
When moistened it is plastic with a greasy feel. 
Optical properties.—The optical properties were very kindly worked 
out by Mr. Ross, who obtained the data given in column 1 below. 
In column 2 are given for comparison the properties of normal mus- 
covite as given in Larsen’s tables.° 
Comparison of optical properties of sericite and muscovite. 
ile 2) 
Sericite (Ross): Muscovite (Larsen): 
Biaxial. Biaxial. 
Negative (—). Negative (—). 
2V 25° 2V¥—)40" 
Xi =e, | se" 
Indices: Indices: 
a = 1.550+ .003 a = 1.561 
B = 1.585+ .003 B = 15590 
y = 1.587+ .003 vy = 1.594 
Birefringence .037. Birefringence .033. 
While the indices of refraction are slightly lower and the birefringence 
is somewhat higher than those given for normal muscovite it 1s note- 
worthy that the optical properties are essentially those of muscovite. 
Pyrognostics.—Heated before the blowpipe this sericite fuses on 
thin edges to a white enamel. This property is important since mus- 
covite and kaolinite are commonly listed as infusible and leverrierite 
sEsper 8, Larsen. The microscopic determination of the nonopaque minerals. U.S. Geol. Sury. 
Bull. 679, p. 252, 1921. 
