726 General Notes. [August, | ‘ 
The formula corresponding to this composition is 3 (Pb, Ag) S 
Bi, S3, which differs from that ‘of kobellite [3 Pb S (Bi Sb), Sli in 
the absence of antimony. W. Cross has recently! adde 
and garnet to the list of well-crystallized minerals found in "the 
lithophyses of the rhyolite of Colorado. The occurrence of these 
two minerals had been referred to before by Mr. J. A. Smith,” but 
it has been left for Mr. Cross to give us a definite description of 
them and their paragenesis. The outer walls of the lithophyses 
as well as their concentric shells are primarily formed of sanidine 
in small stout crystals, of which the transparent pne? possess a 
beautiful blue color’ in the direction of the plane 4 P 3. In iso- 
lated crystals in these lithophyses clear, transparent, dark- red 
garnets occur. They have an average diameter of 2.5 The 
a 20, usually show a striation parallel to the dodecaheail 
edge. An analysis of selected material from parts of thirty 
crystals yielded Mr. L. G. Eakins: 
SiO, Al,O, ra FeO MnO CaO KO a0” HO 
35-66 18.55 0.32 14.25 29.48 1.15 0.27 0.21 0.44 
~ 
The mineral is a typical spessartite, an aluminous garnet in which 
the calcium has been replaced by manganese and ferrous iron. 
_ Topaz appears much less abundantly than garnet. Its crystals 
are prismatic, clear and transparent, either colorless, pale-bluish 
or distinctly wine-yellow. The usual forms are oot, 2 f,c 
Greer es oP eS Pe oP anda Pa The largest crystals 
- measure half an inch in the direction of the ba -axis ; the 
of a pale-bluish tint, while the latter were most frequently oo 
5 fae This difference is ue to be due to bleach ae y 
contain topaz in the manner described. —— An- analysis | eek 
anite pendonor after pyrite yielded Dr. E. G. Smith ? 
oe By. S FeO Fe,O, CaO MgO lossin ignition 
< -675 0.31 0.91 80.21 0.04 0.40 na 
—Mr. S. L. Penfield describes brookite from Magnet cove, 
Arkansas, with a habit differing slightly from Seh usually seen 
æ w P and 2P ġ, with 14 P predominating. 
merican ae of Science, xxxt, J 1886, 
Report on i Development a samp ns * Reontse oi Colorado, 1881-2, P- 3 
Bulletin L U S. Geol. Survey, N , 
Bull. Acad. St. Petersbourg, 4, es 
Amer. sop of Science, XXXI, May, 1886, p- 376. 
ae mineral: The observed forms are P 3, 4P, P, Pa, 
Po 
