1893.] Mineralogy and Petrography. 43 
in these notes because of the full list of rock names contained in it. 
The principal rock types are defined, and under each are given the 
technical names of all its varieties. It is further interesting as an 
indication of the growing importance of lithology in this country, 
since it is quite evident that Prof. Ward would not find it advisable to 
keep in stock such a large quantity of rock material were the demands 
for it rare. The catalogue may well serve the geologist as a table of 
petrographical synonyms. 
A New Occurrence of Ptilolite.—A new occurrence of ptilo- 
lite has been discovered by Cross and Eakins’ in Custer County, Col., 
about three miles southeast of Silver Creek, in the vesicles of a dull 
‘green devitrified pitchstone. The mineral is in very slender needles 
that are optically negative. An analysis made on very carefully 
selected material gave: 
SiO, AlĻO, CaO K,O NaO H,O Total 
67.83 11.44 3.30 .64 263 13.44—99.28, 
which is equivalent to R, Al, Si, O,,+6% H,O, a formula identical with 
that determined for mordenite by Pirsson."” Clarke" regards a part 
of the water in each mineral as basic, and believes that mordenite, the 
ptilolite from Silver Creek and the original ptilolite (which is poor in 
Na,O) are mixtures of the salts. Al, (Si,O,); Ca H, 3Aq, Al, (Si,0,), 
Ca H, 6Aq, Al, (Si,O,), Na, H, 6Aq and Al, (Si,O,), K, H, 6Aq. 
Mineralogical News.—Polybasite and tennantite are reported by 
Penfield and Pearce” from the Mollie Gibson Mine in Aspen, Col. 
The former is the ore of the mine. It occurs massive, often associa- 
ted with barite and siderite. It is of a grayish-black color, and has 
disseminated through it patches of the lighter tennantite. Analyses, 
corrected for impurities, follow: 
As b Ag Pee Ca Za Be 
Polybasite.... 18.13 7.01 .30 56.90 14.85 2.81 
Tennantite... 25.04 17.18 .13 13.65 .86 35.72 6.90 .42 
Crystals of both minerals are known to occur in several of the Colorado 
mines, though they have not yet been described. 
®Amer. Jour. Sci., August, 1892, p. 96- 
Cf. AMERICAN NATURALIST, 1891, p- 372. 
Amer. Jour. Sci., August, 1892, p. 101. 
2 Amer. Jour. Sci., July, 1892, p. 15. 
