Vrans: Ve Vs Accde2. 50 Dee. 5, 
grains, 0.2 to 1.6 mm. long, retaining more frequent and perfect traces 
of their crystalline forms, their sides being often very ragged, curiously 
and deeply eroded into rounded indentations, while within occur numer- 
ous inclusions of the ground-mass and of scales of biotite, long green- 
ish needles of hornblende, and sub-angular drops of a brownish-violet 
glass with one or several fixed bubbles of gas: biotite in abundant 
irregular scales, 0.2 to 1.3 mm. long, brown inclining to maroon or 
brownish-yellow, cloudy to opaque, with some dichroism remaining in 
the striated sections ; hornblende in brownish-green, strongly dichroic, 
fibrous crystalline flakes: opacite, probably magnetite, and ferrite or 
iron-oxide, in dusty particles or groups in the biotite scales and among 
the pumice fibres. The fine ground-mass is mainly composed of minute 
fragments, fibres, scales, etc., of all these minerals: also in large part 
of solid globules of fawn-colored glass, or of thin and apparently hol- 
low shells, or of fragments of quartz or feldspar coated with a glass 
crust. Many of these forms are found adhering in curious aggrega- 
tions or with their sides crushed in. 
The general constitution of this rock is similar to that of the volcanic 
tuff of the El Dorado Cajion, Cal. 
2. Fine green volcanic tuff, of Challis, Idaho. 
A very fine compact rock, with almost the texture of stoneware, with 
a pale, greenish-gray color, and a very thin parallel lamination. A few 
minute scales of biotite can be distinguished by the loup. The surfaces 
of fissures are mottled and spotted with bluish-green and ochreous, 
brownish-gray films. 
The thin sections present the same constitution as that of the coarse 
variety of the rock, without the presence of pumice, the particles of 
quartz and feldspar varying in size from 0.06 to 0.25 mm. Biotite is 
abundant in scales 0.1 to 0.2 mm. in diameter, often of ochreous shades 
of brownish-yellow and maroon, through partial decomposition, and 
with curved fibres or wrinkles as if crushed in by pressure. To its 
abundance are due the fine lamination of the rock and, in part, its 
greenish color. The ground-mass largely consists of globules of color- 
less glass, but in less degree than in the preceding variety, their size 
varying from 0.006 to 0.01 mm. 
3. Fine white pumice-tuff, of Chalizs, Idaho. 
A very fine compact rock, grayish, with a bronze shade, with a lamin- 
ation so decided that it inclines to slaty. Under the loup the same 
constituents are visible as in No. 1. 
The thin sections show a close relationship to those of No. 2. A 
little hornblende is present. Biotite occurs in distinct scales, sometimes 
hexagonal, not so minutely dispersed as in No. 2, generally 0.01 to 0.1 
mm. in diameter. The fragments of quartz and feldspar, as a rule, 
