Trans. X. V. Ac. Sc/. 50 Dec. 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 m 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 Canon, 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 o.i 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 0.0 1 mm. 



3. Fine white pumice-tuff, of Challis, 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. i. 



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 o.oi to o.l 

 mm. in diameter. The fragments of quartz and feldspar, as a rule. 



