Trans. N. Y. Ac. Sc. 52 Dec. 5, 
produced by sorting in the air during their fall, or by later superincum- 
bent pressure while still hot and plastic, or it may be in some instances 
by the influence of overflowing lava-sheets. The cohesion produced by 
such downward pressure and interfusion has produced a structure 
which can hardly be distinguished from that of many obsidians and 
rhyolytes. 
6. Fine white pumice-tuff, from mouth of Bill Williams’ fork of 
Colorado River, Arizona. 
A compact white schist, with almost the fine texture of No. 3, 
traversed in places by brown curved impressions, apparently produced 
by rootlets. 
The thin section mainly exhiisits a very finely felted mass of short, 
straight fibres of pale brownish pumice. Besides these only a very 
few black particles of magnetite, feldspar, etc., were distinguished. 
7. Fine brownish pumice-tuff, from last locality. 
A brownish variety of the preceding, with abundant minute black 
particles. The slaty lamination is decidedly marked, with slight 
adherence over many planes at which the rock breaks easily, presenting 
remarkably flat surfaces. 
The constitution displayed in the thin section is similar to that of the 
preceding specimen. Minute glass globules are abundant, and also 
more numerous angular particles of other minerals: colorless feldspar 
(sanidine ?) showing cleavage: brownish and greenish augite : brown- 
ish and dichroic fibres of hornblende, and black particles of magnetite. 
8. Stratified pumice-tuff, from Black Mountains, Colorado river, 
Artzona. 
A coarser stratified tuff with brown and white layers, in which 
grains of pumice, obsidian, glassy feldspar, and quartz reach a diameter 
of 1 to 5 mm. 
The thin section is rich in pumice in all its fibrous, curving, and 
reticulated forms, and in minute globules, threads, and shreds of vol- 
canic glass: angular grains of finely lamellated plagioclase, water-clear 
quartz, and sanidine with well marked cleavage and often zonal struc- 
ture: particles of biotite, hornblende, magnetite and ferrite: abundant 
grains of augite, angular to rounded, sometimes retaining its optical 
characteristics in spots, but mostly decomposed and isotrope, colorless, 
brownish-yellow, light to deep maroon, etc., finely granular, thready, or 
fibrous, and more or less darkened by opacite even to complete opacity. 
9. Basalt-tuff, or peperino, Chinati Mts., Texas. 
A fine-grained olive-green rock, with white streak, friable to arena- 
ceous, with barely perceptible schist structure in the specimen. Under 
the loup, minute granules of feldspar, quartz, etc., are distinguishable, 
rarely I mm. in diameter, embedded in a grayish-green cement. 
