Trans. N. V. Ac. Scz. 52 jjgc. 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 j)umice-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 exhibits 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 whicli 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 ot magnetite. 



8. Stratified pumice-tuff, from Black Mountains, Colorado river, 

 Arizona. 



A coarser stratified tuff with brown and white layers, in which 

 grains of pumice, obsidian, glassy feldspar, and quartz reach a diameter 

 of I to 5 mm. 



The thin section is rich in pumice m 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. 



