SCIENCE. 



607 



ihese forms are found adhering in curious aggregations 

 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 fis- 

 sures 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 decompo- 

 sition, and with curved fibres or wrinkles as if crushed in 

 by pressure. To its abundance are due the fine lamina- 

 tion of the rock and, in part, its greenish color. The 

 ground mass hrgely consists of globules of colorless 

 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 Challis, Idaho. 



A very fine compact rock, grayish, with a bronze shade, 

 with a lamination so decided that it incline? to slaty. 

 Under the loup the same constituents are visible as in 

 No. r. 



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 diam- 

 eter. The fragments of quartz and /eldspar, as a rule, 

 present their longer axes in the schist plane, varying 

 irom 0.03 to 0.22 mm. in length. The glass inclusions 

 in the quartz, ranging from 0.002 to 0.037 mm. The 

 ground mass appears to be mainly composed of pumice, 

 more or less altered, in very minute fibres and particles. 



This rock strongly resembles the tufa of the lignite 

 beds near Osarisawa, Akita, Japan. 



4. Pumice-tuff, Moore Station, Pancake Range, 

 Moray, Nevada. 



This rock is decidedly ochi^tose, cream colored, nearly 

 white, of a fine grain, intermediate between Nos. 1 and 

 2, most of the constituents being the same as ih No. 1 

 and less than 0.5 mm. in diameter, though occasional 

 grains o f pumice, gray and red obsidian, and perfect 

 crystals of quartz may reach from 2 to 8 mm. in length. 



In the thin section the constituents are found disposed 

 with great regularity : pumice, with its fibres often 

 curved, as if crushed while still soft and plastic : quartz : 

 trichinic feldspar : possibly sanidine : magnetite : ferrite : 

 biotite, salmon colored, sometimes very cloudy : and vol- 

 canic glass in cellular network, often full of gas bubbles, 

 elongated and distorted. In the ground mass, globules 

 of glass and fibres and threads of pumice largely pre- 

 dominate. 



The pumice in all these tuffs is not perfectly isotrope 

 between the crossed nicols, but presents innumerable, 

 though exceedingly minute glittering points, apparently 

 crystallites formed by incipient devitrification. A few 

 minute sphercelites were also detected. 



5. Stratified Rhyolye-iuff, Tempiute, Nevada. 



A snow-white Kaolinic variety, related to the preced- 

 ing, which appears to consist principally of pumice. A few 

 grains of black obsidian and red quartzite cccur, the lat- 

 ter also as a somewhat rounded pebble, 34 mm. in 

 length. 



The thin section, transverse to the schist-plane, pre- 

 sents an interesting structure, made up of granular layers 

 alternating with others possessing strong fibration. 



The material of the former is mostly like that of No. 4: 

 feldspar is sparsely scattered : quartz fragments abound, 

 with the usual glass inclusions, and with sides deeply 

 eroded and indented : also magnetite, ferrite, and minute 



colorless particles of a polarising mineral, perhaps Augite, 

 in a predominant groundmass of particles and fibres of 

 pumice and glass, rich in dark gas-bubbles. 



The alternating fibrous laminae consist of a true rhyo- 

 lyte material, salmon-brown, with a marked fluidal struc- 

 ture around the few quartz-grains, and displaying in 

 spots, and especially next the junction, with granular 

 material, the constituent pumice-fibres whose partial in- 

 terfusion or cohesion seems ordinarily to have produced 

 the solid laminae. 



The arrangement of the glass fibres in parallel planes 

 may have been produced by sorting in the air during their 

 fall, by later superincumbent 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 Wil- 

 liams 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 lew black particles of magne- 

 tite, 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 decid- 

 edly marked, with slight adherence over many planes at 

 wh'ch 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 : brownish and 

 dichroic fibres of hornblende, and black particles of 

 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 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 volcanic glass: angular grains of 

 finely lamellated plagioclase, water-clear quartz, and 

 sanidine with well marked cleavage and often zonal 

 stricture : 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 pepcrino, Chenniti Mis., Texas. 



A fine-grained olive-green rock, with white streak, 

 friable to arenaceous, with barely perceptible schist 

 structure in the specimen. Under the loup, minute 

 granules of feldspar, quartz, etc., are distinguishable, 

 rarely 1 mm. in diameter, embedded in a grayish-green 

 cement. 



In the thin section the constituents are very much the 

 same as in No. 8, with the exception of hornblende, and 

 all the grains are in large part rounded. A few elongated 

 rounded grains of a basaltic lava are also included, 

 highly micro-crystalline with minute ledge of plagioclase 

 scattered through a reddish-brown opaque base. 



This specimen, and perhaps the preceding, represent 

 the basic division of the tuffs, being ejections from an 

 eruption of basaltic lava, though naturally composed of 

 its more fluid, glassy, and acid scoria. 



From these facts it may be concluded that enormous 



