l88l. 51 Trans. N. V. Ac. Set. 



present their longer axes in the schist-plane, varying irom 0.03 to 

 0.22 mm. in length. The glass inclusions in the quartz, range 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 Osar- 

 isawa, Akita, Japan. 



4. P uf nice-tuff, Moore Station, Pancake Range, Moray, Nevada. 

 This rock is decidedly schistose, cream-colored, nearly white, of a 



fine grain, intermediate between Nos. i and 2, most of the constituents 

 being the same as in No. i and less than 0.5 mm. in diameter, though 

 occasional grains of pumice, gray and red obsidian, and perfect crystals 

 of quariz, 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 : tricHnic feldspar, possibly sanidine : mag- 

 netite : ferrite : biotite, salmon-colored, sometimes very cloudy: and 

 volcanic 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 predominate. 



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 devitrifica- 

 tion. A few minute sphasrulites were also detected. 



5. Stratified Rhyolye-tiiff, Tempiute, Nevada. 



A snow-white kaolinic variety, related to the preceding, which 

 appears to consist principally of pumice. A few grains of black obsi- 

 dian and red quartzite occur, the latter also as a somewhat rounded 

 pebble, 34 mm. in length. 



The thin section, transverse to the schist-plane, presents an interest- 

 ing structure, made up of granular layers alternating with others posses- 

 sing 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 polarismg mineral, perhaps 

 Augite, in a predominant ground-mass of particles and fibres of pumice 

 and glass, rich in dark gas-bubbles. 



The alternating fibrous laminae consist of a true rhyolyte material, 

 salmon-brown, with a marked fluidal structure around the few quartz- 

 grains, and displaying in spots, and especially next the junction with 

 granular material, the constituent pumice-fibres whose partial interfu- 

 sion or cohesion seems ordinarily to have produced the solid lam'.nas. 



The arrangement of the glass fibres in parallel planes may have been 



