a88I. 51 Trans. N. Y. Ac. Sez. 
present their longer axes in the schist-plane, varying from 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. Pumice-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. 1 and less than 0.5 mm. in diameter, though 
occasional grains of 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: triclinic feldspar, possibly sanidine: mag- 
netite: ferrite: biotite, salmon-colored, sometimés 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 sphzrulites were also detected. 
5. Stratified Rhyolye-tuff, 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 polarising mineral, perhaps 
Augite, in a predominant ground-mass of particles and fibres of pumice 
and glass, rich in dark gas-bubbles. 
The alternating fibrous lamine 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 lamine. 
The arrangement of the glass fibres in parallel planes may have been 
