HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 



573 



Nevada, as given by King,* and (III) that of a black obsidian from the 

 .Yellowstone National Park, Wyoming, as given by Iddings.t 



Silica 



Alumina 



Ferric oxide 



Ferrous oxide .. 

 Ferric sulphide . 



Lime 



Magnesia 



Soda ... 



Potash 



Phosphoric anhydride. 

 Ignition 



Specific gravity 



Per cent. 

 74 45 

 14.72 



None . . . 

 0.56 



0.28 

 0.83 

 0.37 

 3.97 

 4.53 

 0.01 

 0.66 



100. 38 



TT. 



Per cent. 



74.62 



11.96 



1.20 



0.10 



0.36 



2.26 

 7.76 



99.28 

 2.2 



III. 



Per cent. 



74.70 



13.72 



1.01 



0.62 



0.40 



Trace. 

 0.78 

 0.14 

 3.90 

 4.02 



0.62 



99.91 

 2. 34 17 



Colors. — These are fully as variable as in the quartz porphyries ; 

 white, though all shades of gray, green, brown, yellow, pink, and red 

 are common. Black is the more common color for the glassy varieties 

 of obsidian, though they are ofteu beautifully spotted and streaked with 

 red or reddish-brown as shown in samples 72855 and 72856 from the 

 Yellowstone Park, in 35268 from Glass Buttes, Oregon, 70252 from Ma- 

 raratio, Mexico, 16248 from the Caucasus, aud 11894 from Japan. 



Structure. — The liparites present a great variety of structural fea- 

 tures, varying from holocrystalline, through porphyritic aud felsitic to 

 clear, glassy forms. These varieties can be best understood by refer- 

 ence to the collections and Plates cxxviii and cxxix, prepared from 

 photographs. Pig. 1, PI. cxxviii, is that of the coarsely crystalline 

 variety nevadite from Cbalk Mountains, Colorado; Fig. 2 is that of a 

 common felsitic and porphyritic type ; Fig. 3 is that of the clear, glassy 

 form obsidian; Fig. 4 shows also an obsidian, but with a pumiceous 

 structure ; Figs. 1 and 2 on PI. cxxix show the hollow spherulites or 

 lithophysse, which have been studied aud described by Mr. J. P. 

 Iddings, of the IT. S. Geological Survey.f Such forms are regarded 

 by Mr. Iddings as resulting "from the action of absorbed vapors 

 upon the molten glass from which they were liberated during the 

 process of crystallization consequent upon cooling." A pronounced 

 flow structure is quite characteristic of the rocks of this group as indi- 

 cated by the name rhyolite. (See large sample of obsidian No. 72853, 

 from Yellowstone National Park). The microscopic structure of a lip- 



* Geological Exploration 40th Parallel, vol. i, p. 652. 



t Annual Report U. S. Geological Survey, 1885-'S6, p. 232. 



X Obsidian Cliff, Yellowstone National Park, Annual Report U. S, Geological Survey, 



1885-'86. 



