rpTTT? T TT> A T?TfT'T7'C! 

 X -tl Jcj jUxx ii.xvJL IJCj O 



67 



Chemical Composition. — Below is given the composition of: 

 (I) Nevadite, from the northeastern part of Chalk Mountain, 

 Colorado, as given by Cross.^ (II) That of a rhyolite form, 

 from the Montezuma Range, Nevada, as given by King,^ and 

 (III)' that of a black obsidian from the Yellowstone National 

 Park, "Wyoming, as given by Iddings.^ 



Chemical Composition of Liparite 



Constituents 



Silica (SiOa) 



Alumina (AlgOs) .... 

 Ferric oxide (!Fe208) . . . 

 Ferrous oxide (FeO) . . . 

 Ferric sulphide (FeS2) . . 

 Manganese (MnO) . . . 



Lime (CaO) 



Magnesia (MgO) .... 



Soda (Na20) 



Potash (K2O) 



Phosphoric anhydride (P2O5) 

 Ignition ,....., 



Specific gravity 



74.60% 

 14.72 

 None 

 0.56 



0.28 

 0.83 

 0.37 

 3.97 

 4.53 

 0.01 

 0.66 



100.38 % 



II 



74.62% 

 11.96 



1.20 



0.10 



0.36 



• » « * 



2.26 

 7.76 



* » • * 



1.02 



99.28% 

 2.2 



III 



74.70% 

 13.72 



1.01 



0.62 



0.40 

 Trace 



0.78 



0.14 



3.90 



4.02 



0.62 



99.91 % 

 2.3447 



Colors. — These are fully as variable as in the quartz por- 

 phyries; white, though all shades of gray, green, brown, yel- 

 low, pink and red are common. Black is the more common 

 color for the glassy varieties of obsidian, though they are often 

 beautifully spotted and streaked with red or reddish-brown. 



Structure. — The liparites present a great variety of structural 

 features, varying from holocrystalline, through porphyritic and 

 felsitic, to clear, glassy forms. These varieties can be best 

 understood by reference to Plates 5 and 6, prepared from 

 photographs. A pronounced flow structure is quite character- 

 istic of the rocks of this group, as indicated by the name rhyolite. 

 The microscopic structure of an obsidian is shown in Pig. 4, PL 



1 Geology and Mining Industry of Leadville, Monograph XII, XJ. S. GeoL 

 Survey, p. 349. 



2 Geological Exploration 40th Parallel, Vol. I, p. 652. 



3 Ann. Eep. U. S. Geol. Survey, 1885-86, p. 282. 



