HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 527 



scattered about, forming thus beautifully perfect casts or fossils, as 

 shown in specimen No. 35969. 



(c) Pyrolusite, psilomelane, and wad : These are names given to the 

 anhydrous and more or less hydrated forms of magauese oxides, and 

 which, though wide in their distribution, are found in such abundance 

 as to constitute rock masses in comparative rarity. As with the iron 

 oxides, but a few forms are exhibited here, and the visitor is referred to 

 the economic section for a more complete display. 



(d) Beauxite (so called from Beaux, near Aries, France), is the name 

 given to a somewhat indefinite mixture of alumina and iron oxides, and 

 occurring in the form of compact concretionary grains of a dull red, 

 brown or nearly white color. The origin of the rock is at present some- 

 what obscure, but it is considered by M. Auge * as a deposit from hot 

 springs and geysers. The following analyses given by this authority 

 show the variations in composition : 



Per cent. Per cent. 



Alumina 69. 30 76. 90 



Iron oxides 22.90 0.10 



Titanium oxide 3.40 4.00 



Silica • 0.30 2.20 



Water 14.10 15.80 



The rock is at present represented by specimeus from Thorouet, 

 France (10739), and Floyd County, Georgia (66576 and 66578). 



(e) Silica: Silica, as has already been noted under the head of rock- 

 forming minerals, is one of the most abundant constituents of the 

 earth's crust. In its various forms, which are sufficiently extensive to 

 constitute rock masses, it is always of chemical origin ; that is, results 

 by deposition from solution, by precipitation, or evaporation, as noted 

 above. Varietal names are given to the deposits, dependent upon their 

 structure, method of formation, color, and degree of purity. Siliceous 

 Sinter, or " Geyserite," is the name given to the nearly white, often soft 

 and friable hydrated varieties formed on the evaporation of the silice- 

 ous waters of hot springs and geysers, or through the eliminating action 

 of algous vegetation, as described by W. H. Weed. The specimens 

 from Yellowstone National Park (Nos. 12876, 12888, 17848, 18965, 

 28945, 28946, 28948, 28981, 28982, 35521, 36782, 72844, 72845, and 72S82). 

 Iceland (2548), and New Zealand (70338, 70340, and 70341), are charac- 

 teristic. 



Opal and semi-opal are also hydrous forms of silica occurring in veins 

 and pockets in a variety of rocks. These varieties are shown in speci- 

 mens from the Yellowstone National Park (36150); Buffalo Peaks, Col- 

 orado (OS 185) ; Bapides Parish, Louisiana (38612) ; Mexico (57985), and 

 Hungary (6963). The variety known as wood opal, formed by the silici- 

 ficatiou of wood, is shown in No. 37042 from Wyoming, and in the two 

 pieces from the Pliocene beds on the Madison River, Gallatin County, 

 Montana (38567). 



* Bulletin Geological Society of France, 3d, XVI, 1888, No. 5, p. 345. 



