540 REPORT OF NATIONAL MUSEUM, 1890. 



morphosed as to render the original nature a matter of some doubt. In 

 the series exhibited a large share are of Tertiary or post- Tertiary origin. 

 Among the older and more altered forms attention may be called to 

 those of Needham and Nantasket, Massachusetts (39050 and 38528) ; 

 Scotland (7038$), and Brazil (69977, 09980, and 69984). 



5. Ferruginous group. — This is a small and comparatively unimportant 

 group, comprising only those fragmeutal rocks, the individual particles 

 of which are composed mainly of ferruginous oxides. Such result from 

 the breaking up of the iron ores, hematite and limonite, described under 

 the head of rocks formed by chemical agencies. 



B. — Rocks composed mainly of debris from plant and animal life. 



1. Siliceous group. — Infusorial or diatomaceous earth. This is a line 

 white or pulverulent rock composed mainly of the minute shells, or 

 tests, of diatoms, and often so soft and friable as to crumble readily 

 between the thumb and fingers. It occurs in beds which, when com- 

 pared with other rocks of the earth's crust, are of comparatively insig- 

 nificant proportions, but which are nevertheless of considerable geo- 

 logical importance. Though deposits of this material are still forming,* 

 and have been formed in times past at various periods of the earth's 

 history, they appear most abundantly associated with rocks belonging 

 to the Tertiary formations. 



The celebrated Bohemian deposit is some 14 feet in thickness, and is 

 estimated by Ehrenberg to contain 40,000,000 shell to every cubic inch. 

 The Australian specimeu exhibited (No. 28473) is from a deposit 4 feet 

 in thickness. In the United States beds are known at Lake Umbagog, 

 New Hampshire (specimen No. 29322); Morris County, New Jersey; near 

 Richmond, Virginia (specimen No. 706S9); Calvert and Charles Counties, 

 Maryland (specimen 70089) ; in NeAV Mexico; Graham County, Arizona 

 (specimen No. 72912) ; Nevada (22346) ; California, and Oregon. The New 

 Jersey deposit coveis about 3 acres, and varies from 1 to 3 feet in thick- 

 ness ; the Richmond bed extends from Herring Bay, on the Chesapeake, 

 to Petersburgh, Virginia, and is in some places 30 feet in thickness; 

 the New Mexico deposit is some 6 feet in thickness and has been traced 

 some 1,500 feet; Professor Leconte states that near Monterey, in Cali- 

 fornia, is a bed some 50 feet in thickness, while the geologists of the 

 Fortieth Parallel Survey report beds not less than 300 feet in thickness, 

 of a pure white, pale buff or canary-yellow color as occurring near 

 Hunter's Station, west of Reno, Nevada. (See specimen No. 22346.) 



The earth is used mainly as a polishing powder, and is sometimes 

 designated as tripolite. It has also been used to some extent to mix 

 with nitroglycerine in the manufacture of dynamite. Chemically the 

 rock is impure opal, as will be seen from the following analyses made 



* E. g., ill the marshes of the Yellowstone Park. See on the Diatom Marshes and 

 Diatom Beds of the Yellowstone National Park, by W. H. Weed, Botanical Gazette, 

 vol. xiv, No. 5, p. 117. 



