8 REPORT OF NATIONAL MUSEUM, 1889. 



public, nor so simple and indefinite as to excite the contempt of the 

 specialist. 



As at present arranged all the rocks forming any essential part of 

 the earth's crnst are grouped under four main heads, the distinction 

 being based upon their origin and structure. Each of the main divi- 

 sions is again divided into groups or families, the distinction being 

 based mainly upon mineral and chemical composition, structure, and 

 mode of occurrence. We thus have : 



I. Aqueous rocks. — Rocks formed mainly through the agency of 

 water as (a) chemical precipitates, or as (b) sedimentary beds. Having 

 one or many essential constituents. In structure massive, laminated, 

 or bedded, crystalline, colloidal, or fragmental ; never glassy. 



II. JEolian rocks. — Rocks formed from wind drifted materials. In 

 structure irregularly bedded ; fragmental. 



III. MetamorpMc rocks. — Rocks changed from their original condi- 

 tion through dynamic or chemical agencies, and which may have been 

 in part of aqueous and in part of igneous origin. Having one or many 

 essential constituents. In structure crystalline, bedded, schistose, or 

 foliated. 



IV. Igneous rocks.— Eruptive. Rocks which have been brought up 

 from below in a molten condition, and which owe their present structu- 

 ral peculiarities to variations in composition and conditions of solidifi- 

 cation. In structure massive, crystalline, felsitic, or glassy, or in cer- 

 tain altered forms, colloidal. 



The following shows the type of labels used in this series : 



MICA SYENITE. 



Gallatin County, Montana. 38,600. 



Collected by George P. Merrill, 1886. 



MUSCOVITE-BIOTITE GRANITE with 

 accessory TOPAZ and a LITHIA MICA. 



Stockschneider, in the Erz-Gebirge, Saxony. 



36,161. 

 Obtained from B. Sturtz, 1884. 



PYROXENITE; Websterite. 



Near Webster, Jackson County, North Caro- 

 lina. 38,832. 

 Gift of H. J. Biddle, 1887. 



Quartzite [Novaculite]. 

 Leigh River, Victoria, Australia. 28,321. 



Australian Centennial Commissioners, 1876. 



Note.— The inclosing a name in brackets, as [Novaculite] in the last form, indi- 

 cates that such has gone out of use, or is a local or popular name of little value and 

 not generally accepted. 



