76 EOCKS FORMED THROUaH IGINEOUS AGENCIES 



Chemical Composition. — The average of six analyses given 

 by ZirkeP is as follows: Silica, 58.01%; alumina, 20.03%; iron 

 oxides, 6.18% ; manganese oxide, 0.58% ; lime, 1.89% ; magnesia, 

 0.80%; potash, 6.18%; soda, 6.35%; water, 1.88%; specifle 

 gravity, 2.58. 



Structure. — The phonolites present but little variety in 

 structure, being usually porphyritic, seldom evenly granular. 

 The porphyritic structure is due to the development of large 

 crystals of sanidin, nepheline, leueite, or hauyne, and more rarely 

 hornblende, augite, or sphene, in the fine-grained and compact 

 gromad-mass, which is usually microcrystalline, rarely glassy or 

 amorphous. 



Colors. — The prevailing colors are dark gray or greenish. 



Classification and Nomenclature. — Three varieties are recog- 

 nized, the distinction being founded upon the variation in pro- 

 portional amounts of the minerals, sanidin, nepheline, or leueite. 

 We thus have (1) nepheline phonoUte, consisting essentially 

 of nepheline and sanidin, and which may therefore be regarded 

 as the volcanic equivalent of the nepheline syenite; (2) leueite 

 pJi07ioUte, consisting essentially of leueite and sanidin; and (3) 

 leucitopliyr, which consists essentially of both nepheline and 

 leueite in connection with sanidin, and nearly always melanite. 



So far as now known, these rocks are of comparatively rare 

 occurrence in the United States. The Black Hills of South 

 Dakota and the Cripple Creek district of Colorado are well- 

 known localities. 



4. THE BIORITE-ANDESITE aEOUP 



The rocks of this and the succeeding group differ in a marked 

 degree from those discussed in previous pages, a difference due 

 in large part to an absence of orthoclase or other potash minerals 

 as an essential constituent. The group includes the plutonic 

 type diorite, and the effusive types Jiornhlende porphyrite, and 

 andesite. These may be described as below: 



(1) THE DIOBITES (Greenstones in part) 



Diorite, from the Greek word dcopt^eiu , to distinguish. A term 

 first used by the mineralogist Haiiy. 



Mineral Composition. — The essential constituents of diorite 

 are plagioclase feldspar, either labradorite or oligoclase, and 



^Lehrbueh dor PetrograpHe, II, p. 193. 



