774 PROCEEDINGS OF THE NATIONAL MUSEUM. 



CLASSIFICATION OF THE ROCK TYPES. 



On preliminary examination many of the specimens were found too 

 much decomposed to warrant tlie preparation of thin sections. 



The comparatively fresh rock from which slides were made may be 

 arranged under the following headings: 

 I. Intrusive rock. 

 0. Granite. 

 b. Gabbro diorite. 

 G. Uralite-diabase (proterobase). 



d. Diorite porphyry. 

 II. Effusive rock. 



e. lihyolite. 



/. Ilornblende-mica-andesite. 



g. Augite-andesite and andosite breccia. 



h. Basalt. 



Under Group I are classified rock-types with a granitic-granular and 

 holocry8talline-por[)hyritic structure. 



Granite is composed chiefly of quartz, orthoclase, and biotite. 



Gabbro-dioritc consists principally of augite (diallage), green horn- 

 blende (uralite), biotite, and plagio(dase. 



Uralite-diabase has uralite and plagioclase as chief mineral constit- 

 uents. 



Diorite-porphyry contains plagioclase, hornblende, and quartz as chief 

 ingredients. 



Under Group il are united rock-types of porphyritic structure. They 

 represent the effusive forms of granitic, dioritic, and gabbro magmas. 



Rhyolite is made up of phenocrysts of sanidine, <iuartz, and biotite, in 

 a holocrystalline or vitreous groundniass, and shows in some instances 

 structural phenomena of special interest. (See pp. 777, 778.) 



A peculiar development of this magma is seen in the spherulitic 

 nodules from an obsidian flow near Ai^ua Dulce. (See pp. 778, 779.) 



Hornblende-mica- andesite has essentially the same mineral (constitu- 

 ents as diorite-i)orphyry without (juartz. 



AiKjite-andesite contains augite (dioi)side), plagioclase, and magnetite 

 as chief ingredients. 



Basalt is composed principally of olivine, augite, plagioclase, and 

 magnetite. A rare instance of olivine occurring in two generations is 

 observed in the basalt from Tule Mountains, Gila County, Arizona. 

 (See p. 782.) 



DESCRIPTION OF rilK R<)(JK IN GROUPS. 



I. INTHUSIVK KOCK. 



a. Granite. — Nogales, Mon. No. 122 (No.. '53260). La Osa, Pima County, 

 Arizona, Mon. No. 140 (No. 53207). Pozo Verde, Sonora, Mexi(;o, Mon. 

 No. 141 (No. 53268). Quitobaquita, Sonoyta Valley, Arizona, Mon. No. 

 172 (No. 53209). Tenajas Altus, Gila Mountains, Yuma County, Arizona, 



