NO. 1173. BOCKS FROM VNITED STA TES-MEXJCO BO VNBABY—LORD. 777 



The biotite of the diorite-porphyry is in a highly altered condition, 

 in contrast to the remarkably fresh hornblende; the crystals are in 

 many cases completely replaced by chlorite and eijidote. 



e. Rhyolite. — Fifty-eight miles west of the Eio Grande on parallel 

 31° 47' at Mon. No. 19 (No. 53277). Six miles west of Lake Palo- 

 mas, Mexico, Mon. No. 20 (No. 53278). Oarrizallilo Mountains, south 

 of Oarrizallilo Springs, New Mexico, Mon. No. 33 (No. 53279). South of 

 Dog Spring, Dog Mountains, Mon. No. 55 (No. 53280). West side of San 

 Luis Mountains, Mon. No. 73 (No. 53281). San Bernardino Itiver, Mon. 

 No. 77 (No. 53282). Nogales, Mon. No. 122 (No. 53283). Warsaw Mills, 

 Pima County, Arizona, Mon. No. 132 (No. 53284). Pozo Verde, "El 

 Banorie," Sonora, Mexico, Mon. No. 141 (No. 53285). Sierra Moreno, 

 Pima County, Arizona, Mon. No. 146 (No. 53286). Acjua Dulce Creek, 

 Mon. No. 172 (No. 53287.) 



The specimens of rhyolite vary in character from a pitchy black ob- 

 sidian (Nos. 53281,53287, original Nos. 338, 483) to a light gray porphy- 

 ritic rock with holocrystalline groundmass (Nos. 53278, 53279, 53280, 

 53283, 53284, 53285, 53286, original Nos. 150, 168, 314, 370, 389, 396, 412, 

 419, 432). Intermediate types showing microscopically a felsitic (No. 

 53277, original No. 133), spherulitic (Nos. 53279, 53282, original Nos. 170, 

 343), or glassy groundmass (Nos. 53277, 53280, original Nos. 129, 136, 

 137, 312) are not uncommon. 



The holocrystalline variety is the more common and consists miueral- 

 ogically of orthoclase, quartz, i)lagioclase, biotite, and magnetite of 

 intratelluric origin, and a groundmass of cryptocrystalline quartz and 

 feldspar, through which are scattered minute particles of biotite and 

 magnetite. Many specimens bear evidence of a i)reexisting residual 

 glass-base, now completely devitritied. The products of devitrification 

 are either quartz (Nos. 150, 314, 412, 419, 432), or quartz accompanied 

 by chalcedony (Nos. 53277, 53279, 53280, original Nos. 130, 167, 168, 170, 

 312, 315). Opal occurs in specimens Nos. 129 and 137. In many 

 instances the mica constituent of the rhyolite is completely decomposed 

 (Nos. 396, 412, 419, 432). 



Amygdaloidal types are highly colored by infiltrated liinonite (Nos. 

 136, 168, 170, 315). 



A mineralogical peculiarity of the rock from the region about Mon. 

 No. 19 (Nos. 129, 133, 136, 137), Lake Palomas (No. 150), and from the 

 Dog Mountains, Mon. No. 55 (Nos. 312, 314, 315) is the abundance of 

 triclinic feldspar (andesine) among the porphyritic constituents. In 

 some instances it occurs in sufficient quantity to render the determina- 

 tion of the rock as dacite permissible. 



A holocrystalline rhyolite from Nogales, Mon. No. 122 (No. 370), con- 

 tains among the ])henocrysts large grains of brown tourmaline with 

 stiong absorption — 0> E. 



The successive stages of crystal development from a vitreous obsidian 

 (No. 183) to a spherulitic rhyolite (No. 343) are beautifully illustrated 

 in specimens Nos. 483, 338, 133, 343. l^o. 483 represents a homogene- 



