188 



SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1921. 



places, however, it is very difficult or impossible 

 to determine the age of a particular flow. 



Petrographi/. — Petrographic examination 

 shows that there is considerable similarity in 

 type in these lavas throughout the area. Most 

 of the flows and most of the intrusive rocks that 

 cut them and are associated with them are latites 

 and quartz latites; some are soda rhyolites. 

 The tuffs examined are rhyolitic. Associated 

 with these siliceous and sodic rocks, especially 

 in the upper part of the series, are flows and 

 dikes of basalt. 



The latites are fine-grained rocks, in places 

 porphyritic and commonly showing flow struc- 

 ture and perlitic growths. They are composed 

 essentially of alkali p^agioclase and orthoclase, 

 with hornblende, biotite, and quartz usually 

 present in subordinate amounts. Apatite and 

 epidote were also noted in some specimens. 



Most of the specimens of basalt examined are 

 of the usual types, composed essentially of 

 calcic plagioclase, augite, and olivine, with sub- 

 ordinate amounts of magnetite. They are 

 somewhat porphyritic, all the minerals men- 

 tioned above, except the magnetite, occurring 

 to a greater or less extent as phenocrysts. 

 The groundmass is a fine-grained mass of pla- 

 gioclase laths, showing in places parallel ar- 

 rangement due to flowage, with granular augite 

 and magnetite. Much of the olivine is altered 

 t*o iddingsite. 



Two specimens of basalt from the immediate 

 vicinity of Woolsey Tank, in the Gila Bend 

 Mountains, differ from those above described 

 in that the feldspars are much more sodic. 

 Their composition approaches that of oligoclase 

 or albite-oligoclase. Stratigrapliically these 

 Hows certainly are near the top of the. series of 

 Tertiary lavas, and tliey may even be of Pleis- 

 tocene age. In the Buckskin Mountains near 

 Osborne's Well Blanchard" found a rock which 

 appears to be of a similar type. 



TERTIARY SEDIMENTARY FORMATIONS. 



Distribution^ and ihnnicti r. — Limestone and 

 calcareous conglomerate occur in at least thi-ee 

 widely separated hx^alities in this area. Fur- 

 ther work would probably disclose many other 

 outcrops. The known localities are Osborne 

 Wash, in the vicinity of Osborne's Well, near 

 Parker: Saddle Mountain; and the Clan ton 



" Blanchard. R. C, op. cit., pp. 2()-27. 



Hills and the valley north of them. vSand- 

 stone was found in Antelope Hill, in several 

 places in the Gila Bend Mountains, near 

 Osborne's Well, in the Clanton Hills, and in 

 small amounts elsewhere. Shale is associated 

 with some of the sandstone in the Gila Bend 

 Mountains. 



Antelope Hill, at the south end of the con- 

 crete bridge across Gila River near Wellton, is 

 composed of grayish arkose, a sandstone 

 formed from granitic debris. The rock is, as a 

 whole, somewhat coarser grained near the base 

 of the liill than farther up the slope. The 

 average diameter of the gi-ains ranges from 

 1 to 6 millimeters. The beds have a very 

 gentle southerly dip. The hill is about 580 

 feet high, so that fully 500 feet of sandstone 

 is exposed. Related but coarser sandstone 

 and conglomerate occur farther south.''' 



Red sandstone crops out in several places in 

 the Gila Bend Mountains, notably at and near 

 Woolsey Tank, where there is a bed 30 feet 

 tliick of sandstone interbedded wath the lime- 

 stone. Near the Dixie mine, in the Gila Bend 

 Mountains, red and purplish shale is associated 

 with the sandstone. Plate XLIV, .1 (p. 191), 

 shows Tertiary sandstone in these mountains 

 overlain by Pleistocene gi'avels. 



The relations of these sedimentary rocks 

 to the Tertiary lavas show clearly that they are 

 of similar age. They have been disturbed, 

 like the lavas, by post-Tertiary faulting, so 

 that the beds now dip in various directions. 

 The Clanton Hills, about 25 miles north of 

 Palomas, consist almost exclusively of flat- 

 lying gray cherty fine-grahied limestone with 

 numerous concretions, some of which resemble 

 fossils in superflcial appearance. Some of the 

 beds contain small and mdistinct fossils. 

 (See pp. 189-190.) At the west end of the hills 

 is exposed a bed of reddish sandstone com- 

 posed of quartz grains in a calcareous cement, 

 about 30 feet thick. There has been some 

 faulting accompanied by considerable breccia- 

 tion in the limestone. vSubsequent to the 

 faultmg hot solutions circulated through the 

 fault breccias, as is shown by iron stains and 

 by marked silicitication of the limestone 

 fragments. No definite evidence of valualile 

 mineralization was found. 



> ■ Bryan. Kirk. The Papago counlry, Ariz.: U. S. Geol. Survey Water- 

 Sup|>ly PapiT — (in preparation). 



