184 



SHORTER (CONTRIBUTIONS TO GENKRAL GEOLOGY, 1931. 



four general groups — (1) igneous rocks, (2) 

 highly metamorpliosed schistose rocks, prob- 

 ably in the main of sedimentary origin, (3) 

 thoroughly metamorphosed but much less 

 schistose sedimentary rocks separated from 

 those of the second group by an unconformity, 

 and (4) metamorphosed but not schistose 

 limestone and quartzite, the youngest sedi- 

 mentary rocks in the basal complex. The 

 igneous rocks may be further subdivided into 

 batholithic masses with associated dikes and 

 a group of somewhat younger dikes which 

 cut the less metamorphosed portions of the 

 basal complex. 



Distribution and character. — This ancient 

 complex is present in every mountain range 

 and almost every range of hills in the region 

 under consideration. Even in those moun- 

 tainous areas whei'e it is not shown on the 

 geologic map (PI. XLV, in pocket) outcrops 

 can be found in stream beds that have been 

 cut through the younger formations which 

 elsewhere cover it. In some of the hills, 

 especially those which are composed of ba- 

 saltic lavas, such as the Bouse Hills and Palo 

 Verde Hills, metamorphic rocks do not occur. 



Many of the exposures of the basal complex 

 consist of granitoid rocks. The bulk of these 

 rocks are gray and pinkish gneisses which 

 before their metamorphism were normal gran- 

 ites and intrusive rocks of similar types. 

 These rocks are older than nearly all the other 

 formations in the region, and they crop out in 

 most of the mountain ranges. Plate XLI, B, 

 shows their typical appearance. There are 

 also in the region certain younger granites, 

 not gneissoid, which Bancroft ' considers to 

 be Mesozoic. These are very similar in super- 

 ficial appearance to the ancient granites in 

 th(^ several areas of such rocks mapped by 

 previous workers. (See geologic map, Pi. 

 XLV.) In the Buckskin Mountains near 

 Osborne's Well there are outcrops of a fresh 

 gi'ay granite with no suggestion of gneissic 

 structure. This rock contains specularite in 

 places and perhaps has been otherwise min- 

 eralized, as several shallow prospect holes 

 have been sunk in it. Probably it belongs to 

 the group of Mi^sozoic intrusive rocks. The- 



1 Biincroft, Howland, A roconnaissaiico of the ore deposits in northern 

 Yuma County, Ariz.: U. S. Geo). Survey Bull. 4.51, p. 29, 1911. 



boundaries of this mass were not mapped. 

 Jones ^ reports the presence of Mesozoic intru- 

 sive rocks near Kofa, in the .S. H. Mountains 

 and in the Dome Rock Mountains. Ban- 

 croft ^ found dikes probably of Mesozoic age 

 in the Harcuvar Mountains and Granite 

 Wash Hills. It is probable that there are 

 other areas of igneous rock of this age in the 

 region. 



The basal complex also includes small dikes 

 composed for the most part of diabase and 

 pegmatite. They are of general occurrence 

 but have nowhere been found in large quan- 

 tity. Bancroft ^ describes these rocks and also 

 mentions certain exposures in thc^ region north 

 of that covered by the present report which he 

 considers to be metamorphosed lava flows gen- 

 etically related to the diabasic intrusive rocks. 



In the Buckskin Mountains between Butler 

 Well and Midway there are some good ex- 

 posures of the ancient rocks. At one place in 

 particular the unconformity between the major 

 series of metamorphosed sediments, described 

 below, and the gneiss can be clearly seen. 

 Associated with the gneiss and clearly below 

 the surface of unconformity are intensely 

 metamorphosed schists, mostly somewhat cUo- 

 ritic. In the Gila Bend Mountains also there 

 are small masses of fine-grained mica schists 

 and quartzose schists included in the gneiss. 

 At the southern extreftiity of the Big Horn 

 Mountains, just north of the Palo Verde mine, 

 is a hill composed entirely of dark-green 

 foliated cMoritic schist. (See PI. XLI, A.) 

 This rock is different from any observed else- 

 where in the area, but presumably it is related 

 in age to the rest of the metamorphic rocks. 

 As it is very higlily schistose, it is probably 

 related to the most ancient of the schistose 

 rocks. Blanchard •"' reports inclusions of meta- 

 morphosed limestone and dolomite in a few 

 places in the gneiss of the Buckskin Mountains. 

 In one outcrop of dolomite he found what he 

 considers may be indistinct traces of organic 

 remains. 



2 Jones, E. L., jr., .\ reconnaissance in the Kofa Mountains, Ariz.: 

 U.S. Geol. Survey Bull. 6211, pp. 151-104, 1916; Gold deposits near Quartj- 

 site, Ariz.: Idem, p. 47. 



3 Bancroft, Howland, oj). eil., p. 30. 

 < Idem, p. 2S. 



6 Blanchard, R. C, The geology of the western Buckskin Mountains, 

 Yuma County, Ariz.: Columbia Univ. Contr. Geol. Dept., vol. 26, No. 1, 

 pp. Si-M, 191:i. 



