186 



SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1921. 



absence of fossils is a strong argument against 

 this possibility, for most of the Paleozoic rocks 

 of the region are fossiliferous. The fact that 

 all these rocks except the youngest group are 

 very much more metamorphosed than the 

 known Paleozoic formations to the north and 

 east is another strong reason for believing that 

 they are pre-Cambrian rather than Paleozoic. 

 There is no reason for believing that there has 

 been more metamorphism in this area since 

 Paleozoic time than has occurred in the Ray 

 and Globe mining districts. The limestone 

 and quartzite of the youngest group are not 

 much if any more metamorphosed than similar 

 rocks of Paleozoic age at Ray and Globe. 



TERTIARY FORMATIONS. 

 GENERAL FEATURES. 



Lavas occur throughout the area covered by 

 this report and extend far beyond its limits. 

 The series consists of a number of flows of vary- 

 ing thickness and of widely different super- 

 ficial characteristics, associated with some 

 tuffs and agglomerates and a very subordinate 

 amount of sedimentary rock. It reaches its 

 maximum development in the S. H. Mountains, 

 where the total tliickness is cerainly more than 

 2,000 feet. A number of the individual flows 

 are several hundred feet thick. 



Volcanic rocks similar in occurrence and 

 general characteristics to rocks of this series 

 have been reported from a number of localities 

 in the Southwest. Such rocks are known in 

 the Patagonia district, in southern Arizona;' m 

 Mohave County, Ariz.," to the north of the 

 region covered by this report; in the' Papago 

 country," just south of this region; in eastern 

 California,'" and in southern Nevada." Similar 

 rocks occur at Globe, in central Arizona,'^ and 

 at many other places. These rocks have all 

 been referred to the Tertiary, and most of 



' Schrader, F. C, Mineral deposits of the Santa Rita and Patagonia 

 mountains, Ariz.: U. S. Geol. Survey Bull. 5s2, pp. 70-76, 1915. 



* Schrader, F. C, Mineral deposits o( the Cerbat Range, Black Moim- 

 tains, and Grand Wash Cliffs, Mohave County, Ariz.: U. S. Geol. Survey 

 Bull. 34U. pp. .07-59, 1907. 



"Brj'au, ICirk, The Papago country, Ariz.: U. S. Geol. Sun'ey Water- 

 Supply Paper — (in preparation). 



10 Brown, J. S., The Salton Sea region, Calif.: U. S. Geol. Sinvey 

 Water-Supply Paper — (in preparation). Thompson, D. G., The 

 Mohave Desert region, Calil.: U.S. Geol. Survey Water-Supply Paper — 

 (in preparation). 



11 Ball, S. H., .\ geologic reconnaissance in southwestern Nevada and 

 eastern Cahfornia: U. S. Geol. Survey Bull. 30.s, pp. 31-34, 1907. 



" Rausome, F. L., Geology of the CUobe copper district, Ariz.: U. S. 

 Geol. Survey Prot. Paper 12, pp. (-s-9o, 1903. 



them are supposed to be Miocene. This sup- 

 position is based principally on then- field 

 relations to rocks of known age, the paleonto- 

 logic evidence within the rocks themselves be- 

 ing scanty or lacking. 



Overlying the Tertiary beds and associated 

 with the unconsolidated or partly consolidated 

 Quaternary sand and gravel are basalt flows 

 of early Quaternary age. These will be dis- 

 cussed under the Quaternary formations. The 

 faulted and uplifted basalts that cap many of 

 the momi tains, however, are considered to be of 

 Tertiary age. 



The amount of sedimentary material asso- 

 ciated with the Tertiary lavas is small com- 

 pared to the total thickness of the lavas. The 

 setlimentary rocks are of geologic importance, 

 however, for they furnish clues as to the condi- 

 tions existing at the time these great flows 

 occurred. They comprise sandstone, in part 

 arkosic, shale, and calcareous beds. 



TERTLVRY I_\VAS. 



Distribution and character. — The Tertiary 

 lavas are almost as miiversally present in this 

 region as the metamorphic complex just de- 

 scribed. They were fomid in almost every 

 mountain range examined during this investi- 

 gation, the only exceptions being the Harqua- 

 hala and Little Harquahala mountains. Some 

 of the ranges, such as the S. H., Eagle Tail, 

 and Castle Dome mountains, are composed ex- 

 clusively of rocks of this series resting on a 

 metamorphic basement which is visible in only 

 a few smtdl areas. 



The lavas are for the most part light-colored 

 acidic rocks, but some are basalts. They dis- 

 play a wide range ami variety of coloration. 

 This is particularly striking in the Eagle Tail 

 Moimtains, where more than 1,000 feet of 

 nearly horizontal lava flows, with interbedded 

 tuff, is exposed. Nearly every flow is different 

 in color from those above and below it, and 

 each stands out from the others with clean-cut 

 boundaries. Among the colors are brilliant 

 yellow, soft green, vivid red, somber browm 

 and dun, and creamy white, with streaks of 

 purple, heliotrope, anil other hues. The petrog- 

 rapher who is interested in Tertiary igneous 

 rocks would find much to interest him here 

 and in the other ranges in this region where 

 similar rocks occur. 



