194 



SHORTER CONTRIBUTIONS TO GENEKAT. GEOLOGY, IDSl. 



GEOLOGIC HISTORY. 

 EARLY PRE-CAMBRIAN TIME. 



The remnants of the oklest pre-Cambrian 

 rooks in this area are so few, so widely scat- 

 tered, and SO intensely metamorphosed that 

 almost nothing can be learned from them as 

 to the events of that ancient time. The rocks 

 referred to are the micaceous and chloritic 

 schists, quartzitic schists, and metamorphosed 

 limestones included in gneiss in the Buckskin 

 and Gila Bend mountains. (Seep. 184.) Some 

 of these rocks have the megascopic appearance 

 of liighly altered sediments, but it is by no 

 means certain tliat that is their origin. The 

 large amount of chlorite in some of the schists 

 suggests an igneous origin, but nothing more 

 definite is known regarding them. All that 

 the record shows is that in early pre-Cambrian 

 time certain rocks, principally of sedimentary 

 but perhaps also in part of igneous origin, 

 existed here. These rocks w<'re Inuied, meta- 

 morphosed, and finally intruded by batho- 

 lithic masses of granite and kindred rocks. 

 The period of intrusion was followed by a 

 very long period of erosion. Nearly all the 

 ancient schists were removed, and the granitic 

 rock was exposed. Meanwhile the granites 

 had been rendered gneissoid, and the blocks 

 of other rocks included in them had been 

 highly altered l)y dynamic metamorphism. 



LATE PRE-CAMBRIAN TIME. 



The next event recorded was sinking of the 

 land and infiux of the sea. A tliick series of 

 sandstone and limestone with some sliale was 

 laid down in this sea. 



Many dikes, principally of diabase and peg- 

 matite, are associated with the m<^t amorphic 

 formations. Some of these are to be corre- 

 lated in age with the ancient batholithic intru- 

 sions and are older than the younger pre- 

 Cambrian sedunentary rocks. Others clearly 

 cut and are therefore younger than th<> later 

 sedunentary rocks. The field w<irk Wiis not 

 sufficiently detailed to make it possilile to 

 differentiate these dike rocks. Bancroft has 

 found evidence in the nortliein portion of the 

 area indicating that volcanism occurred during 

 the period of marine sedimentation. (See 

 p. 184.) 



PALEOZOIC AND MESOZOIC TIME. 



No sediments of known Paleozoic or Meso- 

 zoic age have been found in the region. Lime- 

 stones and fjuartzites of possible Paleozoic age 

 occur in the Hanjuahala Mountains and else- 

 where. (See p. IS.5.) These rocks represent 

 either sedimentation near the end of pre- 

 Cambrian time or a continuation of marine 

 sedimentation in the Paleozoic, but the evi- 

 dence at hand is not sufficient to determine 

 their age definitely. If any other Paleozoic 

 or Mesozoic sediments were ever deposited in 

 this region they have since been almost or 

 entirely removed by erosion. It is possible 

 that small amounts of such rocks occur in 

 those pai'ts of the region that were not visited 

 during this investigation. Enough is known, 

 however, to justify the belief that no large 

 areas of such rocks are present anywhere in 

 the region covered by this report. 



The region was again uplifted at some time 

 after the period of marine conditions recorded 

 by the pre-Cambrian sedimentary rocks. Ero- 

 sion was resumed and was long continued. If 

 the marine sediments covered the whole region 

 at the end of pre-Cambrian time, they have 

 been completely removed over large areas and 

 the gneisses once more laid bare. There is 

 abundant evidence, however, that the surface 

 over which the Tertiary lavas flowed was by 

 no means a plain. The country was rolling 

 and hilly.' Some of the small' mountain ranges 

 that are present to-day existed then, although 

 it is probable that they were not as high or as 

 ruggetl as they are now. 



Granitic stocks or small batholiths accom- 

 panied or immecfiately followed by dikes of 

 various types were intruded into the rocks of 

 this region at some period after the pre-Cam- 

 brian and l)efore the Tertiary. The writers 

 who have previously described such rocks 

 consider them to be Mesozoic. This correla- 

 tion seems to be probable and entirely in 

 accord with the facts so far as they are known. 

 Rocks of this type have been reported from 

 the Dome Rock Mountains,^" the S. H. Moun- 

 tains,^' and the Harcuvar Mountains "" and 



M Jones, E. L., jr., Gold deposits near Quartzsite, Ariz.: U. .S. Geol. 

 Survey Bull. 620, p. -is, 1916. 



"Jones, E. L., jr., A reconnaissance in the Kola Mountains. Ariz.: 

 U. S. Geol. Survey Bull. 62(1, p. 15.5, 1916. 



«> Bancroft, Howland, Reconnaissance ot the ore deposits in northern 

 Yuma County, Ariz.: V. S. Geot. Survey Bull. 4.'.1, pp. 29-30, 1911. 



