386 University of California Publications in Geology [Vol. 10 



GENEEAL GEOLOGY 



The geology of this small region is typical of the foothill belt of 

 the entire Sierra Nevada ; all the rocks, except granite, commonly 

 found on the lower slopes of the range, occur within its limits. 



The oldest rocks shown on the map belong to the Calaveras forma- 

 tion. In the folio of the United States Geological Survey covering this 

 region five divisions of the Calaveras have been recognized, but for 

 the purposes of this paper the general term Calaveras suffices. Four 

 separate areas appear on the map. The formation west of New 

 England Mills consists of a highly compressed series of black, clay 

 slates and dark, argillaceous sandstones with several lenses and beds 

 of limestone and bluish or grayish chert. The smaller area northwest 

 of the Bear River is lithologically similar. Intermediate between these 

 two localities on the east bank of the Bear River, directly west of 

 Colfax, is a small outcrop of slate and slightly altered sandstone over- 

 lain by a thick mass of gray limestone which bears several species of 

 lower Carboniferous fossils (see section EP, pi. 28). Forms referable 

 to the anthozoan genera, Clisiophyllum and Lithostrotion, together 

 with round crinoid stems were collected, while two species of Pleuro- 

 tomaria, and one or two spirifiroid brachiopods were observed. In the 

 northeastern part of the area the Calaveras rocks are fissile clay slates 

 of a brownish to black color with a phyllitic development of mica, 

 which gives them a lustrous, silvery aspect when weathered. 



The Mariposa formation of Upper Jurassic age succeeds the 

 Calaveras. It is generally regarded as the latest sedimentary forma- 

 tion of the Bedrock series. The area shown on the map represents the 

 northern limit of a long belt of the formation traceable from Mariposa 

 County. In this vicinity it consists of black slates alternating with 

 dark gray sandstones and a great number of breccia beds. Its strati- 

 graphy will be considered more at length later. 



Igneous rocks are abundant in the area. The amphibolite belt 

 which the American River canon follows closely for some distance may 

 represent flows contemporaneous with the deposition of the Mariposa 

 sediments; long, narrow lenses of black slate included within the 

 amphibolite, near the toll-house on the Forest Hill road, suggest this 

 contemporaneity. Southward, however, and just east of the limit of 

 the map, dikes of a material in every way similar to the amphibolite 

 of the main belt clearly cut the Mariposa slate. 



The diabase west of Bear River is regarded by Lindgren as repre- 

 senting lava flows possibly antedating the Mariposa depositional epoch. 



