398 University of California Publications in Geology ["Vol. 10 



ably in abundance are apparently nowbere so well developed as in the 

 region described. 



The series of breccias of the Colfax area continues into the Placer- 

 ville quadrangle to the south. Lindgren and Turner 3 refer to them 

 here as "a series of dark, partly volcanic sandstones and breccias 

 intercalated among the slates." 



Farther south the breccia becomes less abundant. Near Plymouth, 

 in the Jackson quadrangle, occur two or three beds interstratified with 

 the clay slates and schistose sandstones. Southwest of San Andreas 

 the conglomerate is described 4 as largely replacing the slate. Near 

 Angel's Camp breccia and tuff are interbedded with slate. Ransome 5 

 regards the Mariposa section of Woods Creek in the Sonora quadrangle 

 south of Table Mountain as affording the best exposures in the Mother 

 Lode belt. Here are interbedded slates, schistose sandstone and con- 

 glomerates. The latter are not always persistent, but often pass 

 irregularly into sandstone and slate. These observations tend to show 

 that the breccia phenomena are not to be referred to some local, 

 peculiar condition ; the agencies which produced them were general 

 and widespread. 



On Dry Creek, west of Drytown, Amador County, occurs a ten- 

 foot bed of. brecciated material consisting of fragments of quartz, 

 quartzite. chert, limestone and various igneous rocks, ranging from 

 angular to well-rounded in outline and cemented by a phyllitic matrix. 

 This bed was mapped by Ransome as a part of the Calaveras forma- 

 tion, but as mentioned in the text of the Mother Lode folio, its relations 

 to the Mariposa were not plain. It may belong to the series which runs 

 north to Colfax. Dr. Adolph Knopf, who has first hand knowledge of 

 the occurrence, is of the opinion that this bed represents either a 

 fanglomerate or a tillite with the probabilities favoring the latter 

 hypothesis. 



PALAEONTOLOGIC EVIDENCE 

 The fauna of the Mariposa formation throughout its whole extent 

 is sufficient to place the age of the beds as Upper Jurassic. Dr. 

 Stanton and Professor J. P. Smith concur in this determination. 

 Within the limits of the area studied, however, only two forms have 

 been discovered which are of palaeontologic significance. These are 



3 Lindgren, W. and Turner, H. W., IT. S. Geol. Surv. Folio, no. 3. 

 * Ransome, F. L., U. S. Geol. Surv. Folio, no. 63. 

 s Loc. cit. 



« Personal communication. 



