406 



University of California Publications in Geology [Vol. 10 



developed under the humid cycle is, however, believed to furnish 

 a possible solution of the problem of origin. 



PROBABLE ORIGIN OP THE BRECCIA 



We have here to deal with a normal conformable series of strata 

 progressing in the main from coarse to fine, although occasionally 

 exhibiting a sudden discontinuous change from fine to coarse. The 

 slaty faeies of the series is of marine origin; the breccia, which is 

 interbedded with it, must clearly have been deposited in a marine 

 basin. The problem is then to explain the occurrence of angular 

 pebbles in a fine arenaceous to a silty matrix. 



It is obvious that wave action must have been at a minimum during 

 the period of accumulation of the coarse material. It is equally cer- 

 tain that transportation by fluviatile agencies took place if at all only 

 through short distances. The material of the larger fragments is all 

 derived from the Calaveras rocks exposed in the immediate vicinity, 

 comprising chert, limestone, slate, sandstone, and schists of various 

 types, as well a.s the more common basic igneous rocks, which make up 

 this older series. Quartz pebbles alone seem to have been transported 

 some distance; quartz veins do occur in the Calaveras, but they may 

 belong to the same period of mineralization which affected the Mari- 

 posa at the time of the production of the great Mother Lode belt to 

 the south, but as these pebbles are in general rounded they may be 

 considered to have come from a distance. 



In discussing the evidence for the Mariposa-Calaveras unconformity 

 Turner 30 states that the two distinct belts of the Mariposa in and 

 south of the Jackson quadrangle were deposited in long, narrow 

 troughs between Calaveras ridges, and that later the beds were infolded 

 with the Carboniferous series. Intrusive and extrusive igneous action 

 as well as adjustment of stresses through folding and perhaps fault- 

 ing prepared a Jurassic surface of marked relief in which two long, 

 narrow basins roughly parallel and striking in a northwest-southeast 

 direction, connected with the open sea probably to the southeast. These 

 troughs received the sediments which we now know as the Mariposa 

 beds. 



The opinion of Lindgren 31 as to the origin of the Mariposa of the 

 Colfax region is expressed as follows: "The formation was clearly 



so Turner, H. W., Fourteenth Annual Report, U. S. Geol. Surv., pp. 456-457, 

 1892. 



si Lindgren, W., U. S. Geol. Surv. Folio, no. 66, p. 3. 



