388 University of California Publications in Geology [Vol. 10 



STRATIGRAPHY OF THE MARIPOSA FORMATION 

 General Statement 



The Mariposa formation is familiarly referred to as the "Mariposa 

 slates," and this term is justified when the two long belts of dark 

 sediments, stretching from Mariposa County to well north of the 

 Mother Lode are considered. In many localities, however, coarser sedi- 

 ments are interbedded with the slates ; thus in the Colfax region not 

 over half the rocks of the formation are slate. Sandstones and grits 

 are here well differentiated from the typical black, slaty argillites. 

 Both within the slate and within the sandstone occur numerous beds 

 of angular breccia and subordinate conglomerate lenses which persist 

 on the strike and form bold ridges in the differential weathering of 

 the series. It is estimated that one-fifth the volume of the Mariposa 

 beds shown on the map is made up of this coarse material. 



That the Mariposa formation as mapped is a stratigraphic unit 

 cannot be denied. The relation of the breccia to the slate in the 

 immediate vicinity of Colfax is somewhat obscured by the regolith as 

 well as by minor faulting ; but the sections exposed in Live Oak Ravine, 

 in the American River canon and on the Forest Hill road in Bunch 

 Canon, clearly show a conformable series of slates, sandstones and 

 breccias interbedded and in places grading one into the other. 



Columnar sections were made in the field showing the sequence of 

 beds in Bunch Canon and in Live Oak Ravine. A gradational series 

 from breccia through grit and sandstone to slate is thus made evident. 

 The thicknesses of the various members were estimated by pacing the 

 horizontal distance. In some cases it was necessary to make arbitrary 

 separations of the coarser sandstone from the breccia, and of the slate 

 from the sandstone, where no such lines of separation actually exist. 

 Some doubt must be entertained as to the number of beds indicated 

 because of the possibility of repetition either by folding or faulting. 

 The vertical section taken from Bunch Canon, and shown in figure 2, 

 is believed to be freer from these sources of difficulty as the dips are 

 quite constant, except at R, and the beds are admirably exposed. 

 Some indications of repetition by synclinal folding appear at B. If 

 the section from P to Q is continuous, twenty-four beds of breccia, 

 varying in thickness from two feet to four hundred feet, are inter- 

 stratified with twenty-two slate bands, and fourteen beds of sand- 

 stone. From the anticline at Q to the intersection of Live Oak Ravine 

 with Bunch Canon six breccia beds, eleven sandstone beds and eight 



