392 University of California Publications in Geology [Vol. 10 



of its superior hardness, has caused a marked constriction in the 

 narrow canon. Other thick breccia layers strike across Bunch Canon 

 and in each case the rock shows the same resistance to erosion. So 

 thoroughly indurated is the breccia that artificial fractures as well as 

 joints transect the included fragments. AVeathered-out fragments are 

 rare ; the only places where they are notably developed are on the 

 Colfax road, about a mile north of Ne.w England Mills, and near the 

 point where the county road leading west from Colfax crosses the 

 Bear River. 



Breccia occurs at all horizons throughout the Mariposa, but is 

 somewhat more abundant in the eastern part of the terrane ; slate pre- 

 dominates in the western part. Thus only four well differentiated 

 breccia beds appear in Live Oak Ravine, while twenty-four were 

 counted on the Forest Hill road. 



The several small cuts on the Nevada County narrow gauge line 

 north of Colfax reveal a number of sandy beds which pass into coarser 

 material in places. The second railroad cut south of the Bear River 

 bridge is in a coarse, massive angular breccia in which spheroidal 

 weathering is well-developed. Several coarse, angular fragments 

 form the center of highly silicified sandstone bodies, several feet in 

 diameter. These masses stand out prominently in the weathering 

 of the cliffs, and might easily be mistaken for extra large boulders in 

 the breccia. Their black color and extreme hardness tend to accentuate 

 this possibility. Massive outcrops of rather uniformly angular breccia 

 are to be seen just below the point where the county road crosses the 

 narrow gauge line. Black slates lie above and below this mass. The 

 prominent hill, southwest of Colfax, in the broad embayment of the 

 triangular diabase area, is composed of coarse breccia with some slate 

 beds. In the lower course of the Bear River a few thin beds of hard, 

 angular breccia are interstratified with the typical black slates of the 

 Mariposa. 



The railroad cuts of the Central Pacific south of Colfax furnish 

 excellent exposures of the various rock types of the Mariposa, but 

 difficulty is experienced here in deciding as to the relation of the 

 breccia to the slate; if it were not for sections showing this relation 

 better elsewhere it might be assumed that the breccia was a separate 

 formation. Faulting has contributed to the difficulties, but the chief 

 of these is the wholly massive character of the breccia. In the second 

 cut south of the town, great rounded masses of hard angular breccia 

 project through a red mantle of decayed rock. Spheroidal weathering 



