390 University of California Publications in Geology [Vol. 10 



bluish-black slate occur in Bunch Canon. The slaty cleavage is here 

 wholly conformable with the planes of sedimentation, but this is by no 

 means true in every outcrop. In many cases the lines of stratification 

 run transverse to the direction of cleavage. Several instances of this 

 are to be seen in Live Oak Ravine and in the second right-hand ravine 

 tributary to Bunch Canon. Figure 3 shows this feature in the upper 

 course of Bunch Creek. 



Microscopically the slate is seen to be composed of a very fine 

 aggregate of altered mineral fragments, with an occasional small, 

 angular quartz particle recognizable. The largest grain observed was 



Fig. 3. Mariposa slate showing cleavage transverse to 

 stratification. Bunch Creek. 



less than 0.5 mm. in diameter. The rock is in no way remarkable ; its 

 petrographic features can be duplicated in most of our shale forma- 

 tions. Microscopic organisms were searched for in several slides but 

 none were found. 



Numerous quartz veins traverse the slate in the southern part of 

 the area, but no important mines have been located on them. 



Sandstone 



With an increase in size of particles in the slate, sandstone is pro- 

 duced; gradation of particles from the finest up to the size of a pea 

 are to be observed in hand specimens. These variable sandstones are 



