394 University of California Publications in Geology [Vol. 10 



crosses the stage road a great bed of breccia has been incised and 

 polished by the abrasion of the little stream, so that an extensive sur- 

 face is presented for study. Figure A in plate 31 rather inadequately 

 shows the locality; a number of tortuous bands of black slate occur 

 within a massive bed of coarse breccia. The slate areas which are 

 greatly elongated in the direction of the general strike of the breccia 

 bed represent muds which accumulated and solidified contemporan- 

 eously with the coarse material; some of the slate bands are seen to 

 feather out into the coarser sandstone matrix. 



The American River Canon is replete with bold outcrops of the 

 breccia. Beds interstratified with slate occur well up to the serpentine 

 contact on the west. The exposure is in the bed of the river. Refer- 

 ence has already been made to the thick bed of breccia at the small 

 suspension bridge which spans the American River at a point about 

 one-half mile southwest of Owl Creek. Some of the included boulders 

 are rounded ; they range in size up to fourteen inches in diameter. 



Megascopic Features 



Texturally the breccia grades from fine to extremely coarse; this 

 gradation is both lateral and vertical and, although usually gradual, 

 may be abrupt along the dip. Viewed broadly the breccia is essentially 

 a variable aggregate of angular, subangular and rounded boulders, 

 for the most part unevenly distributed through a matrix varying from 

 coarse sand to fine mud. Sorting action is rare. The variation of the 

 coarse material is in four directions, namely abundance, size, composi- 

 tion, and degree of rounding. 



Nearly every bed has a different ratio between coarse and fine 

 material ; in some cases the fine material is subordinate, in others only 

 a few scattered blocks occur in an otherwise medium- grained argil- 

 laceous sandstone. In no case, however, is there a total lack of fine 

 grained material. 



The average size of the included fragments is approximately one 

 inch. By this is meant that most of the pebbles are of this dimension ; 

 however, the variation both above and below this figure is so great 

 that a wholly inadequate conception of the appearance of the rock is 

 conveyed by any statement as to the average size of the component 

 fragments. In general the size of the fragments ranges from a frac- 

 tion of an inch up to three feet in maximum dimension. The ratio 

 of abundance of fragments between two inches and one-half inch to 



