396 University of California Publications in Geology [Vol. 10 



is this type of fragment that lends distinction to the breccia. In 

 general the pebbles are pockety and unsorted and the longer axes of 

 the flat fragments often transgress the bedding planes. 



Certain phenomena appear in a study of the disposition of some of 

 the coarse blocks which are very difficult to explain without recourse 

 to the transporting action of ice. I refer to the rather exceptional 

 occurrence of single or nearly isolated boulders, some of them ranging 

 in size up to a yard in maximum dimension, entirely within rather 

 thin-bedded slate. Plate 33, figure B, shows a chert boulder three feet 

 in diameter, in a railroad cut just south of Colfax, which is embedded 

 within an arenaceous slate. The boulder is now rather extensively 

 broken down by weathering, but it was evidently somewhat rounded 



Fig. 4. Boulders in finer sediments of Mariposa breccia. North Fork of 

 American River between Owl and Bushy creeks. 



by water wear before its deposition in its fine-grained matrix. Other 

 blocks of a similar nature occur in this outcrop of the breccia. 



The photographs shown in plate 32 were obtained in the bed of the 

 North Fork of the American River between Owl and Bushy creeks. 

 They illustrate the isolated boulders which are rather abundant 

 in the finer sediments prevailing in this cafion. Eight or ten inches 

 is the average diameter of the coarser material here observed. The 

 boulders all show some evidence of water wear ; some of them may even 

 be said to be well rounded. In both the photographs fine-bedded slate 

 is observed to pass around the central boulder ; in figure B this may 

 be due to shearing, although it has the appearance of original sedi- 

 mentation, but in figure A appears a feature which is rather suggestive 



