1917] 



Moody: Breccias of the Mariposa Formation 



397 



of the impact phenomena sometimes observed where floating ice is an 

 important agent of transportation. The accompanying sketch (fig. 4) 

 shows diagrammatically the relations in this exposure. A quartzite 

 boulder (A) ten inches in diameter, elongated in the direction of the 

 bedding of the finer sediments, lies in a depression in a thin slate lens, 

 and is overlain by a second slate stratum which arches over it in a 

 pronounced curve; in the upper stratum a diabase block (B) is dis- 

 tinctly embedded in the fine, slaty material, the sedimentary planes 

 coming squarely up to the subangular block and not passing around 

 it. The attitude of block B is such that it could not possibly have 

 been laid down contemporaneously with the argillaceous material of 

 the slate ; it must have been dropped into its present position after the 

 upper slate stratum was laid down. This occurrence is, however, not 

 typically that observed where evidences of impact are clearly pre- 

 served ; no crater development is to be seen in the slaty material sur- 

 rounding the boulder. 



Microscopic Features 



A microscopic examination of the breccia was undertaken in the 

 hope of gaining some clue as to its origin, as reflected in the nature of 

 the matrix. The accompanying photomicrographs (pi. 30) show the 

 features revealed in thin section. 



Microscopically the matrix is seen to grade from coarse sand down 

 to a fine, felty aggregate which is unresolvable under the microscope. 

 Petrographically it is quite similar to the black slate of the series, the 

 only difference being that it contains larger recognizable particles. 

 These larger particles usually consist of quartz, but some very fine 

 chert granules are to be seen. The recognizable fragments, like their 

 large relatives, are chiefly angular or subangular (fig. A), although 

 somewhat rounded ones also occur (fig. B). The degree of comminu- 

 tion, then, is the only apparent difference between the fine and coarse 

 phases of the rock. 



BRECCIAS IN OTHER AREAS OF THE MARIPOSA 



The area of the Mariposa beds shown on the map represents the 

 northernmost extension of a remarkably persistent narrow belt of 

 similar beds which traverses the entire Mother Lode district. Breccias 

 are present throughout this extent, and although they vary consider- 



