1917] 



Moody: Breccias of the Mariposa Formation 



389 



slaty bands were counted. In the Live Oak 

 Ravine section four heavy breccia beds and 

 seventeen sandstone members appear sub- 

 ordinate to twelve slate layers, some of 

 which are over eight hundred feet thick. 

 Frequent alternations of depositional con- 

 ditions are thus indicated in Mariposa time, 

 but the completeness of the gradations be- 

 tween the various lithologic types repre- 

 sented, and the large number of minor re- 

 currences, make a subdivision of the forma- 

 tion impractical. 



The lithologic distinctness of the Mari- 

 posa from the Calaveras formation is quite 

 marked in the field. The chert, limestones 

 and fissile slates and schists which charac- 

 terize the latter are wholly lacking in the 

 Mariposa. The boundaries are distinct in p 

 most places, and the alteration and defor- 

 mation are decidedly greater in the older 

 series. The conformable, gradational series 

 of slates, sandstones and breccias mapped 

 as the Mariposa, then, is comprised in a 

 well differentiated formation. 



Slate 



The slates of the Mariposa are more con- 

 stant in petrographic character than are the 

 sandstone and breccias. They are nor- 

 mally black, fine-grained shales or slates 

 with a well pronounced slaty cleavage. A 

 high degree of fissility is not attained by 

 the slate ; and arenaceous varieties, grading 

 into sandstone, are quite abundant, particu- 

 larly in the eastern part of the Mariposa 

 belt. The exposures in the ravines and rail- 

 road cuts southwest of Colfax are generally 

 of a rusty brown color, the result of weath- 

 ering; below the regolith the slate is seen to q 

 be firm and black. Splendid outcrops of 



Brecci 



Bri 



Coarse 

 breccia 



Coarse 

 breccia 



Breccia 



Slatv 

 breccia 



Breccia 

 . Breccia 



Slaty 

 breccia 



' Coarse 

 breccia 



Breccia 



Breccia 



Sandy 

 breccia 



Breccia 



Coarse 

 breccia 



Fig. 2. Detailed columnar 

 section between P and Q, 

 structure section KLM. 

 Thickness 5000 feet with 

 possible repetition at R. 



