. Bulletin of the Department of Geology. Geology. 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
. Bulletin of the Department of Geology. Geology. 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 Please
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