1917] 



Moody: Breccias of the Mariposa Formation 



393 



has produced striking exposures ; the larger masses show several ex- 

 foliating layers, the smaller, one or two. 



The heterogeneity of the breccia is perhaps better exemplified in 

 this cut than in any other exposure visited. One small block contains 

 fragments, from one to seven inches across, of chert, quartzite, lime- 

 stone, gabbro, diabase, amphibole-schist, and one rounded pebble which 

 is apparently a normal granite. 



A splendid section of the entire Mariposa belt is obtained by fol- 

 lowing Live Oak Ravine from New England Mills to its juncture with 

 Bunch Canon, thence down the Forest Hill road to the amphibolite 

 contact near the American River. The first breccia bed exposed in 

 this section is well seen on the county road one mile north of New 

 England Mills. Here a fifty foot bed of massive, unsorted material 

 lies between fine black slates dipping 70° E. Throughout Live Oak 

 Ravine coarse material is abundant in the sediments, but only four 

 well-individualized breccia beds appear ; the heaviest of these is per- 

 haps seventy-five feet thick. Reference has already been made to the 

 great number of breccia beds in the lower course of Bunch Canon. 

 All are unsorted, heterogeneous members of the series and are inter- 

 bedded with slate and sandstone. (See detailed columnar section, 

 fig. 2) In thickness the breccia beds range from two feet to four 

 hundred feet. In one exposure the breccia is seen to be lenticular. 

 Considerable mineralization has affected the Mariposa beds in the last 

 two miles of section. Several prospect holes mark attempts to find 

 wealth in the unresponsive rocks. One drift has been run over two 

 hundred feet into the mountain side. It follows a small dislocation 

 plane in the slate ; the gouge has been recemented and sparsely charged 

 with pyrite and less abundant chalcopyrite. Quartz and calcite veinlets 

 cut through slate, sandstone and breccia indifferently, and the above 

 mentioned sulphides are found disseminated in the coarser elastics. 

 The last member of the series, west of the amphibolite belt, is a massive 

 breccia bed which has been so thoroughly silicified and recemented 

 that it presents quite a different appearance from the usual breccia of 

 the formation ; it is hard, dense and almost uniformly aphanitic, except 

 in favorable spots where differential weathering has left a few frag- 

 ments in relief. The rock is pyritized. The Annie Laurie mine, one 

 of the more pretentious prospects of the region, is situated on the 

 amphibolite contact in this pyritized zone. 



An instructive outcrop occurs in Bunch Canon about one mile north 

 of its junction with Live Oak Ravine. At the point where the creek 



