1917] 



Moody: Breccias of the Mariposa Formation 



395 



those having a maximum dimension of three feet is estimated to be 

 about 100:1. Particles less than one-half inch in diameter are of 

 course far more numerous than all the others ; it appeared in the field 

 that the material of this size constitutes from three-eighths to one-half 

 the volume of the entire rock. Many holes or cavernous spaces, with 

 somewhat more restricted limits of size than the included fragments, 

 indicate former limestone fragments which have been dissolved in the 

 meteoric circulation. 



The A'icissitudes through which the Mariposa rocks have passed are 

 not reflected to any great extent in the breccia. The pebbles have 

 suffered very little, if any, deformation ; it is only in the finer facies 

 that any perceptible shearing planes have been developed. The 

 coarser rocks are typically massive ; they fracture indifferently in any 

 direction. 



The argillaceous matrix of the breccia is, in the hand-specimen, well 

 differentiated from the coarser particles of the rock. It is in every 

 way like the black slates with which the breccias are intercalated, and, 

 where sufficiently abundant, shows slaty cleavage in the direction of 

 prevailing schistosity. 



The pebbles and boulders included in the breccia represent a wide 

 range of rock types. Perhaps the most abundant rock represented is 

 chert. Black, gray and green fragments, always wholly angular, occur 

 in nearly every other outcrop ; they are in no way different from the 

 fragments that now mantle the hillsides in the Calaveras area to the 

 west. Next in abundance are limestone pebbles usually of a dark gray 

 color and either angular or subangular in outline. Rounded as well 

 as angular and subangular pebbles of diabase are quite abundant. Of 

 equal prominence are quartz pebbles, which on the whole are moder- 

 ately well water-worn ; few of them can be called angular. Boulders 

 up to a foot in diameter are not uncommon. Subordinately appear 

 blocks of gabbro, basic lava, amphibole-schist, mica-schist, slate, friable 

 sandstone ami felsite. One pebble showing quartz and feldspar 

 twinned on the Carlsbad law was seen in the exposure of massive 

 breccia west of Colfax. It seemed to be a typical granite. Fairbanks 2 

 also reports granite boulders from the Mariposa in the vicinity. 



Rounded, water-worn material is not wanting in the breccia; in 

 fact some of the exposures east of Colfax contain it in some abundance, 

 so that the breccia in places passes into normal conglomerate. In the 

 main, however, angular and subangular material predominates, and it 



2 Fairbanks, H. W., Jour. Geol., vol. 5, no. 1, p. 76, 1897. 



