1917] Moody: Breccias of the Mariposa Formation 391 



more abundant in the eastern part of the area; particularly good 

 exposures appear on the Nevada County narrow-gauge railroad near 

 the Bear River bridge, and in the middle portion of Bunch Canon. 

 The main part of Bushy Canon is cut in this rock. A tendency toward 

 slaty cleavage is evident in all the sandstone examined, but this feature 

 is far from the perfect development that it attains in the slates. In 

 every observed case, this imperfect cleavage is parallel to the strati- 

 fication planes. 



Under the microscope the heterogeneity of the sandstone is quite 

 marked. The individual particles grade in size from the finest up to 

 the largest fragments in the breccia. As has been said, no definite 

 line is to be drawn between sandstone on the one hand and breccia 

 on the other. Angularity is the rule in the fragments large enough 

 to be readily differentiated. Admixed with the angular material, 

 however, are many subangular and even rounded grains of quartz, 

 feldspar and limestone. The larger fragments are usually composed 

 of limestone or chert ; quartzite is commonly present, while fragments 

 of amphibole-schist, diabase, basic lava, and slate appear subordinately. 

 Plate 33, figure A, illustrates the main microscopic features of the rock. 



The fresh sandstone is normally dark in color, but is locally of a 

 greenish cast, due to the presence of chlorite stains. 



BRECCIA 

 Occurrence 



The most interesting and unusual lithologic type of the Mariposa 

 formation is the coarser variety of clastic here termed "breccia." 1 

 Under this head are included the beds in which the larger constituent 

 fragments range from the size of a pea up to a foot or more in 

 maximum dimension. 



The outcrops of the breccia are very striking, due to the great 

 induration of the beds, which are by far the most resistant members 

 of the formation. The ridge southwest of the nameless creek south of 

 Bushy Canon owes its prominence to a massive bed of this rock. The 

 breccia here strikes across the bed of the American River and, because 



i The word "breccia" has never been rigidly defined; it is here employed 

 because of its lack of suggestion as to an hypothesis of origin. The term 

 "conglomerate" is inapplicable because of the peculiar features of the deposit, 

 as well as on account of the unavoidable connotation of ideas of origin which 

 accompanies the use of this common, often too loosely applied, word. "Breccia," 

 as here used, is a term co-ordinate with "glomerate," recently proposed by 

 R. M. Field (Ottawa Nat., vol. 30, p. 31, 1916). 



