1917] 



Moody: Breccias of the Mariposa Formation 



387 



Part of the diabase is certainly irruptive, however ; dikes of a por- 

 phyritic facies clearly intrude the Calaveras limestone shown in sec- 

 tion EF, and other dikes may be seen crossing the Mariposa contact 

 where exposed by the excavations for the Bear River canal. Numerous 

 diabase dikes are included wholly within the Mariposa formation. 



Serpentine, in dikes and masses, cuts the Mariposa and older 

 formations. Its occurrence in dikes along contacts of both igneous 

 and sedimentary rocks suggests that it represents a late phase of 

 intrusion. The planes of schistosity of the Calaveras were, however, 

 seen to pass obliquely across a serpentine dike near the Bear River, 



AM. Amphibolite; DB, Diabase; SP, Serpentine; 

 GB, Gabbro. 



Fig. 1. Generalized columnar section 



though other areas are wholly unsheared ; the possibility of two periods 

 of serpentine intrusion is thus indicated. 



The elliptical area of gabbro, northwest of Colfax, is a coarse 

 granitic rock of basic aspect, which has invaded the Mariposa, the older 

 diabase and the Calaveras. The mapping suggests that this gabbro 

 was brought into its present position by anticlinal folding, but its 

 irruptive nature is indicated by chilled margins and discordance with 

 the strike of the sedimentary beds. 



The only other rocks in the vicinity of Colfax are the volcanic tuffs 

 and river gravels of the superjacent series, which appear as outliers 

 on the higher eminences of the region. 



