1917] Moody: Breccias of the Mariposa Formation 411 



best a high degree of accuracy cannot be attained. In the more open 

 folds exposed along the Bear River and in the railroad cuts south of 

 Colfax comparatively little difficulty of this nature is experienced. 

 The approximate structural relations are shown in the seven sections 

 on plate 29. 



A fault is indicated in the sections but has not been placed upon 

 the map. The long straight contact north of Cape Horn most certainly 

 represents a dislocation of some magnitude. In the Colfax folio, 

 Lindgren 32 states that the angular discordance at this contact is due 

 to an unconformity, but since the younger beds are truncated by the 

 contact plane (see sees. AB, CD, GH) their present position can be 

 due only to a fault. No crushing is to be seen at the contact but this 

 does not seriously weaken the evidence for the existence of the dis- 

 location. 



Faulting within the Mariposa itself is locally abundant but large 

 dislocations were not recognized. Three well-defined minor faults 

 affecting the slates were observed in a railroad cut one mile south of 

 Colfax. In the creek east of Colfax a marked discordance of dip 

 occurs in the slate; nearly horizontal beds are seen lying on the up- 

 turned jagged edges of the lower strata, with considerable kataclastic 

 matter intervening. 



The slaty cleavage that now characterizes the Mariposa beds was 

 superposed on an already folded system of rocks. That this is true is 

 proved by the not infrequent occurrence of planes of slaty cleavage 

 cutting the original bedding planes. This phenomenon was observed 

 only in the slate ; the sandstone and breccia members always conform 

 to the shearing planes. Figure 3 shows the slaty cleavage almost 

 normal to the planes of sedimentation. 



Viewed broadly the Mariposa rocks are to be considered as uncon- 

 formably overlying the Calaveras and infolded with them in the great 

 diastrophic paroxysm that affected the region at the close of the 

 Jurassic. Evidence of structural unconformity is, however, hard to 

 discover in the field. The western contact is obscured by debris north 

 of Howell Hill, and south of this point a serpentine dike has appeared 

 between the two formations. The eastern contact is regarded as a 

 fault and the northern limit is an irruptive mass. When deformation 

 is considered, however, the Mariposa rocks appear much younger than 

 the Calaveras. 



An exposure just east of Colfax station, at the south end of the 



32 Lindgren, W., U. S. Geol. Surv. Folio, no. 66. 



