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University of California Publications in Geology [Vol. 10 



with coarse, and a series of beds, in every way similar to the Mariposa, 

 would be laid down. 



Great persistence on the original dip is not to be anticipated under 

 such conditions, and several instances have been cited where the 

 Mariposa breccias grade laterally into sandstone and slate. Since 

 the dips are all high it has not been found possible to trace many of 

 the beds into fine sediments. The lack of cross-bedding in the Mari- 

 posa series is not considered a serious objection to its estuarine origin 

 under the conditions described, because it is under just such conditions 

 that current action is at a minimum; tidal action being spread over a 

 long shore line nearly equally, would not favor false stratification 

 although the chief role in the intermingling of the differently derived 

 sediments must be assigned to this agency. 



The sporadic distribution of boulders through some of the slate 

 members constitutes the most serious objection to an hypothesis of 

 origin involving alluviation ; their presence must be accounted for in 

 some other way. The alternate hypothesis of ground-ice transporta- 

 tion is the only one which suggests itself as reasonably accounting for 

 this uncommon distribution. 



The assumption of rifting or monoclinal folding as the agency 

 which outlined the Mariposa basin is not violent in the light of later 

 events in the history of the Sierra Nevada. That lines of weakness 

 existed in the earth 's crust in this region, trending in the same general 

 direction, is abundantly witnessed by the great shearing-stresses that 

 imposed a slaty cleavage on the rocks at the close of the Jurassic. 



STRUCTURE 



The Mariposa beds, in the area studied, have a structure imposed 

 upon them which in detail is complicated, but which in its broad 

 features is comparatively simple. The beds have been folded into a 

 series of anticlines and synclines due to eompressional forces acting in 

 a northeast-southwest direction. The general strike of the structure 

 is northwest-southeast. 



The folding does not seem to have been as intense in the northern 

 part of the belt as in the southern. The dips are gentler and the folds 

 are more open in the vicinity of Colfax. In the Live Oak Ravine- 

 Bunch Canon section (sec. K L M, pi. 29) it is very difficult to make out 

 the position of the several folds since the whole series has been closely 

 compressed and overturned ; a careful measurement of dip changes 

 affords the only method of deciphering the structure here ; even at 



