1917] 



Mooch): Breccia* of the Mariposa Formation 



407 



deposited in a gulf or shallow bay, the conglomerates indicating the 

 immediate proximity of the shore line." 



Two alternative hypotheses of origin merit consideration ; the first, 

 is that the breccias are consolidated glacial debris, the second, that 

 they represent alluvial material derived under the humid cycle. The 

 latter theory is essentially only an extension of the views of Lindgren 

 and Turner. 



The Mariposa breccias unquestionably resemble many typical 

 tillites. The admixture of rounded, subrounded and angular boulders 

 in an unsorted matrix of leas coarse material, which is often silty, is 

 a feature which characterizes all tillites from the most ancient to the 

 most recent. Interbedding of such glacial deposits with slates is 

 rather a common feature ; several slate beds are intercalated in the 

 Cobalt tillite, they characterize the Dwyka of Africa, and in the 

 Permo-Carboniferous tillite of Australia, six slate horizons, bearing 

 a marine fauna, alternate with the coarser glacial beds. In addition 

 to these characteristics common to all well authenticated tillites, 

 positive evidences of glacial action, particularly the striating and 

 faceting of pebbles, are observed in the actual deposits, while in most 

 cases a polished, fluted and striated surface is found underlying the 

 morainal material. Such lines of evidence are of course actual demon- 

 strations of extensive glaciation, but their apparent absence cannot be 

 held to disprove any hypothesis of glacial origin. Careful search in all 

 the outcrops of the Mariposa in the vicinity of Colfax have so far 

 failed to reveal any typically glaciated pebbles. Some striations were 

 observed, but all these appeared to be due to cracks or weaknesses in 

 the rock; flattened and snubbed pebbles were nowhere found. Further, 

 the surface upon which the Mariposa beds were laid down is very 

 poorly exposed in the region ; the western Calaveras contact is the 

 only one which might offer an opportunity for observation ; the other 

 contacts are against irrupted masses. Unfortunately this only known 

 normal contact is covered throughout with rock-waste. It is thus 

 unknown whether or not the formation rests on a glaciated surface. 

 However, since marine beds are interstratified with the coarse deposits, 

 if their origin is to be referred to glacial action, the glaciated floor 

 from which the debris was swept would probably be outside the marine 

 basin in which the deposits accumulated, and, as acute deformation 

 and extensive denudation have affected the region, resulting in the 

 removal of a larger part of the rocks which bordered the basin, the 

 discovery of a glaciated surface is hardly to be expected. Dr. Knopf 



