412 University of California Publications in Geology [Vol. 10 



steep cliff facing west, may have an important bearing on the question 

 of unconformity, and moreover, may in time furnish positive evidence 

 supporting the glacial hypothesis. Here a chert body, containing 

 radiolarian remains, and apparently belonging to the Calaveras series 

 exposed to the west, is overlain by the breccia. If a depositional con- 

 tact, this section shows profound erosion between the two formations. 

 The general structure at this point is anticlinal, so that the chert may 

 reasonably be explained as the basement upon which the Mariposa was 

 laid down, now brought to the surface in the axis of the anticline. 

 Professor Lawson and Professor Louderback with a party of graduate 

 students from the University of California visited the locality and 

 exposed a small area of the surface of the chert underlying the breccia 

 in a search for evidence of glacial scouring. Three grooves were found, 

 at least one of which was regarded by Professor Lawson as suggestive 

 of ice sculpture, but as minor faulting has affected the whole region, 

 positive evidence distinguishing the marks from slickenside phenomena 

 was not obtained. 



Angular discordance of dip is shown at the limestone outcrop 

 already mentioned on Bear River, but its significance is probably not 

 far-reaching. The Calaveras slates, which underlie the limestone 

 bearing a Carboniferous fauna, have a north-south strike and a vertical 

 dip ; the Mariposa slates in the immediate vicinity strike northeast and 

 southwest with dips ranging around sixty degrees. This small isolated 

 area of Calaveras rocks is regarded by Lindgren 33 as representing a 

 block torn loose from the main area at the time of the intrusion of the 

 diabase which bounds the Mariposa to the northwest. Movements 

 at this time might account for the discordance, hence the occurrence 

 is not regarded as conclusive proof of angular unconformity. 



The palaeontologic evidence of course indicates a profound break 

 between the two formations, as does the occurrence of the abundant 

 erosional debris of Calaveras rocks in the Mariposa. 



Exact figures on the thickness of the Mariposa in the Colfax region 

 cannot at present be given. In the columnar section shown in figure 2, 

 however, a minimum of 2500 feet may be stated with some confidence, 

 and the probability entertained that this estimate should be increased 

 to 5000 feet. The possibility of a repetition at R has been mentioned. 

 The amphibolite to the east is not to be considered the top of the 

 formation ; the contact may well be a pre-Tertiary fault. With the 

 present data an upper limit of thickness is thus not assignable. 



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



