408 University of California Publications in Geology [Vol. 10 



has suggested that striations might become obliterated during the 

 vicissitudes through which the beds have passed; the Dwyka, Talchir 

 and Australian Permian tillites in which striated pebbles are so 

 abundant, have suffered very little of the intense deformation which 

 has visited the Mariposa. The remarkable preservation of striations 

 on pebbles from the Cobalt tillite, however, shows that once the seal 

 of glaciation is imprinted it becomes well-nigh ineradicable. It should 

 at this point be noted that many Pleistocene moraines yield few or no 

 striated pebbles, so that their original absence does not seriously affect 

 the evidence favoring a glacial origin for the beds. 



In the great glacial horizons of the world the slates interbedded 

 with tillites are generally interpreted as representing interglacial 

 periods or temporary retreats of the ice. At least eight such changes 

 of conditions of deposition are recorded in the Colfax rocks. 



The strongest indications found of the action of ice are phenomena 

 which are usually referred to the agency of ground-ice. The isolated 

 boulders in the slaty beds are rather strongly indicative of the kind 

 of rafting which is, with much reason, attributed to the floating action 

 of icebergs or of river-ice. No other agency seems to account satis- 

 factorily for the appearance of sporadic rounded boulders in such 

 fine-grained sediments. The presence of ground-ice of course implies 

 less rigorous climatic conditions than are entailed in the assumption 

 of extensive glaciation. Average temperatures somewhat below the 

 freezing point of water would be required only during the winter 

 months to produce river-ice in sufficient abundance to account for the 

 transportation of the erratic material observed in the Mariposa slates. 



Should future investigations reveal positive evidence of glacial 

 action within the Mariposa breccias, then the sporadic, disconnected 

 areas of the coarse detrital material may well be explained by the 

 rafting action of icebergs. 



The second hypothesis proposed for the origin of the beds postulates 

 the bounding of the structural troughs, which admitted the late 

 Jurassic sea to the region, by rifts or abrupt monoclinal folds which 

 presented fronts of high relief toward the basins of accumulation. 

 Early Jurassic streams of low grade may be inferred, since the region 

 was in the zone of erosion throughout Pennsylvanian and Triassic 

 time. Some of these streams would doubtless maintain their courses 

 across the structure imposed in the late Jurassic ; it is these antecedent 

 streams as well as the subsequent-consequent streams, i.e., those fol- 

 lowing the trend of the structural troughs, which may be considered 



