384 University of California Publications in Geology [Vol. 10 



The present investigation was undertaken in the hope of securing 

 definite evidence concerning the origin of the uncommon breccia beds 

 which comprise about one-fifth of the formation in this vicinity. The 

 main purpose of this paper, however, will have been accomplished if 

 the attention of geologists is sufficiently called to the peculiarities of 

 these beds to inspire a further investigation as to their origin. The 

 chief stimulus that has lent zest to the investigation has been the sug- 

 gestion by Professor A. C. Lawson that the breccia may be a Jurassic 

 tillite. While the results obtained only partially support this view, 

 its value as a working hypothesis is acknowledged with pleasure. 



Free use of the folio of the United States Geological Survey cover- 

 ing this region has been made during the prosecution of the work. 



LOCATION AND TOPOGEAPHY 



The region studied lies in the southwest quarter of the Colfax 

 quadrangle, California ; it embraces an area of 60.5 square miles, with 

 a length of 11 miles and a width of 5.5 miles. The accompanying 

 map (pi. 28) shows the general features of the geography. Colfax, the 

 only town of importance in the vicinity, is about sixty miles northeast 

 of Sacramento. 



The small area shown on the map is part of the foothill belt of the 

 Sierra Nevada. The relief is pronounced and rapidly increases in 

 ruggedness to the north and east, attaining a maximum at the crest 

 of the range, about thirty miles northeast of Colfax. The highest 

 eminence in the region under consideration is on Cape Horn Ridge 

 northeast of Colfax; its elevation is 3100 feet. The lowest point is 

 at the bottom of the canon of the North Fork of the American River ; 

 the 900-foot contour crosses the stream just south of Owl Creek. The 

 maximum relief is thus 2200 feet. 



The dominant features of the topography are the canons of the 

 Bear River and of the North Fork of the American River. The Bear 

 River takes its course diagonally across the northwestern corner of 

 the area mapped. It occupies a large rugged canon, with precipitous 

 walls where the stream cuts through the plutonic mass northwest of 

 Colfax, but with more gentle approaches in other tracts. Its chief 

 tributary is the Greenhorn River with a small catchment basin west 

 of Dutch Flat. The average grade of the Bear River within the limits 

 of the map is 71 feet to the mile. The North Fork of the American 

 River occupies a deeply incised trench with an average slope of the 



