370 



University of California Publications. 



[Geology 



are found along the streams tributary to Red Rock Canon, and 

 unconsolidated water-rounded gravels and boulders are present 

 in the larger valleys, where they mantle the ridge facets at the 

 lower ends of divides between the lesser tributaries. 



Wide, high-level valleys with their broad bottoms well filled 

 with debris are found in the upland area north of the Mohave 

 River between Barstow and Daggett, in the Calico Mountains 

 north of Borate (pi. 42b), and in the hills south of Mohave River 

 from Daggett to the eastward. These valleys were filled after 

 the latest extrusion of lava, the flows of which they have dis- 

 sected. Their lower courses are rejuvenated by steep V-shaped 

 canons tributary to Mohave River in the sense that the lowland 

 area into which they open is drained by that river. A well 

 defined alluvial terrace can be traced in the valley of Mohave 

 River from east of Daggett to south of Victorville, with its lower 

 and upper limits unknown. 



The dissection of the slopes of the later alluvium and of the 

 bedrock of the Rosamond Series in the locality of the Barstow 

 syncline has probably been caused by a slight upwarp of the 

 peneplained surface during the last period of uplift. The sur- 

 face slope of the surface alluvium and the bedrock surface under 

 it averages 2°, which is the average angle of slope of the debris 

 aprons being formed at present ; where an indurated layer occurs 

 its angle of dip is the same as that of the surface slope (pi. 36b), 

 and the slope of the granite north of the outcrop of the Rosamond 

 Series is also graded and in conformity with that of the dissected 

 alluvium (pi. 35b). 



Probable Antecedence of Black Canon. — There is considerable 

 evidence that Black Canon may be antecedent in that part of 

 its course which cuts through the Black Mountain uplift. After 

 leaving the vicinity of the American Opal Company's prospect 

 the valley has a general southwesterly direction and a shallow 

 V-shaped canon with walls terraced on both sides by resistant 

 basalt. Then changing its course abruptly to the south it cuts 

 through Black Mountain near its western end. By taking a 

 course less than a mile to the westward it would have escaped 

 cutting through the resistant lava cap, and the latter course, in 

 addition to being through much less resistant beds, would have 



