128 University of California Ptiblications in Geology [Vol. 7 



the production in many of the crystals of a reddish-brown margin called 

 by Weinschenk " hyalosiderite. " This covered the entire crystals in 

 some cases. 



Magnetite, in small patches, was also observed. 



This rock, before referred to as an andesite, 7 is without doubt 

 a basalt, and the term "olivine basalt" seems to apply best. 



The topography of the basalt-covered portion of Black Moun- 

 tain is still well within the youthful stage, for much of the 

 original surface of the basalt remains undissected. Canon-cut- 

 ting has exposed the underlying Rosamond, the upper surface of 

 which has been burned red by the basalt, so that it is possible 

 by means of these red streaks to trace the original essentially 

 even surface of the Rosamond at the time of the lava outflow. 



ALLUVIUM 



Alluvium is found mainly as terraces along the drainage 

 courses and as a mantle of the strata of the Rosamond. It is 

 locally partially consolidated and much eroded. 



In the east bluff at the mouth of Red Rock Canon the alluvium 

 is dissected to a depth of forty or fifty feet. It comprises a 

 heterogeneous mixture of fragments, mainly angular in contour, 

 of all the rocks of the Red Rock drainage basin capped by a 

 more resistant layer cemented by calcium carbonate. Bedding 

 planes are not sharply marked, but their attitude is essentially 

 horizontal. Some isolated patches of alluvium have been left in 

 the form of terraces on the walls of the canon of Last Chance 

 Gulch above the present drainage channel. Horizontally strati- 

 fled alluvium, consisting of re-deposited materials of the Rosa- 

 mond, is exposed with a thickness of from ten to twenty-five feet 

 along a drainage channel cutting through the basalt flows about 

 one-half mile east of Ricardo post-office. Unconsolidated water- 

 worn gravels and bowlders are present in the eastern tributaries 

 of the lower Red Rock Canon, where they mantle the ridge facets 

 at the lower ends of divides between the lesser tributaries. These 

 were probably derived from the basal strata of the Rosamond. 



The great development of alluvium is found in the debris 

 slopes of the Sierra, which cover the underlying rocks as far 



7 By H. W. Fairbanks. 



