124 University of California Publications in Geology [Vol.7 



sion comes a light pink spotted tuff-breccia forming one massive 

 bed 100 feet in thickness, and cut by two strike faults of fifteen 

 and fifty feet displacement. The matrix of this bed is a tuff 

 of a lighter shade of pink than the angular fragments of reddish 

 lava which it contains. This is succeeded by beds mainly gray 

 in color with thin interspersed layers of dark red, 150 to 250 feet 

 in thickness. Then come 300 feet of light gray, rather fine, 

 poorly stratified tuff-breccia, capped by a flow of vesicular basalt 

 about fifty feet thick. A second flow of basalt is separated from 

 the first, at an interval of about fifty feet, by beds similar to those 

 just below the lower flow (pi. 8, fig. 2). 



In the writer's first description of this section it was stated 

 that there was but one basalt flow, the outcrop of which had 

 been repeated by a normal strike fault. This statement is now 

 definitely known to have been erroneous. In the first examination 

 these basalt flows were studied only in the vicinity of Ricardo 

 post-office, where their relations are obscured by alluvium. It 

 was noted that about three-fourths of a mile to the west the lower 

 flow came to an abrupt end, and taking into consideration the 

 proved presence of strike faults a short distance below the basalt, 

 it was thought that the best explanation for the outcrop of two 

 layers of basalt in the walls of Red Rock Canon was that a single 

 flow had been repeated by a strike fault fradually diminishing in 

 amount of displacement towards the west and finally coming to 

 an end about three-fourths of a mile west of Red Rock Canon. 

 But in reality a wide stream channel was excavated in the lower 

 basalt flow before the outfloAv of the upper one, for still farther 

 west the lower flow suddenly appears and the space in which the 

 lower flow is absent is filled with much cross-bedded sediments. 

 The relations here are very suggestive of conditions similar to 

 those during which the river-channel sandstones and conglomer- 

 ates were laid down in drainage courses eroded in finer beds of 

 White River Oligocene age in the Big Bad-lands region of South 

 Dakota. 



Conclusive evidence that these basalts are interbedded flows 

 and not intrusive sills is furnished by the fact that their upper 

 surfaces have been eroded and that fragments of the basalt are 

 locally found in abundance in the tuff beds in close juxtaposition 



