348 University of California Publications. [Geology 



western side of Black Canon, where they have been folded into 

 a small dome. The base of the member is not exposed here. 

 At the top of the member is a layer of basalt which gradually 

 thins to the northward, finally becoming attenuated to a knife- 

 edge and disappearing on the north side of the dome. Although 

 this basalt may possibly be a phacolite, 7 the lack of evidence 

 of eontact-metamorphism of the overlying beds, while the under- 

 lying strata are baked a brick-red at the lower contact; the 

 large vesicles in the basalt which are filled with amygdules of 

 chalcedony and agate ; and the fact that the basalt layer does 

 not cut across bedding planes, but is in perfect accordance with 

 the bedding of the contiguous strata, apparently indicates that 

 the basalt is a contemporaneous lava flow. 



Upper Member of Breccia and Tuff. — The ashy and shaly tuff 

 beds are conformably overlain by a coarse granitic breccia, with 

 an arkosic matrix, which is light pink in color and very poorly 

 cemented. The beds are finer higher in the succession, the 

 materials grading into fine gravel and volcanic ash. Near the 

 fault which separates it from the ash-breccia member were found 

 a few indeterminable fragments of bones. 



4 a 31 a 



Fig. 2. Cross-section of the Rosamond Series at Black Mountain. 

 Length of section about five miles. The profile of Black Canon is also 

 represented. (1) Fine ashy and shaly tuff member. (2) Basalt layer. 

 (3) Upper member of breccia and tuff. (4) Tuff-breccia member. (5) 

 Folded basalt flow forming surface of Black Mountain. 



Structure of the Rosamond at Black Mountain. — A northeast- 

 southwest section of the Rosamond Series is given in fig. 2. 

 The beds have been subjected to a minor amount of lateral 

 compression. A minor fault with tilting is shown in plate 

 38b. On the northern side of the dome a red stratum dipping 



1 A phacolite, according to Harker, is a concordant intrusion which 

 occupies crests and troughs of folds, and which disappears entirely or is 

 present only in very attenuated thickness in the flanks. See "The nat- 

 ural history of igneous rocks," p. 77, 1909. 



