62 



University of California. 



[Vol.. i. 



it lies on the upper slopes, and on the crest in the form of small 

 knobs or buttes. 



GEOLOGICAL RELATIONS. 



As a formation the rock is clearly recognized to be a volcanic 

 flow. It forms a comparatively thin sheet, nowhere over one hun- 

 dred feet in thickness, and generally considerably less, spread out 

 on an unevenly eroded surface. Since its extravasation it has been 

 depressed and buried by later sediments, and the whole again 

 uplifted, tilted, faulted, and eroded. 



This erosion has not only removed the greater portion of the 

 mantle of overlying sediments within the area described, but has 

 also dissected the sheet, and carried away very considerable portions 

 of it; so that it is no longer a continuous formation, but merely a 

 succession of patches distributed irregularly on the slope of the 

 ridge. Of the geological continuity of these patches and the 

 essential unity of the formation there can be no reasonable doubt. 

 Our partial knowledge of the underlying and overlying formations 

 does not enable us to fix with precision the age of the extravasation. 

 This much, hov/ever, is clear: The sheet lies indifferently upon the 

 eroded surface of the crystalline schists of the Coast Ranges, and 

 upon the aucella-bearing, Knoxville division of the Cretaceous 

 (Neocomian). It is overlaid by strata in which no fossils have yet 

 been found, but which are probably Pliocene. The age of the flow 

 is thus late Cretaceous, or early or middle Tertiary. The extensive 

 erosion of the aucella-bearing sandstones and shales, together with 

 the total absence of the later Cretaceous rocks, indicates that the 

 sheet is probably post-Cretaceous. 



GENERAL PETROGRAPH ICAL CHARACTER. 



The rock composing this formation may be described under 

 three quite distinct types. The first, which will be known as the 

 "porphyritic fades," represents the most advanced stage of crys- 

 tallization in the flow, and exhibits the features of a quartz-porphyry, 

 viz., a cryptocrystalline ground-mass with quartz and feldspar 

 phenocrysts. The second type presents more the aspect of a rhyo- 

 lite with the features common to surface volcanic flows, viz., a glassy 

 spherulitic matrix, vesicular structure, and well-marked fluxion lines. 



