DIKES IN SAN LUIS OBISPO COUNTY 229 



In tliis paper the dikes of San Luis Obispo county will be taken up 

 first; then those of Santa Cruz county and all other localities observed 

 by the writer will be described and the conclusions concerning them con- 

 sidered. After this discussion a brief review and bibliography of the 

 literature bearing on the subject of clastic dikes is given. While the 

 bibliography may not be complete, it contains references to the papers 

 on the subject that have come under the writer's notice while looking 

 up the literature on clastic dikes. 



Sandstone Dikes in San Luis Obispo County, California 



loca tion 



In San Luis Obispo county sandstone dikes occur about one and one- 

 half miles southwest of Asuncion, a station on the Southern Pacific 

 railroad. They are exposed at intervals along Graves creek, in the 

 Rancho del Encinal, for a distance of about one-half mile. The dike 

 that is farthest upstream is about one mile above the ranch-house of 

 the Rancho del Encinal. 



GEOLOGIC RELATIONS 



The rocks of the locality are Cretaceous sandstones, overlain by diato- 

 maceous shales of Miocene age. Both the general relations of the strata 

 and the manner in which the dikes cut the shales are shown in the 

 accompanying plan and cross-section of the area (figure 2). 



The underlying Cretaceous sandstones, where observed by the writer, 

 are massive, rather fine grained, and of a brownish color. They lie to 

 the west and south of the diatomaceous shales, and the shales in which 

 the dikes occur dip away from the sandstone area. 



The color of the sandstone and its texture where exposed 200 yards 

 above the dikes on Graves creek are somewhat different from that of the 

 sand composing the dike at 1, as well as the other dikes of the Asuncion 

 region, which are for the most part composed of fine grained, hard gray 

 sandstone. It is by no means certain, however, that the texture of the 

 sandstone lying immediately below the dikes is not similar to that of 

 the dikes. The color of the Cretaceous sandstone is evidently due to 

 iron oxide, and it seems quite reasonable to suppose that this iron may 

 have been removed by water circulating through the intruded sands 

 prior to their firm cementation by calcium carbonate. 



The diatomaceous shales which inclose the dikes are brittle and much 

 jointed. They are light in weight ; in color they vary from light gray 



