DIKES IX SAX LUIS OBISPO COUXTY 231 



to white. These shales are but sliD;htly folded along Graves creek where 

 the dikes occur, the recorded dips varying from 9 to 30 degrees. 



CHARACTER OF THE DIKES 



The dikes are composed of fine-grained sand, for the most part 

 firmly cemented by calcium carbonate, w^hile some of them contain a 

 network of calcite veinlets. A fragment of a hard dike, when treated 

 with cold HCl effervesces freely and soon crumbles to a mass of soft 

 sand, owing to the complete removal of its cementing material. 



Both the large and the small dikes are harder, with one or two excep- 

 tions, than the inclosing shales, and they therefore stand out above the 

 shale where exposed in the creek bed and in the banks. This feature 

 of their weathering is well shown in the figures of plates 21 and 22 and 

 in figure 3. 



The contacts between the sides of the dikes and the inclosing shales 

 are sharply defined, and the inclosing walls are quite smooth. In man}^ 

 cases inclusions of the diatom shale occur in the sandstone of the in- 

 trusions. These intrusions are similar in appearance to the inclosing 

 shales and doubtless were detached from the shales and mixed with the 

 sand as the latter was being injected into the fissures where it is now 

 found. 



In thickness the dikes on Graves creek varj" from mere veinlets. a 

 fraction of an inch thick, to intrusions 10 feet thick. AVith one excep- 

 tion those observed stand at high angles, and in some cases they are 

 vertical. In strike they do not vary greatly from an east and west di- 

 rection. 



The variations in dip and strike are indicated in the plan and cross- 

 section shown in figure 2. 



DESCRIPTION OF THE DIKES 



In the following remarks the dikes will be referred to by the numbers 

 which indicate their positions in figure 2. Those that are insignificant 

 or of no especial interest are not described. 



At 1, figure 2, is a dike of hard gray sandstone with an exposed thick- 

 ness of 1 feet. Its entire thickness was not seen, owing to its being par- 

 tially covered by creek sand. It stands in an almost vertical position 

 and has a strike of south 84 degrees west, while the shale near by dips 

 27 degrees north 45 degrees east. This dike is near the base of the 

 Miocene (diatomaceous) shale, which is underlain here by a coarse 

 yellowish Cretaceous sandstone ; the latter outcrops 600 feet up the creek 

 from the dike. 



