DIKES IN SAN LUIS OBISPO COUNTY 235 



4. At two of the dikes the beds of the shale walls have been bent up- 

 ward, suggesting that the sand was forced in from below, under sufficient 

 pressure to cause the upturning of the shale beds. 



5. No bituminous odor is apparent in any of the Graves creek dikes. 



CONCLUSIONS AS TO ORIGIN OF THE GRAVES CREEK DIKES 



A study of the dikes along Graves creek leads to the conclusion that 

 the sand of these dikes was injected from below by hydrostatic pressure. 

 This conclusion is borne out by the following facts : 



1. The only sandstone of the immediate locality (with the exception 

 of the insignificant bed noted on page 234) is that of the underlying 

 beds. 



Whether the dikes were derived from the underlying Cretaceous sands 

 or from sands which may be interbedded with the Miocene shales below 

 the intrusions (as seems more probable) is not known. 



2. The shales at the sides of dikes 12 and 19 could have been turned 

 upward only by the drag of sands intruded from below, or by the in- 

 jected sands forcing the shales apart along a fissure. The edges of the 

 shale could scarcely have been turned upward by sands being injected 

 or falling in from above. 



3. At dike 19 the sandstone is overlain by broken shales as though 

 the sand had not been forced completely through the shales. This, 

 taken in connection with the upturned edges of the shale in the walls of 

 the sand-filled fissure, points strongly to the conclusion that the sand 

 was injected from below. 



The writer is of the opinion that the soft sands were forced up along 

 joint planeSj and that the walls of the fissures were more or less gradu- 

 ally pushed apart by the sands in much the same way as igneous injec- 

 tions might do. There seems no reason for supposing that an open 

 fissure was produced ready to receive the sands, and the contortions of 

 the shale beds at 12 and 19 would oppose such a view. 



After their injection into the shales, the sands were firmly cemented 

 by calcium carbonate precipitated from circulating water. 



Sandstone Dikes near Santa Cruz, California 



location 



The sandstone intrusions of the Santa Cruz region are exposed along 

 and near the sea-cliff from 8 to 13 miles west, and at the bituminous 

 rock quarries 5 J miles northwest of Santa Cruz. These two localities 

 are about 3 miles apart. The general location of this region is shown 



XXXIV— Bull. Geol. Soc. Am., Vol. 14. 1902 



