DIKES ALONG COAST WEST OF SANTA CRUZ 251 



bitumen. Such structure is absetit from the dikes of highly bituminized 

 sands. 



5. There are sandstones underlying the shales in which the intrusions 

 occur, of a character similar to the sand of the intrusions. Except for 

 thin beds of sand which overlie some of the smaller intrusions, the 

 nearest overlying sandstones are of probable Pliocene age and many 

 miles distant. 



6. The intrusions are at the west side of a faulted monoclinal fold. 



7. The sandstones underlying the diatomaceous shales are very soft 

 at some places ; at others they are quite hard. 



8. It is noticeable that the larger dikes along the coast west of Santa 

 Cruz are comparatively free from bituminous matter, while most of the 

 smaller intrusions, and especially those which do not extend to the top 

 of the sea-cliff, are highly charged with that substance. 



9. The absence of overlying sandstones, which could supply the mate- 

 rials for the intrusion, the presence of underlying sandstones, which are 

 bituminous. where they outcrop, and the obvious derivation of the dikes 

 at the bituminous rock quarries from these underlying beds leads to the 

 conclusion that most of the dikes of this region were intruded from 

 below, with one or two exceptions. 



10. The underlying sandstones were probably formerly oil-bearing, as 

 indicated by their bituminous outcrops at the asphalt rock quarries and 

 elsewhere in the district. They are at present practically barren of oil 

 wherever they have been penetrated by borings. The bituminized sands 

 of the smaller dikes along the coast lead to the conclusion that the 

 former oil-bearing sands were forced into joints in the shales and that 

 the residues from the oil entrapped with the sands in these crevices are 

 still found in the dikes. The absence of bituminous matter in the 

 larger dikes along the coast and the presence of a peculiar flow structure 

 in these leads to the conclusion that the cracks into which they were 

 forced opened to the surface, and that the oil sands were forced into these 

 openings from below by hydrostatic pressure, or the pressure of the over- 

 lying beds, OF both. These crevices, with their filling of loose sand, 

 probabl}^ formed the avenues of escape for the petroleum (providing it 

 was originally present in the underlying beds), and afterwards for the 

 water of the underlying sands. The escaping water must have carried 

 the oil and oil residues from the intrusions. 



12. The sands of the dikes may have been intruded rapidly or slowly, 

 and their intrusion may or may not have assisted in the opening of the 

 cracks which received them. 



The cracks were probably formed primarily by the elevation of the 

 coast line, which has been going on more or less regularly in this region 



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



