DIKES NEAR SANTA CRUZ 



237 



supposed Miocene age. Near the base of this sedimentay series the sand- 

 stone beds are intercahited with the shales, while beneath the shales and 

 lying between them and the granites or metamorphic rocks, as the case 

 may be, is a more or less continuous sandstone bed, often as much as 

 200 feet thick. This basal sandstone, as well as the sandstones that are 

 interbedded with the shales, is often slightly bituminized, and at some 

 places it is highly bituminized. 



The sandstones and shales have general westward and southwestward 

 dips of from 10 to 20 degrees. Thus the shales and sandstones in the 

 region where the dikes occur form the eastern side of a monocline or of 

 a broad syncline, the axis of which is beneath the Pacific ocean. 



DIKES AT THE ASPHALT ROCK QUARRIES 



The asphalt* quarries, where some of the dikes described below occur, 

 are 3 miles from the coast and have an elevation of 1,000 feet above tide 



J3 



Diatom Shale 

 (Miocene) 



AsASphoIt Quarries 



B z.'Ben Lomoncf Mountoin 



C= Coast Line vrheie dikes occur 



. Figure 5.~ Cross-section along the Line X-Y, Figure U. 

 Showing the geologic structure and the sea terraces. 



level. The dikes to the west are along the coast line and in the sea-cliff, 

 which is from 50 to 75 feet high. The intrusions along the coast line, as 

 well as those at the asphalt quarries, cut diatomaceous shales. 



Between the asphalt quarries and the coast are a number of beauti- 

 fully cut sea terraces. The character of these terraces is indicated on the 

 accompanying sketch map, figure 10, while the general geologic relations 

 of the region are shown in figure 4, and the cross-section, figure 5. The 

 bituminized sandstones which have supplied the material for many of 

 the dikes occur near the bottom of the shale series. 



The sandstone intrusions at the asphalt quarries of the City Street 

 Improvement Company (see figures 4 and 6), in the Santa Cruz district, 

 pass from the bituminized sand beds into the shale beds above. The 

 sandstones are so heavily charged with bitumen that the fragments in 

 piles of the broken rock soon become firmly cemented. In some cases 



*The terms asphalt quarries used here and on the succeeding pages relative to the Santa Cruz 

 region applies to quarries where bituminized sandstone is obtained, and not to pure asphaltum 

 deposits. No large deposits of pure asphaltum are known to occur in the region about Santa Cruz, 

 California. 



