DIKES NEAR SANTA CRUZ 241 



SUMMARY AND CONCLUSIONS AS TO DIKES AT ASPHALT ROCK QUARRIES 



In summing up the corditions at the asphalt quarries the following 

 are the most noteworthy points : 



1. The dikes vary in thickness from mere films along joint planes to 

 intruded masses several feet thick. 



2. Many of the intrusions are connected with the underlying bitumi- 

 nized sand beds and under such conditions that there can be no doubt but 

 that they were derived from these underlying beds. All of the dikes of 

 this region were apparently injected from below. 



3. The jumbled masses of bituminized sandstone and shale are exactly 

 such as might be expected from the slow intrusion of a plastic substance 

 into highly fractured zones in such rocks as the diatom shales. 



It seems to the writer that the large as well as the small masses, or 

 dikes, of bituminized sandstones which intersect the overlying shale beds 

 at varying angles in the Santa Cruz region owe their present position to 

 fracturing in the shale beds and to the injection of the sands into those 

 fractures, probably at a time when the sands were in a much more 

 plastic condition than they are at present. In view of the extremely 

 large intrusions along the coast (described below), it does not seem un- 

 reasonable to suppose that entire beds of bituminized and plastic sands 

 might be squeezed into the overlying shales. Thus a deposit might re- 

 tain its original simple bedded position at one place, and at another turn 

 up and intersect the overlying beds as a true dike. 



Dikes along the Coast West of Santa Cruz 



location 



With the exception of No. 57, the dikes described below outcrop along 

 3? miles of coast line, reaching from the mouth of Baldwin creek, 5 

 miles west of Santa Cruz, to the mouth of Respini creek, 8 miles west of 

 that city. Number 57 is at the mouth of Scott creek, 13 miles west of 

 Santa Cruz. 



The coast in this region is beautifully terraced. The character of the 

 topography is shown by figure 10. The structure of the region is shown 

 by figure 5 and cross-sections AB and CD, figure 10. In section AB 

 a fault with the downthrow on the west side is shown, while the dikes 

 whose locations are shown and numbered on the coastline are shown in 

 projection on section CD. The dips indicated along the coast line are 

 those of the diatom shales which are cut by the dikes. 



Along the eastern half of the coast line shown in the figure there are 

 occasional thin bituminous sandstone beds interbedded with the shales. 



