252 J. F. NEWSOM — CLASTIC DIKES 



probably since the end of Miocene times. At what period the principal 

 dikes were formed is not known. 



The intrusions at the asphalt quarries show that bituminized sand, 

 which is plastic, may be forced into small cracks for considerable dis- 

 tances. Large quantities of petroleum or water may, or may not, have 

 been present in the sands along the coast when these were injected into 

 the diatomaceous shales in which they are now found. 



The peculiar banded or flow structure in the dikes at 51 and 52 indi- 

 cates that in these cases, at least, large quantities of the liquid medium 

 came up with the sand. It is not believed that plastic bituminized 

 sands would take on that structure. 



In closing the discussion of the Santa Cruz dikes, attention is directed 

 to the fact that the larger of these intrusions afford evidence of the man- 

 ner in which the water or oil formerly held by the underlying sand beds 

 has escaped from those beds. The importance that may attach to such 

 intrusions, which doubtless occur in other districts underlain by oil 

 sands, is therefore apparent. Should sand intrusions occur near the 

 crests of anticlinal folds, in which the hydrostatic pressure is not suffi- 

 cient to force the oil all out from below, the sands of the intrusions might 

 then be oil bearing and also lead downward to the underlying oil sands, 

 thus forming an important source of oil supply. When, however, the 

 intrusions are large, when they have low outlets to the surface and occur 

 near the lower edge of the oil reservoir in a region of low hydrostatic 

 pressure (or gas pressure from above), or when they occur at the upper 

 edge of the oil reservoir in a region of high hydrostatic pressure, they are 

 in a position to have completely drained the underlying beds of their 

 oil. 



Clastic Dikes observed Elsewhere by the Writer 



near stanford university, california 



A number of very small dikes of coarse sands occur in much jointed 

 basalt at the quarries just west of Stanford University. 



At the quarry on the east side of San Francisquito creek, the joint 

 blocks near the top of the quarry are angular and irregular. Farther 

 down, the irregular jointing gradually gives way to poorly defined and 

 then to well defined basalt columns. 



Overlying the basalt and resting on an old erosion surface is a coarse 

 gray sandstone. Many small dikes occur in this quarry along joint fis- 

 sures immediately below the overlying sandstone, and also in the basalt 

 mass, as much as 25 feet below the sandstone. In some cases films of 

 sand occur between the basalt columns. Most of the dikes are very 



