262 J. F. NEWSOM — CLASTIC DIKES 



creek, west of Red Bluff, California (the locality is indicated at 1, figure 1 

 of this paper). One of these intrusions, the Great dike, is of micaceous 

 sandstone, somewhat harder than the adjacent shales, and can be traced 

 more or less continuously for 9i miles. 



The dikes stand about vertical (varying from vertical to 60 degrees) 

 and have a general northeast-southwest trend, which varies from north 

 20 degrees east to north 71 degrees east. 



Regarding the relations between the dikes and joints in the adjacent 



strata, Diller says : 



" The dikes are parellel to the joints in their vicinity, and so related to them as 

 to indicate that the joints have not been produced by the dikes, but that, on the 

 contrary, the position of the dikes has been determined by the joints." 



The dikes frequently contain small shale inclusions; they are occa- 

 sionally banded near the edges, the banding being vertical and " due to 

 streaks of finer and coarser sand." 



The dikes often have two sets of fractures, one set being transverse 

 and the other being parallel to the sides. The intrusions weather more 

 slowly than the inclosing rocks in many cases and project from the hill- 

 sides as stone walls. 



The dikes are composed of gray micaceous sandstone, which weathers 

 brown. Besides mica and quartz sand, the rock contains a number of 

 other minerals, among which is carbonate of lime, which is the cement- 

 ing material of the rock. The mica scales of the dikes were found to be 

 arranged parallel to the sides of the dikes, a position which they would 

 assume if carried there by ascending currents of water. 



After a careful examination of the phenomena presented by the dikes 

 west of Red Bluff, Mr Diller reaches the conclusion that the sand was 

 forced into the fissures from below, and thinks the dikes " record seismic 

 movement during the Tertiary.-' Concerning the force by which the 

 sand was injected into the fissures, he says: 



*' It appears that if by any means a fissure were suddenly formed from the sur- 

 face down to the sand saturated with water the latter would rise in the fissure, 

 and if the hydrostatic pressure were sufficiently great the water would rush forth, 

 carrying the sand with it to fill the fissure, and, like an artesian well, overflow 

 upon the surface." 



In support of this view, the phenomena of sand and water being ejected 

 from fissures during earthquakes in various regions are cited. 



Hilgard, 1890. In 1890 E. W. Hilgard described the deposits of the 

 Ventura Asphalt Company, situated 5 miles in a northwesterly direction 

 from San Buena Ventura, California.* 



*E, W. Hilgard : Report on the asplmltum mine of the Ventura Asphalt Company. California 

 State Mining Bureau, Tenth Annual Report of the State Mineralogist, 1890, pp. 7t53-772. 



