Ransom k. | 



Eruptive Rocks of Point Bonita. 



below the sedimentary series, the latter everywhere dipping away 

 from it in a general easterly direction. Moreover, the beds of 

 pyroclastic rock, and all structural planes which were presumably 

 at one time parallel to the original surface of the flow, show a sim- 

 ilar easterly inclination, which would carry them under the sedi- 

 mentary rocks. But here another factor enters into the problem 

 in the occurrence of a fault, extending, as shown on the map, from 

 G to the stream mouth near A, and possibly further north. The 

 curtailment of the western limb of the syncline, and the reappear- 

 ance, west of K, of only a small part of all the sandstone beds, 

 cannot be explained otherwise than by the assumption of such a 

 fault, and cumulative evidence pointing to its existence might 

 readily be adduced. The" faulting, in all probability, accompanied 

 the intrusion of the diabase, but it is not at all unlikely that the 

 differential movement continued after the solidification of the latter, 

 and was the cause of much of its subsequent shattering. 



Admitting the fault into the problem, it is at once seen that two 

 alternatives are suggested, — either the spheroidal basalt was poured 

 out anterior to the deposition of the sedimentary strata, and was 

 afterwards elevated to its present position, or else it is a flow of 

 much more recent date extra vasated upon the top of the sedi- 

 mentary series, and owes its present inferior position to the fact 

 that it is upon the down-throw side. The former view is most in 

 harmony with the present position of the spheroidal basalt, its 

 easterly inclination, the general tilting to the east of the beds along 

 the top of the cliffs north of A, and the upturning of the faulted 

 edges of the sandstone beds at G, accompanied by the crushing at 

 K, while the latter is chiefly supported by the fact that the sphe- 

 roidal basalt and the diabase are so nearly identical in chemical com- 

 position, and appear to have suffered equally from the effects of 

 time. On the whole, the former view seems to have most in its 

 favor, and to be most in agreement with what is known of other 

 occurrences of similar spheroidal basalt in the vicinity, the latter 

 being, as far as noted, always closely associated with jaspers and 

 the San Francisco sandstone, indicating a contemporaneous rather 

 than a subsequent origin for the basalt. 



Spheroidal basalt, apparently similar to that described, has been 



