104 



University of California. 



[Vol. i. 



and are imbedded in a matrix of more finely comminuted volcanic 

 material. 



At F the exposure is more extensive and more readily acces- 

 sible, the fragmental rock occurring as a sheet of very irregular 

 thickness, reposing upon an uneven surface of spheroidal basalt, and 

 having a general inclination to the east or southeast. (See Fig. 3.) 

 It is here composed of fragments of light gray aphanitic amygda- 

 loidal rock, similar to that forming the spheroids below it, and rang- 

 ing in size from a fraction of an inch to a foot or more in diameter 

 the average size being about three inches. These fragments are 

 imbedded in a rust brown matrix, which is apparently made up of 

 finely comminuted volcanic ejectamenta. In general the pieces are 

 more or less rounded, but sharply angular fragments are also 

 abundant. This bed is overlain by the diabase, a portion of the 

 latter rock being also intrusive below it in what is apparently a 

 small apophysis. Although the fragmental rock was evidently laid 

 down upon an uneven surface, much of its present irregularity, 

 especially of its upper surface, appears to be due to subsequent dis- 

 turbances, probably coincident with the intrusion of the diabase. 

 The testimony afforded by this bed of agglomerate as to the divisi- 

 bility of the eruptive rocks of the area into those belonging to at 

 least two periods of volcanic activity, is of the most emphatic and 

 positive nature, and is enough to establish the surface character of 

 the spheroidal basalt. 



RELATIONS OF THE ERUPTIVE ROCKS TO EACH OTHER. 



It has been shown that the eruptive rocks are of two periods of 

 eruption, inasmuch as one portion was extravasated as a surface 

 flow, while the other is intrusive. Of these two periods that of the 

 surface flow preceded the intrusion of the diabase, for the spheroidal 

 basalt is generally observed underlying the diabase, and has been 

 invaded by it. The only exception to the inferior position of the 

 spheroidal basalt is found in the place held by the comparatively 

 large mass forming the eastern side of the point. The latter rock 

 differs from the normal spheroidal basalt in two other respects than 

 that of relative position; — it contains abundant inclusions of jasper, 

 and at one place shows augite in thin sections. Moreover, being 



