86 



University of California. 



[Vol. i. 



The next opportunity for a close inspection is afforded where the 

 path makes the sharp turn to the east, about one hundred yards 

 south of the old tower, the rock here forming the backbone of the 

 ridge. Its contact with the spheroidal basalt can be followed down 

 on the west side almost to the beach, and seems to indicate its 

 intrusive character, although the very close macroscopic and micro- 

 scopic resemblances of the two rocks in some cases, make the writer 

 cautious about too positive an assertion. It may, however, be safely 

 affirmed that there is here either a case of very complete intrusion 

 of the diabase into the spheroidal basalt, or else a remarkably rapid 

 transition of one rock into the other; the contact is certainly not 

 one of faulting. 



At the turn of the path above referred to, this linear exposure of 

 the diabase apparently terminates, and vain endeavors were made in 

 the field to trace it further. A microscopic slide, from a specimen 

 collected about half way between this point and the mass of the 

 diabase lying west of point G, showed, however, the characteristic 

 features of this rock, and indicates the probable continuity of the 

 two portions. The nature of the ground is such that the connec- 

 tion might easily escape detection in the field. 



The line separating the sandstone of point G from the eruptive 

 rock on the west, is defined by a little gully running down to the 

 beach, but the contact between the two is not exposed to view. The 

 contact of the diabase with the spheroidal basalt, as shown in the 

 cutting beside the path, appears to indicate that the former is intru- 

 sive, but the evidence on this point is again not as conclusive as 

 might be desired, it being similar to that already adduced at the 

 turn of the path. At one place a roughly rectangular mass of the 

 basalt, about one foot wide and three feet long, was observed, sur- 

 rounded on at least three sides by the diabase; but this, in the ab- 

 sence of other evidence, might with equal propriety be regarded as 

 a small apophysis of the basalt, or as an included fragment. At 

 any rate, it indicates an igneous contact and not a mere sudden gra- 

 dation of one rock into the other. 



Farther north the diabase occurs between A and F, but it was 

 impossible to determine its precise extent, owing to the steepness 

 and insecurity of the crumbling slopes. At F it occurs as shown 



