Ransome.] 



Geology of Angel Island. 



the beach about five hundred feet south of the wharf, having a 

 bright blue color, with a very dense jaspery texture, was examined 

 microscopically. It proved to be a mass of glaucophane microlites 

 and recrystallized quartz, the former so small and so thickly 

 crowded that the highest power can only resolve them in the thin- 

 nest edges of the section. Possibly other minerals arc present, but 

 these two are the only ones recognized. Scattered through this 

 dense groundmass are numbers of the small circular areas of clear 

 quartz, which have been identified as the probable remains of radio- 

 laria. These are fringed and transfixed by the sharp needles of 

 glaucophane projecting into them from the groundmass. 



THE SERPENTINE. 



Occurrence. — As has previously been indicated, this rock occurs 

 as a large dyke traversing the western portion of the island, and 

 attaining in its broadest exposure a width of over five hundred feet. 

 In its northern portions it stands out boldly above the surrounding 

 soil as a massive belt of ragged gray fragments and j utting outcrops. 

 Toward its southern end, however, the dyke loses prominence and 

 finally pinches out in a little rav ine, reappearing again in an exten- 

 sive outcrop just beyond, and also in three places along the shore 

 to the west of the main exposure. That a dyke of such size should 

 really be discontinuous at the point indicated upon the map is 

 extremely improbable. It is a more reasonable supposition that 

 the original top of the dyke was not far from its present exposed 

 surface, and that at the point where the map shows an apparent 

 break, its original top has never been revealed by erosion. It is easy 

 on such a hypothesis to account for the several small exposures of 

 serpentine,and for the wide extension of the metamorphic rocks about 

 the southern extremity of the dyke. The altered rocks can be 

 regarded as resting upon the serpentine, the latter being at no great 

 depth below. 



The trend of the dyke is nearly northwest and southeast, and 

 its hade, as revealed by the mapping, and indicated by certain 

 rough laminations within its mass, though probably very variable, 

 is to the southwest, at a maximum angle of about 35 °. Its north- 

 easterly boundary is marked by a fairly simple line of contact with 



