Ransomk.1 



Geology of Angel Island. 



197 



examination showed that the rock contains only 70.5 per cent of 

 silica, and is accordingly very far from being a pure quartzose 

 sandstone. The rapid disintegration of this rock, when employed 

 as a building stone in San Francisco, is doubtless due to the relative 

 abundance of the plagioclastic and other non-quartzose granules. 



At Quarry Point the sandstone lies in very massive beds, but 

 this appears exceptional. From Point Blunt westward to the ser- 

 pentine, the exposures along the shore generally show very thin 

 beds or lamina;, separated by slight partings of shale. In these 

 sections there is little or no conglomerate, although higher up on 

 tlie hill, just below the intrusive sill, some of the beds become 

 quite pebbly. Similar pebble bands outcrop on the beach, about 

 half way between the north end of the serpentinized dyke and Point 

 lone. The pebbles are usually disposed as bands in the sandstone, 

 and not as separate sharply defined beds. They are generally rather 

 flattened and of all sizes up to ten inches (25 centimeters) in diame- 

 ter, and four inches (10 centimeters) thick. They consist of quartz, 

 dark chert, basic eruptive, sandstone, and a peculiar noncrystalline 

 rock showing idiomorphic decomposed feldspars. The last is very 

 abundant, and is remarkable for containing fine needles of a blue 

 amphibole apparently identical with that presently to be described, 

 but which cannot have had a contemporaneous origin with it. The 

 rock from which the pebbles are derived has evidently been consid- 

 erably altered, but is not known to occur in si///. 



Certain portions of the sandstone, notably that exposed between 

 Point Blunt and Quarry Point, contain numerous fragments of 

 chert, identical with certain varieties of the bedded cherts of the 

 sedimentary series. These fragments are derived from some pre- 

 existing terrane of cherts, the occurrence of which in place is 

 not yet known. At certain places, also, chert}' bands, from a 

 fraction of an inch up to three inches (7.5 centimeters) in thickness, 

 occur in the sandstone, parallel with the bedding of the latter. 

 This is well shown on the western side of Point lone, where they 

 form numerous thin parallel bands on a cliff section of rather 

 massive sandstone. 



All the sandstone on the island, with the exception of that at 

 Quarry Point, and possibly a few of the thicker beds elsewhere, is 



