Ransom k.] 



Geology of Angel Island. 



201 



The largest mass of fourchite occurs in the form of the intrusive 

 sheet or sill, already referred to as invading the San Francisco sand- 

 stone. There are no satisfactory exposures of the contact between 

 the eruptive and the sandstone, but the irregular shape of the 

 former, and the accompanying contact metamorphism, prove it to 

 be a true sill and not an interbedded flow. 



Another mass of considerable size occurs to the west of the 

 serpentinized dyke, and is most conspicuously exposed in the 

 bluffs near Point Stuart, where it weathers to a characteristic rust- 

 brown color. Its intrusive character is plainly shown, both near 

 Point Stuart and Point Knox, by the presence of abundant altered 

 inclusions of chert, and by actually observed igneous contacts. 

 The best example of the latter may be seen on the beach to the 

 northeast of Point Stuart. The hill, of w hich the point is a part, is 

 capped by a mass of the thinly bedded chert resting upon the four- 

 chite. This mass of strata has been deformed and twisted by the 

 intrusion, so that a portion in which the beds have been thrown 

 nearly vertical, continues down the northern slope of the hill to the 

 water's edge, at which the eruptive rock' can be seen in sharp 

 igneous contact with the chert, and crowded with disrupted frag- 

 ments of the latter. The alteration brought about in the chert at 

 this point will be described in detail in the sequel. 



Between the wharf of the military post and Point Knox, the 

 eruptive rock presents several facies. Portions consist of a breccia 

 others show an imperfect spheroidal structure, while still others 

 appear to be identical with the massive fourchite of the sill. These 

 different varieties cannot be sharply differentiated; they were evi- 

 dently all simultaneously formed during one period of intrusion. 

 The spheroidal facies appears to be intermediate between the brec- 

 ciated variation and the ordinary fourchite. 



The metamorphic action of the eruptive west of the serpentine 

 is everywhere much less intense than that brought about by the sill 

 rock to the east, indeed, it often seems to be wholly absent or con- 

 fined to mere mechanical disturbance. This tact, taken in connec- 

 tion with the presence of brecciated and glassy facies, indicates 

 that this portion of the fourchite was intruded much closer to the 

 then existing surface than that forming the large sill; yet the abso- 



