Ransome.] 



Geology of Angel Island. 



2 I I 



colors, have been referred to augite; they form, however, but a small 

 part of the rock. The larger part of the slide is occupied by a 

 cloudy gray glass. The specific gravity of the rock was determined 

 to be 2.80, it being somewhat lighter than the preceding. 



Contact Metamorphism. — If, as seems most probable, all the 

 eruptive rock of the island, with the exception of the serpentine, is 

 the product of one eruption, then a stud)' of the map is enough to 

 demonstrate its intrusive character. Hut additional evidence on the 

 latter point is amply supplied by the pronounced alteration that the 

 invading rock has produced in the sedimentary formations through 

 which it forced its way. Where the alteration has been most 

 complete, the resulting rock is in general a holo-crystalline blue- 

 amphibole schist. As the existing literature on Coast Range 

 geology makes no mention of "glaucophane schists" arising from 

 local contact metamorphism, and generally assigns them, together 

 with the radiolarian cherts and much of the serpentine, to wide- 

 spread regional metamorphism, the results arrived at in this paper 

 have been to a certain extent forced upon the writer against certain 

 preconceived notions drawn from reading. Consequently, more 

 than usual care has been taken in establishing what, from the map- 

 ping alone, would in ordinary cases be accepted as a true zone of 

 contact alteration. One familiar with the geologic conditions about 

 San Francisco Bay will readily understand that nothing like a con - 

 tinuous belt of schist is exposed along the contact between the 

 fourchite and the sandstone. On the contrary, in walking along 

 the boundary line between the two rocks, the schist is encountered 

 in isolated masses of varying sizes, outcropping above the soil, and 

 in loose fragments, so disposed as to indicate the presence of a nar- 

 row zone of the same rock beneath. Such outcrops are rigidly 

 confined to a narrow strip between the sandstone and the fourchite, 

 or they are entirely surrounded by the latter. In no single instance 

 has any schist been found upon the island that does not bear the 

 above relation either to the fourchite or the serpentine. Further- 

 more, although the former has broken through the sandstone and 

 contains abundant inclusions, they are all more or less perfect 

 amphibole schist, and an included fragment of sandstone is unknown 

 except in the case of one or two very small sporadic intrusions 



