Ransome.] 



Geology of Angel Island. 



233 



like all the large masses of serpentine with which the writer is 

 familiar 111 the vicinity of San Francisco Bay, it has resulted from 

 the serpentinization of a holocrystalline basic eruptive rock, and is 

 in no sense a metamorphosed sediment. In the case of Angel 

 Island this original rock appears to have been of rather an excep- 

 tional character, consisting mainly of diallage. No indications 

 of the former presence of olivine have been detected, nor has 

 entirely satisfactory evidence of rhombic pyroxene been obtained. 

 Although the two last-named minerals are preeminently those 

 most likely to form serpentine, yet the serpentinization of 

 diallage is by no means unknown. Turner* mentions a dyke in 

 which "the pyroxene is plainly altering to serpentine," the context 

 indicating that the pyroxene is monoclinic. 



As to the fourchite, little remains to be said, save, perhaps, 

 that it affords in its various facies an example of magmatic differ- 

 entiation similar to many that have been described of late years, 

 although the occurrence is not sufficiently favorable to throw much 

 light upon the general law according to which the differentiation 

 has taken place. 



It may also be well to lav a parting emphasis upon the state- 

 ment that the fourchite, together with the eruptive rocks of Point 

 Bonita described in a preceding paper, are identical with, or are 

 included in, various rocks of eruptive character described by earlier 

 writers as metamorphosed sediments, and to which Becker has 

 given the names of pseudo-diabase, pseudo-diorite, etc. 



In conclusion, the writer desires to express a grateful sense of 

 obligation to Professor Lawson, who has always proved an unfailing 

 source of aid and encouragement. Acknowledgments are also due 

 to Lieutenant L. P. Brant, Post Adjutant, and to Dr. C. T. Peck- 

 ham, of the U. S. Marine Hospital Service, for courtesies of which 

 the author was the recipient while at work upon the island. 



Geol ogical L a bora lory. 



University of California, October S, 189J.. 



^Geological Notes on the Sierra Nevada, Am. Geologist, Vol. XIII, 1894, 

 p. 299. 



