Ransome.] 



Geology of Angel Island. 



231 



smaller size than the serpentine, or, possibly, as a series of plugs, 

 filling vents situated upon a line of disturbance. It is considered 

 probable that from this fissure, or from these vents, came a portion 

 of the fourchite magma, although it is admitted that the only direct 

 evidence for such a view is the identity in character of the augite 

 in the included masses and in the fourchite. After this first intru- 

 sion had taken place, and the fourchite had solidified, it is supposed 

 that the intrusion of the rock from which the serpentine has been 

 derived, followed, forcing aside the walls of the fissure, so as to 

 give it a much greater width than before, and catching up and 

 including within its mass the fragments of the earlier dyke. 



The fact that the rock from which the serpentine was formed 

 was probably made up largely of diallage, while the fourchite is 

 highly augitic, points to a possible genetic relation between the two 

 rocks, suggesting that the rock at present represented by the ser- 

 pentine was the final basic residuum erupted from the reservoir that 

 had previously furnished the fourchite. Through the lack of sub- 

 stantial evidence as to the relative age of the two rocks, this can be 

 taken as little more than a suggestion. 



CHEMICAL ANALYSES. 



The following analyses were made by the writer in the minera- 

 logical laboratory of the University of California, upon specimens 

 representative of typical facies of the rocks whose petrographical 

 characters have been described in the preceding pages. 



I. II. III. IV. 



SiO., 46.98 42.06 80.21 70.50 



ALA 17.07 ) 7.99 



Fe,0 3 I.85 } 2 V- included with FeO 



FeO 7.02 2.88 3.35 



CaO 12.15 1. 10 



MgO 8.29 39.53 r . 54 



K. 2 .53 not estimated .22 



Na 2 2.54 " " 5.97 



PA .09 



Loss on . o/i 



ignition (H,0) 4 ' 5 ° I2 -°4 -74 



Sp. gr 



101.38 



3.20 



99-23 

 2.61 



101. 12 



