2j2 



University of California. 



[Vol. i. 



I. Fourchite of main sill, same facies as shown in PI. 13, Fig. 1. 



II. Serpentine, hard sound nodule in crushed matrix. 



III. Contact-metamorphic schist next to serpentine. 



IV. Silica determination of sandstone from Quarry Point. 



CONCLUSION. 



The most important general result that has been arrived at from 

 a study of the rocks of Angel Island, is the establishing of the fact 

 that holocrystalline glaucophane (blue amphibole) schists can be 

 formed by the contact metamorphism of the San Francisco sand- 

 stone and associated cherts through the intrusion of certain basic 

 igneous rocks. It follows from the foregoing statement, that the 

 attempt to assign all of the glaucophane schists of the Coast Ranges 

 to a general regional metamorphism, or to a simultaneous meta- 

 morphism of any kind, must be abandoned. Direct evidence of the 

 n on- contemporaneity of at least two rocks characterized by the 

 presence of blue amphibole was afforded by the occurrence, in the 

 unaltered San Francisco sandstone, of abundant pebbles of a pecul- 

 iar rock not known in situ, bearing well- developed needles of the 

 mineral, while the same sandstone has itself been metamorphosed 

 into a perfect glaucophane schist, at the contacts with the fourchite 

 and serpentine. In the light of these facts, the remarkable and 

 often perplexing .manner in which various varieties of crystalline 

 schist are associated with areas of serpentine about San Francisco 

 Bay, becomes susceptible of explanation. It is believed that in many 

 such occurrences the accompanying schist can be shown to be the 

 direct result of local contact metamorphism. 



It is also believed that the finding of radiolarian remains abun- 

 dantly present in the cherts of Angel Island, and in those of other 

 localities, is an important step toward an understanding of the true 

 nature of these interesting rocks. It shows that a certain corre- 

 spondence exists between them and the radiolarian cherts of other 

 portions of the globe, and is an addition to the mass of evidence 

 that is gradually accumulating to controvert the view that they repre- 

 sent ordinary shales silicified during regional metamorphism. 



In regard to the serpentine, it is now scarcely necessary to 

 emphasize the fact that, like the serpentine of the Potrero, and, indeed, 



