14 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



According- to Rosenbusch, chlorites are common minerals as sec- 

 ondar}' products in altered tuffs, being derived from the aluminous 

 members of the biotite and phlogophite micas, pyroxenes and am- 

 phiboles. Just what minerals the chlorite in these tuffs was devel- 

 oped from is indeterminable from any slides made, but it represents 

 the presence of some of the ferro-magnesian minerals previous to 

 metamorphism. 



The presence of albite, microperthite, orthoclase and quartz 

 places the tuff with the acid group, but not definitely. 



This tuff shows three periods of metamorphism : first, the for- 

 mation of pyrite and chlorite ; second, the introduction of the 

 secondary quartz veins, which show recrystallization and wavy 

 extinction, and which have cut through the chlorite ; third, the intro- 

 duction of veins of calcite that do not show wavy extinction and 

 which are later than the quartz. 



Jaspers. — The hand specimens of the jasper need little descrip- 

 tion as they are already well known. Very little petrographic work 

 was done upon them but a resume is here given of the results of 

 the examination made by Dr. Hinde.^ 



Sections were made from the chert from Angel Island and from 

 the Buri-Buri ridge, San Mateo county. The radiolaria appear as 

 rounded outlines, with diameters of .055 mm. to .3 mm., set in a 

 matrix of silica. The structure is not retained, the walls having 

 graded into the silica matrix. In general the silica of the cast 

 differs from that of the main rock mass, which shows a minute 

 aggregate form. 



All the forms were poorly preserved, making the exact determi- 

 nation unsatisfactory, and specific correlation impossible with other 

 known radiolaria. In general the types resemble those from the 

 Cretaceous and Jurassic of Europe. Dr. Hinde has given a partial 

 identification of certain species, and plates illustrating these accom- 

 pany his paper.- 



III. Merced Series (Pliocene). — The Merced series is well 

 exposed along Seven Mile beach, presenting a measured section 



^The Radiolarian Chert of Angel Island, Bulletin Department Geology, 

 University of California, I, No. 7, p. 235, 

 ' Loc. cit., p. 240. 



