60 



University of California Publications. 



[Geology 



blasting in quarrying is unnecessary. The fractures are so 

 open in places that the overlying sands are washed into them 

 and in the rainy seasons considerable trouble arises from sliding. 



The petrographic character and the chemical composition of 

 the various phases of the diorite were fully set forth in Reid's 

 paper, to which the reader is referred. 



FRANCISCAN SERIES. 



Franciscan rocks, together with the serpentine bodies to be 

 described, occupy much of the northern part of the area mapped. 

 This complicated terrane shows great erosional resistance and 

 hence forms a prominent northwest-southeast ridge extending 

 far beyond the limits of the map to the northwest. In the 

 area mapped, the southern end of this "core" of Franciscan 

 rocks plunges beneath the Tertiary sedimentaries of the Santa 

 Clara Valley. 



Detailed mapping within the Franciscan terrane was not 

 attempted and the formation as a whole was roughly divided 

 into two parts : the sandstones and radiolarian cherts, and the 

 foraminiferal limestones. The latter are probably younger in 

 age than the former. 



The older division of the formation is composed in the 

 main of a hard, dark gray sandstone. Interbedded seams of 

 black foliated shale are not uncommon, and in one place was 

 noted a rather large mass of a very coarse white sandstone with 

 a scattering of large pebbles. The great mass of sandstone is 

 underlain by a thick accumulation of coarse conglomerate. (See 

 pi. 14, fig. 1.) The radiolarian cherts are abundant throughout 

 the terrane, but in no place do they outcrop favorably for the 

 study of their stra ti graphic relations. 



The foraminiferal limestone is exceedingly well developed in 

 the region and forms extensive outcrops, in most of which the 

 intercalation of layers of chert is a characteristic feature. It 

 is thought unnecessary to enter into any detailed discussion of 

 the microscopic characteristics and minute structural relations 

 of the rocks composing the Franciscan series. This phase of the 



