1918] Davis: The Radiolarian Cherts of the Franciscan Group 241 



occur as intrusive masses and also as lava flows interbedded with 

 sandstones or cherts. Intrusive bodies of peridotite, now altered to 

 serpentine, form a conspicuous part of the Franciscan terrane, and 

 with these are intimately associated gabbros, pyroxenites and related 

 rocks. 



In the vicinity of intrusive masses of pillow basalt or serpentine 

 there are at many places small irregular areas of metamorphic rocks 

 produced by the contact action of the intrusives. These include glau- 

 cophane and crocidolite schists, mica schists, lawsonite schists, actino- 

 lite schists, garnetiferous schists, and many other varieties. The 

 schists containing blue soda amphiboles are the most characteristic 

 and are very common. In addition to glaucophane and crocidolite, 

 several other blue amphiboles are found in these rocks. It is custom- 

 ary to speak of them in a general way as ' ' glaucophane schists. ' ' 



THE RADIOLARIAN CHERTS OF THE FRANCISCAN 



GROUP 



The sandstone of the Franciscan group is, in the region around 

 San Francisco Bay, 2 divided into three formations by the presence 

 of two horizons of radiolarian chert. The following tabulation will 

 indicate the disposition of these formations : 



Bonita sandstone 1400+ feet 



Ingleside chert 530 



Marin sandstone 1000 



Sausalito chert 900 



rCahil sandstone 2000 



J Calera limestone member 60 



I Cahil sandstone 5004- 



Base not known. 



It is not possible to separate the two chert formations lithologic- 

 ally; they can be distinguished only by stratigraphic relations and a 

 description of the varieties of chert in one formation will apply 

 equally well to the varieties found in the other. 



2 Lawson, A. C, San Francisco Folio, U. S. Geol. Surv. Folio no. 193, 1914. 



