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University of California Publications in Geology [ VoL 11 



Sausalito chert is another formation of sandstone, known as the Marin 

 sandstone, about 1000 feet thick. This is succeeded by another forma- 

 tion of radiolarian chert — the Ingleside chert — having a thickness of 

 530 feet. The upper member of the group is the Bonita sandstone, 

 about 1400 feet thick. The three sandstone formations are so much 

 alike that they cannot be distinguished on the basis of lithology, and 

 the same thing is true of the two chert formations. Except for the 

 general stratigraphic relations, one cannot tell with which formation 

 of chert or sandstone he has to deal. 



Near Corral Hollow, there are three formations in the Franciscan 

 group.' - ' The lowest formation is a dense blue sandstone, cut by many 

 intersecting quartz veinlets. Above this are the Corral Hollow shales, 

 which contain crumpled beds of radiolarian cherts. The upper mem- 

 ber of the group is the Oakridge sandstone, which is slightly meta- 

 morphosed. The top and bottom of the section are not exposed. 



In the San Luis Quadrangle, 7 the Franciscan — called there the 

 San Luis formation — is represented by about 10,000 feet of sandstone, 

 in which there are numerous scattered lenses of radiolarian chert. 



Igneous Rocks 



Beside the sedimentary rocks of the Franciscan group there are 

 certain igneous rocks, intimately associated with the sediments, which 

 appear to have been intruded into the sediments shortly after their 

 deposition. 



The first of these intrusives occurs in irregular masses and con- 

 sists of basalt and diabase, often characterized by a " pillow-form, ' ' or 

 ellipsoidal structure. These rocks are commonly referred to as the 

 ' ' pillow basalts. ' ' Numerous well exposed sections at Hunter 's Point, 

 Angel Island, Point Bonita, Mount Diablo, and in the Santa Ynez 

 Mountains, and other parts of the Coast Ranges, leave no doubt as to 

 the intrusive nature of a large part of these pillow basalts. Certain 

 other occurrences show no evidence of intrusive action and represent 

 contemporaneous lavas, extruded during the deposition of the Fran- 

 ciscan sediments. 



The basalts and diabases are associated with intrusions of peri- 

 dotite. These peridotites are now largely altered to serpentine, which 

 forms a conspicuous feature of the Franciscan terrane. Gabbros, 

 pyroxenites, and related rocks are found intrusive into the Franciscan, 



e Tolman, op. cit. 



"San Luis Folio, U. S. Geol. Surv., vol. 101, 1904. 



