46 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



work on the Franciscan series, are, that it is at least pre-Cretaceous 

 in age, but how much older, is as yet unknown.^ 



Formation of the Merced Series. — The mode of formation of the 

 Merced series seems from all the field evidence to have been a local 

 deposition in an arm of the sea, or an outlet to the bay correspond- 

 ing to the present Golden Gate. In the area south of this are large 

 deposits of fresh water gravels that are considered by Dr. Arnold to 

 be the fresh water equivalent of the Merced series. If this is true 

 and these fresh water gravels represent a fresh water lake which 

 emptied into the Pacific Ocean by way of the Merced outlet, then 

 there should be faunal evidence of the change from fresh to salt 

 water forms. Work of the nature that would demonstrate the plau- 

 sibility of this theory has never been attempted. There remains, 

 however, the conclusion that this great thickness of Pliocene beds 

 exposed at Seven Mile beach is a local deposit formed in one of the 

 two ways suggested and undoubtedly not extending in any such 

 thickness over the main San Francisco Peninsula. 



Origin of the Jaspers. — The Franciscan jasper was considered 

 bv Becker as metamorphosed silicious shale, ^ and by Lawson as a 

 deposit from submarine springs.^ The wide spread extent of the 

 beds shows it to be a true bedded deposit, as it is known to extend 

 from near the Oregon line to Santa Barbara county. 



The presence of radiolarian remains in the jasper also show it 

 to be a marine deposit. These fossils do not prove that the jaspers 

 are composed of radiolaria but show that it is similar to present- 

 day deep sea deposits. The agencies that have metamorphosed the 

 sandstones have probably helped in the secondary silicification of 

 the jaspers. 



There is another possible origin for the large beds of jaspers 

 in the Franciscan series and this is suggested by similar beds in later 

 deposits. The siliceous Miocene shales resemble very closely the 

 jasper beds under discussion except that they do not show so much 

 folding and distortion. These Miocene shales are diatomaceous 



^The Pre-Cretaceous Age of the Metamorphic Rocks of the California 

 Coast Ranges, American Geologist, IX, 153. Monograph XIII, U. S. Geo- 

 logical Survey, p. 211. 



2 Monograph XIII, U. S. Geologi'^^^l Survey, p. 106. 



'A. C. Lawson, 15th Ann. Report U. S. Geological Survey, p. 440. 



