1918] 



Davis: The Franciscan Sandstone 



29 



formed rocks were eroded and their detritus deposited in new places. 

 Otherwise, one is forced to assume that there is an older assemblage 

 of rocks, somewhere in the Coast Ranges, that is identical in most 

 respects with the Franciscan. 



The presence of breaks within the Franciscan has been suggested 

 by Roderick Cranclall in a paper on the geology of the San Francisco 

 Peninsula. 45 He noted the presence of chert boulders within the 

 Franciscan sandstone, and points to these as evidence of the presence 

 of an erosion interval. 



THE ORIGIN OF THE FRANCISCAN SANDSTONE 



The mineralogical composition of this rock, its angularity of 

 grain, and its uniformity of character over wide areas and through 

 great thicknesses all combine to make it an unusual formation. The 

 lithologic uniformity points to persistence of conditions over a large 

 area and during a long period of time. 



Lack of Fossils 



The absence of fossils in the greater part of the sandstone strongly 

 suggests a continental origin, though as Kindle points out, this may 

 not be an infallible criterion. Kindle 46 shows that in certain regions 

 of the sea bottom, where currents are strong, the shifting of the sands 

 prevents colonies of mollusks from getting a foothold, or covers them 

 and prevents their growth. As a result there are many large areas 

 of the sea bottom over which no shells are now found. Grabau, 47 who 

 also discusses this criterion, states that marine elastics are usually 

 well bedded, and are as a rule, fossiliferous. He says : 



. . . Indeed, it may be seriously questioned if marine elastics are ever wholly 

 free from organic remains, though for considerable distances off certain shores 

 organisms may be so rare as to escape detection. Thus Kindle reports dredging 

 off the coast of Alaska for a hundred miles or more along the shore, without find- 

 ing any organic remains whatever. This of course does not prove their absence 

 but only indicates their scarcity, and indeed at another point off the same coast 

 organisms were abundant. Moreover, such dredging affects only the surface layers 

 of the sea floor, and does not prove the absence of remains in somewhat deeper 

 layers. 



« Proc. Am. Philos. Soc, vol. 46, 1907. 



4(1 Kindle, E. M., Cross-bedding and Absence of Fossils Considered as 

 Criteria of Continental Deposits, Am. Jour. Sci., vol. 32, p. 225, 1911. 

 *7 Grabau, A. W., Principles of Stratigraphy, p. 641, New York, 1913. 



