1918] 



Davis: The Franciscan Sandstone 



Sedimentary Rocks 



The total thickness of the sedimentary portion of the Franciscan 

 group varies considerably in the different areas where it has been 

 studied. Professor Lawson measured a section over 6000 feet in 

 thickness in the region of San Francisco. 2 C. F. Tolman, Jr., de- 

 scribed a measured section having a minimum thickness of 15,000 

 feet at Corral Hollow near Livermore. 3 Here neither the top nor the 

 bottom of the section is exposed. Templeton reports a thickness of 

 15,000 to 20,000 feet in the region of Mount Hamilton. 4 Fairbanks 

 measured a thickness of about 10,000 feet in the neighborhood of San 

 Luis Obispo. 5 



The sedimentary portion of the Franciscan group includes arkose 

 sandstones with some shale and small amounts of conglomerate, radio- 

 larian cherts, and foraminiferal limestones. 



The sandstones are, as a rule, poorly stratified and have yielded 

 only a few indeterminate fossils, so that the details of the stratigraphy 

 are often difficult or impossible of determination. In the region 

 around San Francisco, the alternation of sandstones with formations 

 of radiolarian chert enables the sandstone to be separated into dis- 

 tinct formations. 



The lowest portion of the Franciscan group, exposed in the region 

 of San Francisco, is a sandstone called the Cahil sandstone. This is 

 divided by a limestone member about sixty feet thick. This lime- 

 stone — the Calera limestone — shows numerous foraminifera. It is 

 usually white to dark gray in color, but is sometimes black and fetid. 

 It contains considerable amounts of silica in the form of nodules, 

 patchy lenses, and anastomosing, blanket-like masses of chert. The 

 long direction of the blanket-like masses is approximately parallel to 

 the bedding. The chert is often black in color, sometimes pale gray, 

 and rarely greenish. Below the limestone member there are 500 feet 

 of sandstone. The base of the formation is nowhere visible. Above 

 the limestone member there are 2000 feet of sandstone, making a total 

 exposed thickness of 2560 feet for the Cahil sandstone. Above the 

 Cahil sandstone is the Sausalito chert, composed of radiolarian chert 

 of various kinds, having a thickness of about 900 feet. Above the 



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



3 Nature and Science on the Pacific Coast. San Francisco, Elder, 1915. 



* Templeton, E. C, The Mount Hamilton and San Jose Quadrangles, Bull. 

 Geol. Soc. Am., vol. 24, p. 96, 1913. 



■• San Luis Folio, U. S. Geol. Surv., Folio no. 101, 1904. 



