1918] 



Davis: 



The Franciscan Sandstone 



17 



the black shale shows a decided lenticular character. Some lenses 

 are fifteen to twenty feet long and a foot or so thick in the central 

 part. 



There are occasional lenses of conglomerate in the Franciscan 

 sandstone. These occur at various horizons but are not persistent and 

 usually cannot be traced more than a few rods. 



Very small lenses of chert are found occasionally in the sand- 

 stone. One occurrence of this sort is seen at a point about a third 

 of a mile above the mouth of Perkins Canyon, Mount Diablo. The 

 surface of the sandstone has been polished by the stream, and on this 

 surface there are shown a number of anastomosing veins of white 

 quartz and calcite. The chert is green, with a faint bluish cast ; it has 

 a waxy luster, and is much like green radiolarian chert in appearance, 

 though it contains no radiolaria. It occurs in irregular lenticular 

 bands two or three feet long and two inches in maximum thickness. 

 There are several lenses one above the other separated by sandstone. 

 The general relations are shown in the photograph (plate IB). 



Ransome 3 ' 5 has described similar occurrences on Angel Island, 

 where he found numerous thin parallel bands of chert in a cliff section 

 composed of rather massive sandstone. 



Occurrence of Fossils 



One of the significant features of the Franciscan sandstone is the 

 lack of fossils. It is well exposed in many quarries as well as in road 

 cuts and sea cliffs ; but notwithstanding the numerous excellent ex- 

 posures, only a very few fossils have been discovered in it. Whitney 37 

 reports the finding of an Inoceramus in the sandstone of Alcatraz 

 Island in the bay of San Francisco. A fragment of an Inoceramus in 

 the collection of the University of California bears the following 

 label : 



"Found in a schooner load of broken rock, brought from Goat 

 Island or Angel Island for the construction of the iron draw-bridge 

 over the tidal canal between Alameda and Oakland, F. L. Ransome. ' ' 



A fragment of either an Inoceramus or an Aucella was found two 

 and three-quarter miles south of San Mateo. 38 Turner 39 reports find- 

 ing a few fossils in the Franciscan sandstone on the north side of 



3« Univ. Calif. Publ., Bull. Dept. Geol., vol. 1, pp. 197, 198, 1894. 



=7 Geo!. Surv. Calif., Geology, vol. 1, p. 77, 1865. 



38 Lawson, A. C, U. S. Geol. Surv. Folio no. 193, 1914. 



so Turner, H. W., Jour. Geol., vol. 6, pp. 492-93, 1898. 



