16 University of California Publications in Geology t VoL 11 



certainty regarding the range of radiolaria, but points out the 

 remarkable similarity between the geological columns of the two sides 

 of the Pacific. 



The foregoing statements summarize the various ideas which have 

 been held, and are now held, regarding the place of the Franciscan 

 group in the geological column. The great uncertainty as to the age 

 of the Franciscan is not wholly due to our lack of knowledge of the 

 relations of the Coast Range granite and the absence of characteristic 

 fossils in the Franciscan itself. The uncertainty regarding the age 

 of the Knoxville adds greatly to the difficulty. 



THE FRANCISCAN SANDSTONE 

 Stratification 



The Franciscan sandstone is usually massive, although there are 

 certain facies which are stratified. In the cliffs at Land's End, 

 on the south side of the Golden Gate, sections from one to two hun- 

 dred feet thick may be seen with only an occasional indication of bed- 

 ding. Similar sections may be seen in Marin County and also in the 

 recently constructed road cuts on Point Richmond. In certain cpiar- 

 ries, faces nearly one hundred feet high are exposed, in which the 

 sandstone is wholly massive and shows no bedding. Looking at such 

 exposures from a little distance, one would be inclined to regard them 

 as composed of igneous rock on account of their massive character. 



When the sandstone is stratified this is usually due to the inter- 

 calation of fine grained shale, black to gray in color, or of lenses of 

 conglomerate or conglomeratic sandstone. There are no variations 

 in color or lithologic nature of the sandstone itself to mark out strati- 

 fication planes. Occasionally one may see a bed of shale from fifteen 

 to twenty feet thick, but usually the shale is not more than one to 

 three feet in thickness. In certain localities the sandstone is associ- 

 ated with considerable shale, though this is exceptional. Near San 

 Bruno Mountain, excellent sections are seen in which the sandstone is 

 fairly evenly bedded with black shale. Some of these sections are 

 plainly visible from trains coming into San Francisco in the neigh- 

 borhood of Visitacion Valley. Near Fleming Point, south of Point 

 Richmond, the waters of the bay have cut low cliffs in which even 

 alternations of sandstone and shale are observable. In some instances 



