1918] Davis: The Badiolarian Cherts of the Fmncisocm Group 245 



There are perhaps three interpretations which could be put upon 

 these small isolated areas of chert : 



1. They may be exactly what they appear to be — local depositions 

 of chert in lenses which were finally surrounded and buried by the 

 accumulation of sandstone. 



2. They may be merely residuals of erosion, left as a result of the 

 removal of an overlying body of chert. 



3. They may represent small blocks which have been let down into 

 or thrust out upon the sandstone by faults. 



It is very difficult in certain instances to be sure of the correct 

 interpretation. This uncertainty is brought about by the lack of good 

 exposures in which to observe the nature of the contact of the sand- 

 stone and the chert, and by the lack of bedding planes over large 

 areas of the sandstone. That some of these isolated outcrops are due 

 to faulting is unquestionable, since fault contacts may be seen. Fair- 

 banks believed, at one time, that the apparent lenslike distribution 

 was due to intense crushing and faulting of the Franciscan, causing 

 the chert to appear at many points in separated areas. This would 

 require an immense amount of minor faulting, and as Professor Law- 

 son pointed out in objection, the disturbance of the earlier formations 

 of the Coast Ranges has certainly not been so great as this. At a 

 later time, writing in the San Luis Folio, Fairbanks specifically states 

 that the cherts of the San Luis quadrangle occur in lenses. The sec- 

 ond explanation would undoubtedly prove to be true in some 

 instances. 



While the two explanations above suggested — faulting and differ- 

 ential erosion — are surely correct in certain instances, it seems im- 

 probable that all the minor areas of chert can be explained in this 

 way. In localities where these lenses are quite numerous, as in the 

 San Luis quadrangle, their explanation by faulting would involve an 

 extremely complicated system of cross faults, which is not borne out by 

 the general character of the Franciscan where well exposed. In the 

 region south of San Francisco, the mapping of the lines of contact 

 between the larger chert formation and the sandstone shows that there 

 is none of this intricate cross faulting in that area. Yet in that region 

 there are great numbers of small isolated areas of chert in addition 

 to the larger body. 



To explain them all as due to a combination of infolding and 

 erosion would be incorrect. In many of these small masses, the dip 



a Fairbanks, Bull. Geol. Soe. Am., vol. 6, p. 85, 1894. 



