244 University of California Publications in Geology [Vol. 11 



end of the San Mateo quadrangle, this sporadic distribution of iso- 

 lated chert masses in sandstone is quite marked. The mapping shows 

 scores of small isolated areas of chert within sandstone. In the area 

 around Mount Tamalpais there is no horizon of chert persistent 

 enough to be mapped as a definite formation, but there are many 

 lenslike masses scattered through the sandstone. 



This peculiar tendency to lenslike distribution has been noted by 

 many geologists who have studied the radiolarian cherts. Professor 

 Lawson described it in his first report on the San Francisco Penin- 

 sula. 4 Ransome found the same distribution at Angel Island, 5 where 

 there are numerous small, isolated masses of chert within the sand- 

 stone. In the Santa Cruz Polio, the statement is made that the 

 Franciscan cherts of the Santa Cruz quadrangle occur in lenses. 

 Smith has described the occurrence of lenses of radiolarian cherts in 

 the Philippine Islands. Perhaps the most extreme case of this len- 

 ticular occurrence of radiolarian chert is found in the San Luis 

 quadrangle. Fairbanks 7 mapped and described the radiolarian cherts 

 of that area as being altogether in the form of discontinuous lenses. 

 The map shows a large number of small lenses in the sandstone. 

 These are described as reaching a maximum thickness of 100 feet. 

 It is stated that occasionally an individual outcrop may be traced 

 for a mile or more, but most of them are much shorter. It is sug- 

 gested that some of them may be continuous for longer distances 

 but ai*e not traceable on account of the narrow outcrop. The number 

 of lenses in a given stratigraphic section varies from place to place. 

 In one locality six lenses occur in about 2000 feet of strata. 



In spite of the suggestive distribution of these isolated patches, 

 when they are actually examined in the field one feels that there may 

 be room for doubt in interpreting all of them as lenses, deposited as 

 such and surrounded by sandstone. The doubts are somewhat in- 

 creased when one remembers that in Scotland, 8 where occurrences of 

 radiolarian chert were studied with much care and in the greatest 

 detail, no mention is made of lenslike bodies of this sort. 



* Lawson, A. C, Sketch of the Geology of the San Francisco Peninsula, 15th 

 Ann. Eep. U. S. Geol. Surv., pp. 401-476, 1895. 



5 Bansome, F. L., The Geology of Angel Island, Univ. Calif. Bull. Dept. Geol., 

 vol. 1, pp. 193-240, 1894. 



e Branner, Newsom and Arnold, U. S. Geol. Surv. Folio no. 163, 1909. 

 ' Fairbanks, op. cit. 



8 The Silurian Eocks of Britain. I, Scotland, Mem. Geol. Surv. United King- 

 dom, 1899. 



