246 University of California Publications in Geology [Vol. 11 



of the bedding of the cherts is nearly vertical and this relation is 

 found everywhere across the outcrop of the lens. In order to explain 

 these uniform steep dips in the small chert masses by the hypothesis 

 of infolding, it would be necessary to assume tbat the folds were of 

 the nature of very closely appressed anticlines or synclines. Nothing 

 in the Franciscan justifies this assumption. While the cherts them- 

 selves are often intensely contorted, this is due to incompetence of 

 the cherts and the consequent local readjustments within their mass. 

 The contact planes between the larger formations of chert and sand- 

 stone, as revealed by field mapping, show absolutely no sign of such 

 close folding. Where sandstone is bedded with shale a similar con- 

 dition is found. While the dips may be steep there is no evidence 

 of the intense plication required by the hypothesis under discussion. 



Moreover, small lenses of radiolarian chert with dimensions of only 

 a few feet are sometimes exposed in sections. On Mount Tamalpais 

 an instance of this sort may be seen on the curve of the railroad 

 near the saddle between the two summits. Here a small lens of radio- 

 larian chert showing shale partings is exposed in the side of the rail- 

 road cut. It is not over five feet in diameter and is entirely sur- 

 rounded by sandstone. 



In the quarry across from the cemetery in the region of the Clare- 

 mont Country Club, in Piedmont, there is a sandstone-chert contact. 

 A few feet above the top of the main chert body and entirely sep- 

 arated from it there are lenticular bodies of rather massive chert, 

 embedded in the sandstone. 



Roderick Crandall 10 has described such an occurrence in San 

 Francisco : 



Besides the large areas of jasper, there are small areas in various places 

 throughout the Coast Eanges that do not seem to belong to the main beds. These 

 may possibly be erosion remnants left by the removal of the larger masses, or it 

 may be that the organisms forming the jaspers survive in colonies that form small 

 lens-shaped masses. This latter suggestion receives support by evidence found at 

 a place in the cliffs near Golden Gate Strait, where about six or eight inches of 

 jasper appears in the sandstone several feet below the bottom jasper beds. The 

 small bed appears to show the arrival of the first condition that allowed the devel- 

 opment of the organisms followed by a period of slight change with sandstone 

 and final settling down to the quiet conditions in which the larger deposits were 

 laid down. 



10 Geology of the San Francisco Peninsula, Proe. Am. Philos. Soc, vol. 46, 

 p. 48, 1907. 



