1918] Davis: The Radiolarian Cherts of the Franciscan Group 359 



tent of the Franciscan shales with the silica content of various ana- 

 lyzed specimens of red clay a notable difference becomes apparent. 

 Red clay, on the average, contains between fifty and fifty-five per cent 

 of silica. The analyses of various Franciscan shales, previously given, 

 show that they usually contain over sixty per cent of silica. Only 

 exceptional varieties are low in silica. The alumina in the red clay 

 is also higher than in the Franciscan shales. 



Terrigenous Minerals. — The presence of grains of quartz and rare 

 flakes of mica in the red shales is opposed to the idea that they are 

 the equivalents of the red clay, since these minerals are almost never 

 found in the red clay. 



Absence from Shales of Certain Things Characteristic of Bed 

 Clay. — Due to the slowness of accumulation of the red clay, it con- 

 tains considerable numbers of shark's teeth, ear-bones of whales, 

 meteoric particles, and manganese nodules. These are not found in 

 every dredging, to be sure, but they are very common. In the con- 

 siderable amounts of red shale that have been examined by the writer 

 at many places, nothing corresponding to any of these has been en- 

 countered. The ear-bones of whales should of course be missing in a 

 Mesozoic deposit, but no other explanation of the absence of shark's 

 teeth, meteoric particles, and manganese nodules, seems forthcoming 

 save that the red shale partings do not represent the red clay. 



Rate of Accumulation. — Another point to be borne in mind is 

 the extreme slowness of accumulation of the red clay, the rate being 

 such that the teeth of sharks, extinct since Tertiary time, have not 

 yet been buried. The statement has been made by one authority, 123 

 that a thickness of 500 feet would probably contain the whole accu- 

 mulation of red clay in all geological time. 



Assume that the average bed of chert is two inches thick, and that 

 the shale partings will average one-quarter inch. This gives a thick- 

 ness of shale equal to one-ninth of the thickness of the chert formations. 

 The two principal formations of radiolarian chert on the San Fran- 

 cisco Peninsula have a thickness of 1430 feet, and on the above esti- 

 mate, 160 feet of this would be red shale. Such a thickness of red clay 

 could hardly be expected to accumulate during a portion of one geolog- 

 ical period. 



Rhythmic Bedding. — If the cherts represent radiolarian oozes and 

 the shale partings represent beds of red clay, it is impossible to account 

 for their rhythmic alternation. It is very improbable that in the open 



123 Lane, A. C, Science, n. s., vol. 37, p. 073, 1913. 



