1918] Davis: The Radiolarian Cherts of the Franciscan Group 389 



fine silt would be carried out beyond its usual place and deposited 

 above the accumulation of silica of the preceding season. As the sea- 

 sons changed a regular alternation of sediments would be produced. 



There is an effective check on the notion of annual variation, in 

 the thickness of material which must accumulate in order to explain 

 the bedding on this hypothesis. 



The Franciscan terrane has an average width of fifty miles in the 

 California Coast Kanges. Chert does not cover the whole area, so we 

 might assume that the width of a belt of chert was twenty-five miles, 

 measured transversely to the shore line. Assume that all the silica 

 of the cherts was originally brought down in the form of dissolved 

 silica by river waters. Each linear mile of this deposit of silica — 

 twenty-five miles in width — will then correspond to an area 25 x 5280 x 

 5280 = 696,960,000 square feet. This area must be covered, in one 

 year, to such a depth that when the rocks are consolidated there shall 

 be beds as thick as the average Franciscan chert. Assuming two 

 inches as the average thickness of chert beds, then each layer of result- 

 ing chert must contain a volume of chert equal to 696,960,000 6 = 

 116,160,000 cubic feet. The final density is about 2.5. Each cubic 

 foot of chert weighs 2.5X62.3 pounds = 156 pounds. The total 

 weight of silica in one layer of the dimensions specified is equal to 



116,160,000 X 156 nnn inn , . . 



= 9,600,480 tons of silica. Clarke, m the Data of 



2000 . 



Geochemistry, estimates that 319,170,000 metric tons of silica are car- 

 ried each year into the ocean by all the rivers of the world. 319,170,- 

 000 metric tons is in round numbers equal to 352,000,000 tons. Ac- 

 cording to this calculation the whole of the silica of the rivers of the 

 world in one year would produce a layer of chert twenty-five miles 

 wide, two inches thick, and having a length a little less than thirty- 

 nine miles. 



'352,000,000 . 



) 



\ 9,060,480 



The linear extent of the Franciscan terrane is several hundred 

 miles; the amount of river-borne silica coming into the basin could at 

 most be only a small fraction of the total amount contributed to the 

 ocean by all rivers. It is obviously impossible to believe that seasonal 

 alternations could give the rhythmic bedding, on the hypothesis of 

 chemical precipitation. Even the most liberal estimates of conditions 

 would indicate that a great many years would be necessary for the 

 formation of one bed of chert. 



