388 University of California Publications in Geology [Vol. ll 



particular ease is explained as being due to slight differences in the 

 deposited material, as a result of the regular variation of summer and 

 winter. 



APPLICATION OF HYPOTHESIS TO RADIOLARIAN CHERTS 



If we attempt to apply this hypothesis to the radiolarian cherts 

 there are two special cases which might be discussed: (1) hypothesis 

 that cherts are radiolarian oozes; (2) hypothesis that cherts are 

 chemical precipitates. 



Hypothesis that Cherts are Radiolarian Oozes. — We might think 

 first of an area of the ocean bottom, at some distance from land, on 

 which the skeletons of radiolaria are accumulating. Think of the 

 climate on land as of such a nature that it shows a marked variation 

 in its seasons, as for example, an alternation of a dry summer with a 

 rainy winter. During the dry season the rivers flowing from the land 

 would be contracted and would carry their load of sediment out to a 

 certain distance from the shore. Beyond this limit radiolarian oozes 

 would accumulate. During the rainy season the streams would be 

 swollen and much sediment would be carried out to a greater distance 

 than in summer. Under such conditions a transition zone would be 

 formed between the terrigenous sediments on the landward side, and 

 the pelagic sediments on the ocean side, within which the seasons 

 would be marked by a characteristic alternation of sediments. 



On the assumption that the change was annual, the radiolarian 

 oozes would accumulate at the rate of one to three inches per year. 

 No definite figures are available on this point, and since radiolaria 

 may obtain their silica from the decomposition of clay particles we 

 have no absolute check on the amount of silica coming down by organic 

 precipitation. It seems, however, highly improbable that siliceous 

 oozes could accumulate at this high rate. 



Hypothesis of Chemical Precipitation. — Adopting for the discus- 

 sion, the idea of a chemical precipitation of silica from river water, 

 we might suppose that all the silica comes down in the form of silicic 

 acid. This silicic acid might be assumed to be flocculent, so that it 

 would travel out beyond the finest mechanical sediments before 

 deposition. On such an assumption there would be a zone in which 

 mechanical sediment of very fine grain was being deposited. Further 

 out there would be a zone of chemically deposited silica. As long as 

 conditions of river supply remained constant, the boundary between 

 these zones would remain fixed. If annual floods occurred silica 

 would be carried out further at one season than at another and the 



