1918] Davis: The Radiolarian Cherts of the Franciscan Group 365 



tidal currents and its exposure to air during times of low tide. They 

 believe this was effected by a barrier of vegetation on the outer rim 

 of the lagoon. 



Beside the difficulty of maintaining such a peculiar arrangement 

 of land and sea, it is difficult to see why vegetation does not ulti- 

 mately possess the whole area of shallow water. Fresh water com- 

 ing into the basin, also, should render it brackish in the absence of 

 free communication with the sea, and thus prevent the growth of 

 radiolaria. Of course, the presence of a land barrier might deflect 

 all the streams save the very small ones whose contribution would 

 be compensated by evaporation. In the special ease of the Franciscan 

 the arid conditions suggested by the character of the sandstone would 

 obviate the latter objection. 131 



Absence of Calcareous Organisms. — The absence of calcareous 

 organisms is hard to understand on any theory of deposition of radio- 

 larian oozes in shallow water. In all places in the present ocean 

 where the water is clear and free from mechanical sediment, there 

 are great numbers of lime secreting organisms. They make up the 

 bottom deposits in such clear water, save where the depths are so 

 great that the shells are dissolved. In all other cases, the calcareous 

 organisms are so abundant that they mask the accumulation of the 

 siliceous organisms. 



We have, then, a strange contradiction : the radiolarian cherts of 

 the Franciscan and also the radiolarian cherts in many other locali- 

 ties, by reason of their association with rather coarse mechanical sedi- 

 ments, appear certainly to have been deposited in shallow water; yet 

 such conditions of shallow water, with freedom from mechanical 

 sedimentation, are the ideal conditions for the accumulation of de- 

 posits of carbonate of lime. 



Variation in Ocean Waters. — Dixon and Vaughan are aware of 

 this difficulty in their hypothesis and attempt to explain it by sug- 

 gesting temperature differences and chemical differences in the water 

 of the sea during the deposition of the cherts. 



It hardly seems possible that the temperature and chemical com- 

 position of the sea could alternate with the rapidity necessary on this 

 hypothesis to explain the accumulation of a few feet of radiolarian 

 ooze. In the beds above and below the cherts of Gower there are 

 considerable numbers of ordinary calcareous fossils. 



Enclosed Basins. Tropical Weathering. — Scrivenor presents an 



i3i Davis, op. cit. 



