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University of California Publications in Geology [Vol. ll 



hypothesis of local variation in the composition of the water of an 

 enclosed basin. Under conditions of tropical weathering, large 

 amounts of silica are brought down to the sea by rivers. He believes 

 that the waters of closed basins in the tropics might become highly 

 charged with silica and thus favor the growth of radiolaria but not 

 of calcareous organisms. They might become so changed as to pre- 

 vent the growth of calcareous organisms altogether. As Clarke has 

 pointed out, the waters of tropical rivers are high in silica in pro- 

 portion to the other salts which they contain. Waters coming from 

 granitic areas are high in silica and low in lime and magnesia. By 

 the combination of a granitic area and tropical weathering one might 

 expect large contributions of silica to the waters of the ocean, and if 

 there were enclosed basins, notable chemical changes might be brought 

 about. However, it seems certain, from the character of the sand- 

 stone, that Franciscan time was not characterized by humid tropical 

 weathering. 



Solution of Calcareous Remains in Shallow Water. — It is known 

 that in deposits of calcium carbonate the remains of siliceous organ- 

 isms tend to go into solution, probably because of the alkaline reaction 

 of the water above such an ooze. This suggests the possibility that 

 where siliceous remains are abundant there might be some cause at 

 work, tending to dissolve calcareous organisms and prevent their 

 accumulation with those of siliceous organisms, even though the water 

 was not deep. However, no reaction can be imagined, which would 

 do this. It seems extremely unlikely also because of the occurrence, 

 in the cherts and diatomaceous shales of the Monterey, of great num- 

 bers of foraminiferal shells. 



Change in Habits of Life. — Another possibility is that the habits 

 of life of the calcareous organisms were different at the time of deposi- 

 tion of the chert. Jukes-Brown 132 says : 



It is also clear that it must be unsafe to draw conclusions as to the condi- 

 tions under which any Paleozoic deposit was formed by assuming that the 

 organisms whose fossil remains it contains had the same habits of life as 

 their modern representatives. 



Hill, 133 also, has discussed this question of the association of radio- 

 larian rocks with rather coarse sediments, or with terrigenous muds, 

 and the difficulty of accounting for the absence of calcareous organ- 

 isms. He states that it was only in Tertiary time that deposits of cal- 



132 Jukes-Brown, A. J., Building of the British Isles, p. 12, London, 1911. 



133 Rocks containing Eadiolaria, Proc. Geol. Assoc., vol. 23, p. 90, 1912. 



