1918] Davis: The Badiolarian Cherts of the Franciscan Group 383 



Silica, as being a colloid, has not a definite solubility ; its existence as a 

 hydrosol is limited only by the coagulating action of the electrolyte solutes of sea- 

 water or by its precipitation in combination with a base. As to the former effect, 

 we have no data except that sodium chloride is comparatively feeble as a coagu- 

 lant. It is remarkable that no silica seems ever to reach the bottom as a chem- 

 ical precipitate of calcium or magnesium silicate, although magnesium silicate, is 

 known to be soluble to only one part in 100,000 of water. This perhaps indicates 

 that the silica in solution in the sea is always below saturation point, so that 

 a local concentration large enough to determine precipitation never occurs. Or 

 again, excess silica perhaps combines with what little alumina there is in sea water 

 and is deposited as clay ; if that were the case the limit of dissolved silica would 

 be set by the solubility of this substance, which may well be less than that of mag- 

 nesium silicate. 



Occurrence of Chert in Lenslike Bodies. — The hypothesis as above 

 presented possesses the further disadvantage that it fails to provide 

 any explanation for the local occurrence of chert in lenses. Under it, 

 one would expect the deposition of silica to take place over rather 

 large areas. 



A modification of Scrivenor's idea, set out above, might perhaps 

 meet the objection. We might think of conditions of humid tropical 

 weathering of granitic rocks, with rather shallow enclosed basins along 

 the margins of the land. In the water of these basins, the continued 

 accession of waters rich in silica and poor in lime and magnesia would 

 finally produce important modifications. The magnesia would ulti- 

 mately be used up in combination with silica, and waters would be 

 produced which would be rich in alkaline chlorides. From these 

 waters one would get precipitates of silicic acid together with the 

 deposition of tests of radiolaria — provided radiolaria could exist in 

 waters of this peculiar composition. The skeletons of the radiolaria 

 would be perhaps the only fossils, since calcareous organisms might 

 not thrive in such water. 



In objection to this, it may be pointed out that the Franciscan 

 sandstone indicates that the conditions in Franciscan time were not 

 those of humid tropical weathering. 



HYPOTHESIS THAT THE SILICA OF THE CHERTS IS DERIVED FROM 

 SUBMARINE SILICEOUS SPRINGS 



The idea that the silica of the cherts comes from siliceous springs 

 is one which was held by certain of the earlier geologists in Europe. 

 It was held up to the time when radiolaria were found in certain of 

 these cherts. Since that time most European geologists appear to 

 have regarded the silica of the cherts as entirely of organic origin. 



Professor Lawson, in his study of the Franciscan cherts, came to 

 the conclusion that there was more silica in them than could be attrib- 



