394 University of California Publications in Geology [Vol. 11 



radiolaria through their being entangled in the gelatinous silica. 

 However, since it seems necessary to assume that a number of years 

 are required to accumulate enough silica for a bed of chert, it may be 

 necessary to invoke another factor. It may be possible that the radio- 

 laria would thrive to an unusual degree in water which contained a 

 high concentration of silica. It might then be expected that their num- 

 ber would be very great at a time just before the precipitation of the 

 chert layer. 



In any case of gradual addition of silica to an ocean basin by the 

 slow contribution of rivers we would expect that, during the long time 

 necessary to accumulate enough silica for a single bed of chert, the 

 ocean currents would gradually mix the water and render its com- 

 position very uniform. Obviously it would be a very difficult matter 

 to supersaturate the ocean, so that this explanation does not seem very 

 probable if one regards the silica as coming in from river waters. 



If the hypothesis of siliceous springs or the hypothesis of solutions 

 emanating from igneous rocks be accepted, the idea of supersaturation 

 and rhythmic precipitation has more to recommend it. The more 

 rapid addition of silica to a limited volume of the sea water would 

 give much better opportunities for supersaturation of smaller vol- 

 umes and the precipitation of silica from them. Even here it would 

 appear that this idea of rhythmic precipitation would fail to explain 

 the rather uniform thickness of the chert layers. Due to such causes 

 as variations in currents, one would expect variations in the volume 

 of water which would become supersaturated at one time, and a con- 

 sequent variation of the thickness of the deposited chert. 



POSSIBILITY THAT THE EHYTHMIC BEDDING MAY BE DUE TO 

 SEGEEGATION IN A COLLOIDAL OOZE 



Another possibility which requires consideration, is that finely 

 divided gelatinous silica and shaly material were deposited together 

 in a colloidal mixture and that, at a later stage, there was a tendency 

 for segregation in the ooze and the gelatinous silica then aggregated 

 itself into the definite layers or beds which are now found. 



An idea, somewhat similar to this, has been advanced by Lang, 161 

 in an attempt to explain the peculiarities of certain limestones. He 

 studied the Liassic rocks of the Charnmouth Cliffs in England, where 

 a series of blue-gray marls are traversed by indurated bands and 



lei Lang, W. D., The Geology of the Charnmouth Cliffs, Beach and Fore-Shore, 

 Proe. Geol. Assoc., vol. 25, p. 293, 1914. 



