1918] Davis: The Radiolarian Cherts of the Franciscan Group 331 



by rocks which are of Cretaceous age. The relations in Borneo are 

 thus similar to those in California, since in both places the series 

 containing the radiolarian cherts lies below recognized Cretaceous 

 rocks. 



Molengraaff regards these cherts and their associated shales as 

 the equivalents of the radiolarian oozes and red clays of abyssal 

 depths. Their freedom from terrigenous material, their association 

 with manganese, and their great extent, cause him to believe that they 

 were laid down in deep water far from any land. On account of the 

 size of the basin of deposition, he regards it as highly improbable 

 that the cherts could have been deposited near land through the cut- 

 ting off of terrigenous material by a barrier of any sort. He regards 

 the cherts as representing a long period of time, pointing out that 

 their minimum thickness is 100 meters, and stating that only a few 

 centimeters of material have collected on the ocean bottom since the 

 end of Tertiary time. 



MALAY PENINSULA 



Radiolarian cherts also occur in the Malay Peninsula and archi- 

 pelago. 84 These cherts vary in color from green or gray to bright red 

 or yellow. Other varieties contain abundant carbon and are black. 

 The carbon in some of the black cherts is so abundant that thin sec- 

 tions are nearly opaque. They are different in this respect from the 

 cherts of Borneo. The shales interbedded with the cherts may also 

 contain much carbon. 



Scrivenor regards the cherts of the Malay Peninsula as shallow 

 water deposits on account of their high carbon content. He admits 

 the possibility that the cherts of Borneo, which cover large areas, 

 might be abyssal deposits, but points out that they do not contain 

 such large quantities of carbon as the cherts of the Malay Peninsula. 



Scrivenor is inclined to accept the lagoon hypothesis of Dixon, 

 and believes that the waters from which the cherts were deposited 

 contained considerable amounts of dissolved silica. This increase -in 

 silica might be due to volcanic emanations, but he points out that, 

 under conditions of tropical weathering, large amounts of silica get 

 into solution in river waters. If such siliceous waters emptied into 

 closed basins, the sea water would become highly charged with silica. 

 The high concentration of silica would tend to hinder the growth of 

 calcareous organisms and favor the growth of organisms which 

 secreted siliceous skeletons. 



s* Scrivenor, J. B., Kadiolaria-bearing rocks of the East Indies Geol 

 vol. 49, p. 241, 1912. 



