384 



University of California Publications in Geology [Vol.11 



uted to the radiolaria. He invoked the aid of siliceous springs to 

 account for the excess silica. The details of his statement have been 

 previously given. 



This hypothesis possesses the advantage that it readily accounts 

 for the occurrence of chert in lenses, a fact which presents difficulties 

 on most other hypotheses. 



F.anning believes he has confirmation of this hypothesis in the 

 occurrence of radiolarian chert in the Philippines. Here the cherts 

 occur in small isolated masses and, where the andesite below the 

 cherts is exposed, it is found to be extensively silicified, Fanning be- 

 lieves this was caused by siliceous springs which came up through it. 



One objection, which is sometimes made to the application of this 

 hypothesis, is the fact that the conduits or channels through which 

 these springs reached the surface are not found. It might be expected 

 that occasionally one would be encountered, since there are many 

 lenses of chert. By analogy with known siliceous springs we should 

 expect the rocks around such a conduit to be silicified. "While it may 

 not be possible to say definitely that we find the conduits of siliceous 

 springs, there are certain occurrences which are at least suggestive. 

 Occasionally the Franciscan sandstone shows irregular fissures which 

 are bordered by narrow rims of intensely silicified rock resembling a 

 green chert. No large or extensive occurrences of this sort have been 

 found, but it is reasonable to siippose that these fissures represented 

 channels of circulating siliceous waters. 



One very serious objection to the hypothesis of siliceous springs 

 was raised by Rothpletz 148 in discussing the kieselschiefer of Saxony, 

 and this objection appears to have been instrumental in discrediting 

 the hypothesis. Rothpletz regarded the rhythmic bedding of the 

 kieselschiefer as due to regular alternations in deposition. If the 

 silica comes from springs, he could see no reason why there should be 

 such regular alternations in the supply. 



HYPOTHESIS THAT THE SILICA OF THE CHEETS IS OF MAGMATIC 

 OEIGIN AND CHEMICALLY PEECIPITATED 



The jaspers of the Lake Superior District contain no radiolaria, 

 but are otherwise very much like the radiolarian cherts. Van Hise 

 and Leith, as set out above, believe that these jaspers and the associ- 

 ated iron ores are due to solutions derived from the ellipsoidal basalts 

 which were extruded in large quantities in this region during pre- 

 Cambrian time. 



1*8 op. cit. 



