364 University of California Publications in Geology [Vol. 11 



12,000 feet before radiolarian oozes would accumulate on the conti- 

 nental deposits. As already pointed out, the twofold repetition of 

 this movement with a twofold reversal during one geological period 

 seems improbable. Also the nature of the contacts of chert and sand- 

 stone are not what might be expected on this hypothesis. 



If the Franciscan sandstone were marine it might be possible to 

 clear up the anomaly of the association of coarse sandstones with 

 cherts by postulating some such condition as those suggested. 



The occurrence of chert in lenses, however, could not be explained 

 in this way. 



Interbcdcling of Cherts with Coarse Mechanical Sediments. — Evi- 

 dence that radiolarian cherts may be deposited in rather shallow 

 water is found in the Ordivician cherts of Scotland, where there is a 

 very close association of cherts with coarse arkose sandstones. In 

 some instances sandstones are actually interbedded with thin beds of 

 chert. In passing it may be remarked that a similar argument ap- 

 plies to the Monterey cherts, since their interbanding with sand- 

 stones is often very marked. 



Bipple Marks. — That radiolarian rocks may accumulate in shal- 

 low water is also shown by the occurrence of radiolarian mudstones 

 and associated radiolarian cherts in Australia. There the mudstones 

 in certain localities show ripple marks. The radiolarian novaculites 

 of Texas also show markings which have been interpreted as ripple 

 marks. 



Conclusion. — When all the facts regarding the Franciscan cherts 

 and identical rocks elsewhere are considered, the only possible con- 

 clusion appears to be that the cherts did not accumulate as abyssal 

 sediments. 



HYPOTHESIS OF LAGOON DEPOSITION 



Dixon and Vaughan in describing the radiolarian cherts of Gower 

 advance the hypothesis of lagoon deposition, certain details of which 

 are set out above. 



The Lagoon Phase. — Dixon and Vaughan believe that the pres- 

 ence of the remains of pelagic organisms is not proof of a deep sea 

 origin. They postulate some sort of barrier between the basin of 

 deposition and the source of sediment. In the region thus screened 

 from land contributions pelagic sediments are formed. They find 

 it necessary to postulate a barrier between the open ocean and 

 the lagoon in order to prevent the scouring of the deposit by strong 



