1918] Davis: The Radiolarian Cherts of the Franciscan Group 319 



off from communication with the deeper portions of the sea, and no 

 strong currents sweep over it. The rise and fall of the tide, if large, 

 would be a source of strong currents, but it is assumed that along the 

 margin of the lagoon, swamps and vegetation would exist which would 

 cause such a retardation of the tidal flow that there could be no 

 effective communication. It is necessary to postulate further that 

 some sort of barrier cut off the coarse sediments derived from the 

 land. 



The lagoon phase further implies, as pointed out by Dixon, a 

 continuous and very gentle subsidence at a rate sufficient to com- 

 pensate for filling by sediments. In such an unusual area there 

 might be peculiar types of sediments and faunas. 



It is found that some of the calcareous mudstones associated with 

 the formation of chert contain Modiola and other fossils, indicating 

 shallow and brackish water. The peculiar lenslike bedding of cherts 

 and shales (wedge-bedding) described before in connection with the 

 Franciscan radiolarian cherts, is well developed in these beds in 

 Gower. Dixon explains it, and also the fine lamination of the sep- 

 arate beds, as due to the gentle scouring of currents laden with the 

 finest sediments. 



Dixon notes the lack of calcareous fossils in the cherts. He be- 

 lieves that this was due to some peculiar condition of the sea at the 

 time of the deposition of the cherts. Either a change in the lime con- 

 tent or a change in temperature are suggested as competent to account 

 for the absence of calcareous organisms from the cherts themselves. 

 In this type of lagoon the deposition of radiolarian skeletons gave rise 

 to the radiolarian cherts ; and it would appear from the discussion 

 that the interbedding with shale is believed to be due to the action of 

 currents which were transporting the material. 



MULLION ISLAND 



Fox and Teall ul have described radiolarian cherts from Mullion 

 Island, which lies off the Lizard Peninsula in southwestern Cornwall. 

 The cherts are associated very intimately with an ellipsoidal basalt, 

 which is the dominant rock of the island. The stratified rocks make 

 up a small part of the island, and consist of radiolarian cherts, and 

 some limestones in a narrow band, entirely surrounded by igneous 

 rocks. 



61 On a Eadiolarian Chert from Mullion Island, Quar. Jour. Geol Soc 

 vol. 49, p. 211, 1893. 



