520 MR. E. E. L. DIXON AND DR, A. VAUGHAN ON [Nov. I9H, 



J. Hinde, been regarded as deep-sea deposits (on account of their 

 exceedingly fine-grained texture as well as the abundance of 

 radiolaria in some recent deep-sea deposits), before proceeding we 

 must show why we conclude that the Gower cherts, which are 

 similar in both fauna and texture, have been formed under 

 diametrically different conditions. This is the more necessary, 

 on account of the weight attaching to the opinion of Dr. Hinde, as 

 well as the importance of the farther conclusions based by other 

 geologists on his work. 



The way has been prepared by many observers, who have shown 

 that radiolarian rocks have not necessarily been deposited at, the 

 depths at which the radiolarian oozes above-mentioned are found ; 

 in the case of some there is strong evidence of a shallow-water 

 origin. Thus, at Garn-bica in Carmarthenshire, on the same 

 horizon as the Gower cherts similar rocks have been found by 

 Mr. T. C. Cantrill and described by Dr. G. J. Hinde 1 which, 

 though of finer grain, if anything, than the Gower cherts, may be 

 inferred to have been deposited in shallow waters, on account of their 

 occurrence as a thin, conformable group between two shallow-water 

 formations — the llottenstone Beds at the top of the Carboniferous 

 Limestone below and the Millstone Grit above. 



As may be gathered from the conclusions of Dr. Hinde & 

 Mr. Fox as to the conditions of deposition of the Lower Culm 

 radiolarian rocks of Devon, 2 of Prof. David & Mr. Pittman in the case 

 of the similar Devonian rocks of New South Wales, 3 and of others, 

 as well as from a remark of Prof. Watts in discussing the Australian 

 example (op. cit. p. 64), the most characteristic feature, next to 

 their fauna, of all radiolarian rocks is their freedom from any, except 

 the finest, terrigenous material. This character is irreconcilable 

 with an origin in shallow waters of an open sea. A clue, however, 

 to conditions of semi-isolated though shallow deposition is afforded 

 by the calcite-mudstones of the Modiola phases previously described. 

 These rocks, though differing from them in composition, resemble 

 radiolarian cherts in being, essentially, exceedingly fine-grained. 4 

 Further, some contain an abundance of unicellular plankton — 

 Calcisphcera (?). We may, therefore, find parallels for the peculiar 

 features of radiolarian cherts among those of the calcite-mudstones 

 of Modiola phases. These mudstones have, undoubtedly, been 

 deposited in shallow waters, but the chief feature wherein they 

 resemble radiolarian cherts, namely their fine grain, is anomalous 

 in shallow-water deposits ; as we have seen on pp. 517-18, 

 it is necessary to suppose that the waters were under lagoon- 

 conditions. Similarly, it is concluded that those radio- 

 larian rocks that appear, from independent evidence, 

 to have been formed in shallow waters, owe their 



1 ' The Country around Ammanford ' Mem. Geol. Surv. 1907, pp. 72-76. 



2 Q. J. a. S. vol. li (1895) p. 662. 



3 Ibid. vol. lv (1899) p. 36. 



4 The terrigenous material in the mudstones is as fine-grained as the 

 calcareous. 



