524 MR. E. E. L. DIXON AND DR. A. VATJGHAS ON [Nov. 191 I, 



the Barnstaple district of North Devon, 1 so far as they go, suggest 

 that the cherts cropping out there are lagoon-deposits rather than 

 those of a fairly deep sea. Consequently, as we need consider only 

 these two alternatives, it is concluded that the Carboniferous radio- 

 larian cherts of Devon, which continue through a wide extent of the 

 West Country, constitute a lagoon-phase. This conclusion is not 

 impugned by any of the known features of these rocks in the rest of 

 their outcrops. Various particulars are given in the comprehensive 

 work of Hinde & Fox, and those that bear on the question are 

 mentioned in Table IV (p. 523). There is, however, much to be 

 learnt concerning this highly interesting group, and it is well that a 

 detailed examination of the Lower Culm and Upper Devonian has 

 been undertaken by my colleague, Mr. H. Dewey. 



(4) Meaning of the Difference between the Modiola Phases 

 and the Radiolarian Phase of Gower» 



Although the radiolarian phase of Gower appears to have been 

 deposited under the same conditions of depth as the Modiola phases, 

 in facies it presents a great contrast. Each phase, however, reflects 

 the character of the fauna of the sea adjacent to the lagoon-area 

 in which it was formed, as the terrigenous material is, typically, 

 similar in all; the difference resolves itself, therefore, into one 

 between the facies of the contiguous open-sea fauna. 



The open sea during the Modiola phases was rich in forms 

 possessing calcareous skeletons and giving rise by detrition to much 

 calcareous mud ; some of this, doubtless, passed into the lagoon- 

 areas with fine terrigenous material, to form, with the detritus of 

 the indigenous fauna and, possibly, calcareous material chemically 

 precipitated, the characteristic mudstones. But the radiolarian rocks 

 were formed at a time (the commencement of P) when, in the South- 

 Western Province, the sea underwent a great and persistent change 

 in character. Calcareous organisms disappeared almost completely 

 from that province, being chiefly represented throughout P and higher 

 parts of the Carboniferous by thin-shelled goniatites and lamelli- 

 branchs. Siliceous forms, however, assumed greater relative im- 

 portance, both in the ground-living fauna, — sponge-spicules being 

 noticeable in the radiolarian cherts and at some higher horizons 

 themselves forming cherts,— and among the free-swimming groups, 

 where radiolaria became conspicuous. 



But, although it is evident that in our area the sea from the 

 commencement of P onwards was inimical to so rich a calcareous 

 fauna as it had, up to that time, supported, yet not to a siliceous 

 one, we do not know what precisely was the change in its character ; 

 for apparently that effect on its fauna might result from any one 

 of several changes, such as a diminution in its soluble calcium- 



1 I take this opportunity of thanking Mr. J. G. Hamling, F.Gr.S., most 

 heartily for placing his extensive knowledge of this district at my disposal. 



