244 THE LITHOLOGICAL SUCCESSION [vol. Ixxvii, 



Discussio:n\ 



Prof. E.J. Gaewood conoTatnlated tlie Autlior on the interestino- 

 results wliicli he had obtained from a detailed study of the petro- 

 logical characters of the rocks of the Avon gorge, and expressed 

 his admiration of the beautiful photographs of rock-structures 

 thrown on the screen. He was especially interested to hear of the 

 numerous horizons at which calcareous algae had been found, and 

 the important part that thej had played as rock-builders, together 

 with the problematic form Spongiostroma, which he had found to 

 have a widespread distribution in the Lower Carboniferous rocks of 

 Britain and Belgium, associated with algal remains. He would 

 like to ask the Author whether he had been able to differentiate 

 between different species of Mitclieldeania, and whether these 

 species were characteristic of different horizons, as at Mitcheldean 

 and in North Cumberland, where JT. nicliohoni occurred in K and 

 M. gr eg aria in C^ or the top of Cy Also Avh ether the interesting- 

 little bodies characteristic of the china-stones, usually designated 

 Calcispher(S, were sufficiently well preserved in the Avon rocks to 

 be capable of specific identification, and whether they threw any 

 new light on the true nature of these organisms. He was in- 

 terested in the Author's description of the occurrence of ' spotted 

 beds,' which recalled those described by the speaker from Grreat 

 Bundle Beck in the Pennines ; quartz-grains Avere conspicuous in 

 the matrix of these beds, though almost absent from the spots. 



Prof. 0. T. JoxES remarked that they could not have too man}' 

 papers on the conditions of deposition of sedimentary rocks — a 

 subject which was still far from being well understood ; but it was 

 not easy to j^i'esent the necessarily detailed observations in an 

 interesting manner. The Author, with the aid of his fine series 

 of lantern -si ides, had succeeded admirably. It was of considerable 

 interest to note that the Bristol area showed such close resem- 

 blance to the Gower area, so abl}^ described b}^ Dixon & Yaughan, 

 and that all observations on the Carboniferous Limestone Series 

 agree in indicating that the series from top to bottom was formed 

 under relativeh' shallow- water conditions. Not many years ago, 

 following the descriptions of deep-sea deposits by the Challenger 

 Expedition, the pure limestones of the South of Britain were . 

 regarded as deep-sea deposits, of which the Lower Carboniferous 

 rocks of the North of England were the shallow-^vater repre- 

 sentatives. The Coddon Hill Shales with their -radiolarian cherts 

 were even supposed to represent a still-deeper water development. 

 The fallacy of these views in regard to the limestone facies had 

 now been amply demonstrated ; but the lesson, so far as the 

 North Devon rocks were concerned, had not been driven home as 

 thoroughly as it should have been. He also asked the Author 

 Avhether it was now proposed to draw the line between the Upper 

 and the Lower Avonian at the top of the Caninia Oolite, where 



