Vol. 67.] THE CAKBONIFEROUS SUCCESSION IN GOWEK. 569 



The sudden change from the ' calcite-mudstones ' of S 2 to the 

 limestones with a deep-water fauna of D 1? as seen at Lydstep in 

 Pembrokeshire, was difficult to reconcile with the suggested lagoon- 

 formation of the ' calcite-mudstones.' 



Dr. T. P. Sibly remarked that numerous interesting questions 

 had been raised by this paper. Obviously, a careful study of the 

 evidence would be necessary, in order to appreciate the Authors' 

 conclusions with regard to ' complete cycles of intermittent change,' 

 arrived at by comparing the sequences of lithological types in the 

 three districts. The speaker, however, pointed out that, in view of 

 the small extent of the area under consideration, and the distinctly 

 shallow-water facies of the Avonian as a whole in this area, cor- 

 relation of movements of the sea-floor in different parts of this 

 area with similar disturbances in different parts of the much 

 larger Bristol area, must be open to considerable error. He asked 

 Mr. Dixon for a definition of the distinctive lithological facies of a 

 ' lagoon-phase.' 



It would have been instructive to hear a summary of the special 

 features of the faunal succession in this area. The Avonian fauna 

 appeared to have maintained, on the whole, its normal progression, 

 despite remarkable vicissitudes. With regard to the very wide 

 question of the limiting horizon between Lower and Upper Avonian, 

 was this area, which clearly suffered a maximum of disturbance, a 

 favourable one in which to deal with the question on a faunal 

 basis ? The description of the D 2 -D 3 phase of the Dibunophyllum 

 Zone would doubtless furnish many important facts, and would be 

 especially valuable to those interested in the difficult subject of the 

 correlation of the highest Avonian beds in different areas. 



Mr. J. H. Collins regretted that the Authors had not supplied a 

 cross-section, and that no statements had been made as to the 

 thicknesses of the various zones and their subdivisions. 



Mr. Cosmo Johns welcomed this account of the lithology and 

 physiographic conditions of deposition of the Carboniferous Lime- 

 stone in a most interesting district. The Authors' careful observa- 

 tions confirmed the evidence obtained in other areas, and demon- 

 strated the shallowness of the Lower Carboniferous Sea. The 

 non-appearance of dolomites at horizons where, from the evidence 

 of shallowing, they would be expected, might be explained if a 

 critical temperature rather than depth be the factor determining 

 their occurrence. It would appear that the Authors, by ' lagoon- 

 phase,' simply meant partly land-locked shallow waters, free from 

 disturbance by currents. In such restricted areas the water might 

 easily be appreciably warmer than in the outer sea, and thus 

 dolomites for this reason might have a local development only. 

 This suggestion could be put to experimental proof without much 

 difficulty, and temperatures near 30° C. would, from such evidence 

 as is available, seem most promising. He regretted the absence of 

 Dr. Vaughan, in view of the important faunal considerations that 

 arose out of the work done in the area. 



Mr. G. W. Young enquired whether the Authors had formulated 

 any theory as to the cause of the frequent oscillation of the sea- 



