162 THE CARBONIFEROUS LIMESTONE [vol. Ixxiii, 



The Carboniferous Limestone also, immediately below the junction, 

 is penetrated to a distance of several feet below its upper surface, 

 by pipes and veins of hard grit, pebbly in places, continuous with 

 the overlying, evenl3^-dipping beds of grit but firmly welded to the 

 enclosing limestone and, on some unweathered surfaces, difficult to 

 distinguish from it at first sight. It is evident that the limestone 

 has been eroded subaerially, and the cavities filled with Millstone 

 Grit. But the even lie of the beds of grit above, the consolidated 

 state (due to siliceous cementation) of the grit-infilling, and the 

 close union of limestone and infilling make it equally evident that 

 the grit, both that infilling the limestone and the bedded grit, has 

 been deposited from the first in the position in wdiich it is now 

 found. That is, the subaerial erosion of the limestone preceded 

 the deposition of the grit. The junction is, therefore, truly 

 unconformable. 



The other point mentioned by the speaker was the possibility 

 that knoll-reefs would be found in the Barry area, where, he under- 

 stood from the Authors, highly-fossiliferous well-bedded limestones 

 attained a great development. For, in a similarly-placed outcrop 

 in Pembrokeshire, small knoll-reefs had been discovered at about 

 the junction of the Lower with the Upper Avonian, the adjacent 

 beds — in fact, the gi'eater part of the Main Limestone — -being 

 highly-fossiliferous well-bedded limestones with the fauna distinc- 

 tive of Zaphrentid phases. Similar phasal deposits appear to be 

 the invariable concomitants of knoll-reefs. 



Dr. T. T. GrROOM said that the Fellows of the Society would see . 

 with gratification that the valuable zonal work initiated in the 

 British Carboniferous Limestone by the late Dr. Yaughan was 

 being continued by the Authors, As to their application of the 

 results to the structure of the Forest-of-Bean Coalfield, the speaker, 

 accepting Mr. Kidston's view that the Middle and Lower Coal 

 Measures were absent from this area (a view confirmed by Dr. Arber), 

 had, like the latter, infeiTed the existence of an unconformity 

 beneath the Coal Measures of the basin. But, when Prof. Sibly 

 later maintained that this was accompanied by extensive overstep 

 within the area, the speaker was unable to agree with him. He asked 

 for evidence in support of the statement that such overstep occurred 

 in the northern part of the basin, and said that, with reference to ' 

 the south-eastern side, much of which he had mapped on a large 

 scale, he hR d stated that the supposed overstep of the Coal Measures 

 on to the Old Red Sandstone was due to faulting. He was quite 

 unable to accept Prof. Sibly's interpretation of the section in the 

 Blackpool Valley, where the Carboniferous Limestone dipping at 

 about 70° was supposed to be directly overlain by gently-inclined 

 Coal Measures. The theory of overstep in this district was beset 

 with difficulties, one of which was that, at the time of the deposition 

 of the Coal Measures, the Limestone and associated beds must have 

 shown over the area of overstep a dip nearly equal to that seen at 

 the present time ; while the corresponding beds bej^ond the area were 

 still undisturbed : also that the limestone of the patch underwent 



