Vol. 6$.~] BASEMENT-BEDS OF THE BRISTOL COALFIELD. 409 



Figs. 19 a Sc 19 6. Orthoceras cf. culmdraeeum, Fleming. Natural size. 



(See p. 464.) 

 Fig. 20. Orthoceras cf. conquestum, de Koninck. Natural size. (See p. 465.) 



21. Glyphiocems cf. nitidum (Phillips). Portion of outer whorl showing 



the suture-lines. X 2. (See p. 465.) 



22. Gastrioceras coronatum, Foord & Crick. Example showing the wide 



deep umbilicus and the broad flattened whorls. Natural size. 

 (See p. 466.) 



Discussion. 



Dr. A. Stkahax remarked on the value of such a record of tem- 

 porary exposures as was contained iu this paper. The strata 

 yielding this interesting assemblage of fossils were seldom exposed 

 to view, and, but for the energy of a local observer, would have 

 escaped detection in this underground exploration. Marine fossils 

 had long been known to occur in the lowest productive Measures of 

 Monmouthshire, thanks to the labours of Mr. Adams, Dr. Bevan, 

 and others. Since the working of the local iron-ores had been 

 discontinued, few further discoveries of marine fossils had been 

 made, probably owing to lack of opportunity rather than to their 

 non-existence. The observation that the brachiopod-fauna con- 

 tained forms identical with, or closely approximating to, species 

 occurring in the Cyailiaxonia- and Dibiuiophi/Iliim-Zones suggested 

 that the definition of Carboniferous zones largely depended on the 

 nature of the fossils selected for zoning purposes. 



Mr. Walcot Gibson wished to congratulate the Author on his 

 interesting discovery of a marine band in the Coal-Measures of 

 Somerset. The Author's experience in Somerset, that the marine 

 fossils were found in a particular kind of rock, held good for every 

 coalfield and for every marine band. Hecent investigations on the 

 eastern side of the Pennine Chain proved that these marine episodes 

 in the Coal-Measures were general, of considerable duration, and of 

 frequent occurrence. 



The Author thanked the Fellows for their kind reception of his 

 paper, and pointed out that the exploration -branch proved the 

 Millstone-Grit Series to have a thickness of about 950 feet, a close 

 approximation to that estimated by the officers of the Geological 

 Survey. The lowest workable seam in the Bristol area was the 

 Gays Mine, found in the branch ; but it was not at Ashton Vale 

 sufficient in itself to justify working. The uppermost limit of the 

 Millstone Grit had been drawn below a black shale containing 

 typical Coal-Measure fossils. The breaking-in of water and ex- 

 haustion of the seams had caused the mine to be closed down. 



