632 EDWARD HULL ON THE CLASSIFICATION (10) South-Wales Coal-basin. Notwithstanding the great thickness which the coal-measures of this district attain, amounting to 11,650 feet, still higher beds have once been superimposed on those now remaining, which must have constituted Stage G, either in whole or iu part. It is impossible to say whether the uppermost known beds of Glamorganshire — those above the " Mynydd Isslwyn" coal — are in part the representatives of this stage, as there are no peculiarities by which to distinguish those beds from Stage F ; the probabilities are, however, in fa- vour of this supposition. As in the case, also, of the Somersetshire coal-field, the Pennant grit must be considered as a member of the Middle Coal-measures. The Lower Coal-measures, especially along the southern borders of the field, form a well marked zone, very rich in coal and ironstone, and distinguished by a remarkable group of fossil shells of marine genera, as in the case of the same beds in the northern counties*. The higher beds are only characterized by shells of the genus An- thracosia and its allies ; and whatever may have been the conditions under which this genus of molluscs lived, the vertical extent of its range, as compared with that of the marine species in the Lower Coal-measures, seems to point to some marked physical difference in the conditions under which the Lower and Middle stages were depo- sited f. Section in Glamorganshire. feet. Stage G ? Upper Coal-measures.— -Sandstones and shales down to the Mynydd Isslwyn Coal (?) 1000 Stage F. Coal-measures. — Sandstones, shales, and coal-seams, in the lower part of which is the Pennant Grit series, 3240 feet near Swansea. Plants, fish, Anthracosia, Anthracomya, JPhillipsia, Esther ia, and Leaia 564(5 Stage E. Gannister Beds. — Principally shales rich in ironstones and! o™ coal-seams with Gannister floors. Fish, Anthracomya, Athyris I planosculata, Spirifer, Productus, Orthis Michelini, O. resupinata, f -irvprv Chonctcs, Nautilus falcat us, 8fc I Stage D.~ Millstone Grit, including the " Gower Series" 200 to 350 Stage C. — Yoredale Shales (unimportant) 100 to 200 Stage B. — Carboniferous Limestone 800 to 900 Stage A. — Lower Limestone Shale 1G0 Old Bed Sandstone* The shales of the " Hosser vein " arc especially rich in marine shells, consisting of 24 genera, of which a full list, with plates, is given by the late Mr. Salter. Somo of the species are found in the " Pennystone " beds of Coalbrook Dale, and are undoubtedly on, or
- Determined by the late Mr. Salter from the collection of I)r. Beyaii,
' Iron-ores of Great Britain,' Part iii. t Coal-fields of Great Britain, 3rd edit, p, 92.
