Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/760

650 EDWAKD HULL ON THE CLASSIFICATION On the other hand, Stages E, D, and C form one natural group, somewhat similar in mineral composition, and palseontologically connected throughout by marine molluscan forms. Their association under the term " Middle Carboniferous," seems to be a proper way in which to recognize this relationship. Lastly, it is proposed to retain Stages A and B in the Lower Carboniferous group as generally accepted — representing the pre- dominance of marine conditions, and of mineral characters generally differing from those of the overlying beds. Discussion. Prof. Ramsay, after referring to the exceptional opportunities which Prof. Hull had enjoyed of becoming intimately acquainted with the Carboniferous rocks of these islands, remarked that in the present paper he had correlated these beds with singular ability, but added that, for his own part, he was averse to drawing hard and fast lines without any distinctly evident unconformity. There was, however, evidence of great up-and-down physical changes, slowly substituting estuarine for marine conditions, and vice versa ; and these were associated with small palaeontological breaks. He did not feel sure that the Scaur Limestone of the north-east is the entire representative of the Lower Carboniferous Limestone of Derbyshire. He thought that the only philosophical way of map- ping the Carboniferous deposits of England was upon lithological principles, colouring the sandstones, limestones, &c. separately, and, when coal-beds occur, indicating the underclays &c. In this way a good physical map of the whole series would be obtained. Prof. Ramsay also stated that he had been informed by a Russian gentle- man that the Scotch Coal-measures are identical with those of Southern Russia. Prof. "W. Boyd Dawkins, referring to the break which Prof. Hull had put at the top of the Gannister series, stated that he was unable to recognize any decided physical or palseontological break at this point. Taking Lancashire and the north, the Coal series appears to be continuous. Erom the presence of remains of sharks he inferred that the upper deposits were not exclusively estuarine or freshwater, but at least partly marine. Prof. Hughes objected to the assumption of a sharp line of demarcation at the top of the Mountain Limestone. He doubted the identification of the so-called Irish Yoredale beds with those of Yoredale, and stated that the latter in the typical locality thin out from 2200 to 609 feet within a short distance. In the Carboniferous series we have local alternations of marine and freshwater condi- tions ; and such a correlation as had been attempted by the author, implying synchronism in England, Scotland, and Ireland, did not seem to him to be useful. Prof. Peestwich did not doubt the correctness of Prof. Hull's Addresses to the Geological Society of London, Quart. Journ. Geol. Soc. vols, xix. and xx.