BRITISH ASSOCIATION MEETING. 



455 



of land stretched across central England from "Wales to the German Ocean. To 

 the north of this barrier, or isthmus, the coal-fields of Salop, Staffordshire, 

 Warwickshire, and Leicestershire, and all the Carboniferous strata of the north 

 of England were accumulated ; while on the south of the isthmus were found 

 the coal-fields of South Wales, Eorest of Dean, Somerset, and possibly of the 

 Thames Yalley. 



It was then argued that a broad distinction is to be drawn between lime- 

 stones and all sedimentary strata, the former having been the production of 

 living animals, which generally required clear water for their proper develop- 

 ment, the latter being due to deposition from more or less muddy or sandy 

 seas. The agencies in each case have been in some degree antagouistic ; and 

 where the ocean has been, or is at present, highly charged with sediment, lime- 

 stones cannot be found to any great extent. This view — which is exemplified 

 by many of our seas — is borne out by the relative distribution of the calcareous 

 aud non- calcareous members of the Carboniferous group. The sedimentary 

 strata (sandstones, shales, and clays) were shown to attain their greatest ver- 

 tical development in Scotland, the Borders, and Lancashire, and from thence to 

 thin away towards the south, till they finally terminate against the barrier. On 

 the other hand, it was shown that the Carboniferous limestone reached its highest 

 development in Derbyshire, and from thence thins away westward and northward 

 into Scotland, where it is almost entirely replaced by sedimentary strata. Thus 

 where the one group of beds is most fully developed, there the other is least. 

 These phenomena had long since been pointed out by Professor Phillips in York- 

 shire, but are applicable to the whole of Britain north of the barrier. On the 

 south, it was shown that the sedimentary strata are in greatest force towards the 

 W.S.W. — thinning away northwards and eastwards — while the limestones be- 

 come most fully developed towards the east. 



Reverting to America, the author reminded the Section how similar pheno- 

 mena had been shown by Profevssor Rogers, Sir C. Lyell, and others, to hoJd 

 good in that country, but under somewhat different circumstances. They all, 

 however, indicated the existence of land in the North Atlantic during the Car- 

 boniferous period. The thickening of the sedimentary strata in Britain to- 

 wards the north-west, Mr. Hull attributed to the prevalence of a great current 

 (an old Gulf-stream), carrying the sediment from the shores of this North 

 Atlantic continent, and spreading it over submerged Britain. The sea thus be- 

 came purer as the distance from the source of the sediment increased — that is, 

 towards central England — and here, on the other hand, the crinoids and corals 

 were most active in forming limestones. 



The variations in the thicknesses of the two kinds of strata were represented 

 by curved lines drawn on maps (termed isometric lines), each of which marked 

 a given thickness from 1000 to 10,000 feet. One set of lines showed the 

 development of the sedimentary beds, the other of the calcareous. There 

 were thus two systems of concentric lines intersecting each other from opposite 

 directions, and gradually dying out like waves emanating from a focus of 

 disturbance. 



The author had previously shown that the sedimentary strata of the Lower 

 Mesozoic series reach their greatest development towards the north-west of 

 England, and thin away in the direction of the estuary of the Thames, where 

 they are altogether absent, as proved by the Harwich boring. All this showed 

 that the sediment had been drifted from the north-west — the waste of pro- 

 bably the same continent which had furnished the materials for the Carbo- 

 niferous Hocks. Here, then, we have evidence of a great North Atlantic con- 

 tinent existing throughout the Upper Palaeozoic and Lower Mesozoic periods, 

 whose shores were swept by an oceanic current, which carried off the sand and 

 mud of its shores and rivers to form the materials of future geological for- 

 mations. 



