Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/278

 fields round the margin of the Triassic plain of Cheshire, and the extension of the Coal-measures below the Triassic and Permian rocks of the plain. The position of the axis of Lower Carboniferous rocks from west to east is represented by the unshaded band ranging under the banded portion (representing the Trias and Permian area), and emerging at each side in the tract now formed of Lower Carboniferous beds in North Wales and East Cheshire. The shaded portion banded with close lines shows the extent of the Coal- measures to the north and south of the axis covered by Permian and Triassic formations ; and it will be observed that the whole Coal- tract, both at the surface and concealed, resolves itself into two great coal-fields to the north and to the south of the supposed barrier of Lower Carboniferous rocks, that to the north being continuous with the Coal-fields of Flintshire, Lancashire, and Cheshire, that to the south with the Coal-fields of Denbighshire and North Staffordshire. A southerly prolongation of this tract probably connects it with the Shrewsbury Coal-field.

It will be observed that at the northern part of the map, and extending inland for some distance from the coast of Lancashire, there is a large banded tract of ground covered by Triassic rocks, under which, if penetrated, I consider that only the Lower Carboniferous beds (below the coal) would be found, the whole of the Coal-measures having been removed by denudation before the Permian period. To this tract I have referred in my paper on the relative ages of the physical features and lines of elevation of Lancashire (Quart. Journ. Geol. Soc. vol. xxiv. p. 333).

Discussion.

Prof. Ramsay considered that the lithological differences in the Permian rocks of the two areas referred to were hardly so great as was supposed by Mr. Hull.

Mr. Prestwich remarked that the nearly equal thickness of the Permian deposits in the two areas was in favour of their having been deposited in continuity.

Mr. W. W. Smyth considered that the difference between the Permian beds in question was not so great as the author supposed, but that the undoubted existence south of Chester of a breadth of 5 or 6 miles of Bunter Sandstone lying immediately upon the Mill- stone Grit, although observed only at one point, was strongly in favour of the author's hypothesis.

Mr. Hull stated in reply that the difference between the two groups of Permian rocks to which he had referred was so great as to render their identity at first sight very doubtful. The extent of the anticlinal at Rushton Spencer is so great that it must be inferred to have extended far both to the east and west.