Section 10 is one of Mr. Binney's, given by Mr. Hull[O 1]. I insert it because of its resemblance on a restricted scale to that of Ardwick on the one side and that of Ifton on the other. It is separated from the last by a distance of about forty miles; and the country between consists of a trough filled up with Triassic beds which hold the great salt-deposits of Cheshire.
Section 11 is that of Ifton, which I have already described, and to which I shall have occasion to refer again.
Section 12 is one which, in its middle portion, group 3, is well known and often quoted. It was, I think, first described by Prof. Sedgwick[O 2], in the year 1832, though alluded to by Messrs Conybeare and Phillips in 1822; and it has subsequently been noticed by Sir R. I. Murchison, Prof. Ramsay, Mr. E. W. Binney, Mr. Hull, and others. Prof. Sedgwick described it, in descending order, as follows:—
Group 3. "A very fine magnesian conglomerate, in mineral structure like the Bristol and Devonshire conglomerates."
Group 2. "Coarse reddish sandstones, in character intermediate between a coarse coal-grit and a true red or variegated sandstone." Between this last and the upper coal-measures may now be added various green rocks, breccias, and conglomerates."
Group 1. "Coal-measures."Sir R. I. Murchison places the conglomerate group 3, on the same horizon as the Ardwick limestone bands and the Magnesian Limestone beds of the north-east counties. Mr. Hull, however, is inclined to place it, along with the conglomerates of Shiffnal, South Staffordshire, and Enville, in the lower part of the Permian series. In deciding a point of this kind, the position the strata occupy is of more importance than their mechanical structure and the derivation[O 3] of the materials of which they are composed. Seeing, then, that
- ↑ Geology of the Country around Bolton le Moors, p. 17.
- ↑ Geol. Transactions, ser. 2, vol. iv. p. 398 et seq.
- ↑ A few words concerning the derivation of the limestone fragments and boulders of this conglomerate may not be out of place here. They are derived fragments, and not concretions formed with the rock like those of Coedyrallt. Mr. Hull, in the work quoted below[I 1], assigns some of them, as does also Prof. Ramsay, to the Mountain Limestone, and others to the thin band of Spirorbis-limestone which, at some depth, lies between them and the coal. In considering this explanation several difficulties meet us:—1. While in colour the fragments have a general resemblance to the lower cream- and buff-coloured beds of the Mountain Limestone, they are of a paler cast, and often have a greenish tinge. 2. There is an entire absence of the gritty, reddish, and greyish beds of the middle portion, and also of the bluish-grey beds of the upper series of the Mountain Limestone of North Wales. 3. Hitherto I have failed to detect in them any Mountain-Limestone fossils.
Then the Spirorbis-limestone, at the most, consists of only thin bands, and it would take the denudation of a large area to supply the materials for this conglomerate. It is also closely associated with coal-strata; and if it supplied the limestones, we should naturally expect to find fragments of the coal also, which we do not. There is also, as far as I am acquainted with them, an absence in the limestone fragments of the characteristic fossils of the Spirorbis-limestone. From what source or sources, then, were they derived? I offer the following suggestion towards a solution of the question. By a reference to group 3 of the vertical sections, it will be seen that a large quantity of calcareous matter was deposited during the accumulation of the red marls, in the shape of concretions and interstratified beds. If we imagine any considerable area of these, which were older than the conglomerate itself, exposed to denuding agencies, it is easy to conceive how in a rough shallow sea the denuded limestone boulders would be rolled up along the shore-line and become cemented together by such portions of the denuded red sands and marls as did not float back into still water. May not the conglomerate therefore be made up of denuded Permian limestones redeposited with, and cemented together by, Permian marls?
- ↑ Triassic and Permian Rocks of the Midland Counties, p. 21.
