Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/37

Rh of certain rock structures in the associated beds. So in 1873 I was led to describe the Ifton Coal-measures as coal-seams in the Permian.

Since the publication of that paper I have been constantly engaged in boring, sinking, and other mining operations in the district under consideration, and I have thus had great facilities for the minute study of the peculiarities of its strata. More recently, in an attempt I have been making to correlate the Coal-measures of North Wales with those of other coal-fields, I have been led to compare the Ifton section with all the known sections of Permian and Upper Coal-measure strata which I could procure. The result is that the conviction at which I have inevitably arrived is this:—Call them by what names we will—Magnesian Limestone, Permian, Dyas, Permo-Carboniferous or Upper Coal-measures, the strata described by modern writers as Upper and Lower, and by the older writers as Middle and Lower Permian are identical with the groups 2 and 3 of the Ifton section No. 11.

As this is a question of general interest, I will ask you to accompany me somewhat closely through the process by which I have arrived at this conclusion.

In the diagram of vertical sections (Pl. I. A) I have collected a number of accredited Permian sections from the typical Permian country, from the earlier English home of the strata, from Saxony, Bohemia, North America, and from various counties in Great Britain; I have placed the Ifton section in the midst of them, together with one or two others from North and South Staffordshire which may heretofore have been considered doubtful. In most of the sections the true thickness, as given by the respective authors, is given; but in one or two cases no thickness has been assigned. I have divided the sections into groups of "Upper Coal-measures," Lower, Middle and Upper Permian, and "Bunter Sandstone;" I have taken as my base-line the Spirorbis-limestone. In dividing the sections horizontally I have taken as the extent of each group the greatest known thickness of that group; and I have placed the strata of the other sections, as nearly as I could from the descriptions given, in their corresponding place in the respective groups.

It will be observed how very fragmentary in many sections are the representatives of each group. The gaps between these fragmentary portions in each division serve to show either, first, the attenuated condition of the strata at that point, or, secondly, the amount of denudation that has taken place, and, thirdly, the amount of real unconformability there is between the various strata of each section as compared with the typical section of the group.

With these preliminary observations I will now ask you to consider with me each section separately.

Section 1 is that of the typical Permian district as described by the late Sir R. I. Murchison. Sir Roderick states that the strata