STRATA OF SHROPSHIRE AND DENBIGHSHIRE. 15 



semblance 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 en- 

 gaged in boring, sinking, and other mining operations in the district 

 under consideration, and I have thus had great facilities for the mi- 

 nute 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 accom- 

 pany me somewhat closely through the process by which I have ar- 

 rived at this conclusion. 



In the diagram of vertical sections (PI. 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 hereto- 

 fore 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 1 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 frag- 

 mentary 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 f . Sir Roderick states that the strata 



* " On Coal-seams in the Permian at Ifton, Shropshire," Proceedings of the 

 Geologists' Association, vol. iii. 



f 'Russia in Europe,' p. 146; also ' Siluria,' 1854, p. 29G-97. 



