STRATA OF SHROPSHIRE AND DENBIGHSHIRE. Z{ 



If this be so, it will be seen that while tho strata we haro been 

 considering, in their conglomerates, breccias, sandstones, limestones, 

 and marls, indicate the widespread prevalence of marine conditions 

 throughout an immense period of time, yet, in the strata .charged 

 with plant-remains and in the better-defined coal-seams, we have 

 evidences that during the whole of that period there were, as, indeed, 

 we ought to expect there would be, land surfaces on which the sur- 

 vivors of the coal-measures proper lingered amid the growth of new 

 forms of life. The carbonaceous and fossiliferous shales are the 

 memorials of estuaries and lagoons into whicli were drifted the frag- 

 ments of the fauna and flora of the land; the coal-seams are the relics 

 of land-surfaces themselves, especially of those favoured spots where 

 a dense and luxuriant vegetation could flourish. 



It would be easy by numerous quotations to show the difficulty 

 which all geologists who have made these strata their study find in 

 drawing the exact line where the Coal-measures end and the Permian 

 strata begin. By some that line is marked by the absence of Stig- 

 maria ficoides ; but gradually that fossil is found higher up in 

 the series. Then it is the distance above the /S^'ror&is-limestone*. 

 Often, as in Germany, it is marked by the first conglomerate, and in 

 Scotland, Lancashire, Staffordshire, and Shropshire by the green rock 

 and breccias. The truth is, as it appears to me, that there is no well- 

 marked boundary-line; tho change is gradual, and it is locally dif- 

 ferent. That change is marked generally by a decadence of the density 

 and luxuriance of the coal-measure flora, by the greater paucity of 

 land-surface, and by the wide spread and long continuance of marine 

 conditions. 



Taking the whole of the sections together, it will be seen that, 

 looked at comprehensively, there is no real general break in the 

 sequence of the strata or in the continuity of life, but that there are 

 only local and great unconformabilities of strata, marked by equally 

 great local gaps in the succession of life. 



It is also interesting to note how very similar in many respects 

 were the birth and growth of the great Carboniferous flora to its de- 

 cadence and final exit. In the midst of the marine conditions of 

 the deposition of the Carboniferous Limestone in Scotland, in the midst 

 of similar conditions during the formation of the Yoredale Rocks 

 and Millstone Grit of Yorkshire, there were here and there land-sur- 

 faces on which grew the harbingers of that flora which was destined 

 to attain to such magnitude and extent in the Coal-measures proper. 

 So also, in the midst of the marine conditions under which were 

 formed the marls, limestones, and breccias of the strata we call Per- 

 mian, there were, as we have seen, land-surfaces on which that flora 

 still lingered, changed somewhat in character by the appearance of 

 new forms in its midst. A stray plant found here and there tells us 

 how the survivors lingered on during the deposition of the upper 

 sandstones, group 4. Mr. Binney f tells us that a Sternbergia has been 



* Would not this limestone form a good base-line for all these Upper Car- 

 boniferous strata ? 



t Triassic Strata of Cumberland and Dumfries, p. 355. 



