Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/49

Rh If this be so, it will be seen that while the strata we have 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 survivors 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 which were drifted the fragments 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 Stigmaria ficoides; but gradually that fossil is found higher up in the series. Then it is the distance above the Spirorbis-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; the change is gradual, and it is locally different. 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 decadence 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-surfaces 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 Permian, 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 tells us that a Sternbergia has been