The Geological History of Plants

by Sir John William Dawson

[BV] “Transactions of the Edinburgh Botanical Society,” 1870.

Messrs. Jack and Etheridge have given an excellent summary of our present knowledge of the Devonian flora of Scotland, in the Journal of the London Geological Society (1877). From this it would appear that species referable to the genera Calamities, Lepidodendron, Lycopodites, Psilophyton, Arthrostigma, Archæopteris, Caulopteris, Palæopitys, Araucarioxylon, and Stigmaria have been recognised.

The plants described by these gentlemen from the Old Red Sandstone of Callender, I should suppose, from their figures and descriptions, to belong to the genus Arthrostigma, rather than to Psilophyton. I do not attach any importance to the suggestions referred to by them, that the apparent leaves may be leaf-bases. Long leaf-bases, like those characteristic of Lepidofloyos, do not occur in these humbler plants of the Devonian. The stems with delicate “horizontal processes” to which they refer may belong to Ptilophyton or to Pinnularia.

In conclusion, I need scarcely say that I do not share in the doubts expressed by some British palæontologists as to the distinctness of the Devonian and Carboniferous floras. In eastern America, where these formations are mutually unconformable, there is, of course, less room for doubt than in Ireland and in western America, where they are stratigraphically continuous. Still, in passing from the one to the other, the species are for the most part different, and new generic forms are met with, and, as I have elsewhere shown, the physical conditions of the two periods were essentially different.[BW]

[BW] “Reports on Devonian Plants and Lower Carboniferous Plants of Canada.”

It is, however, to be observed that since—as Stur and others have shown—Calamities radiatus, and other forms distinctively Devonian in America, occur in Europe in the Lower Carboniferous, it is not unlikely that the Devonian flora, like that of the Tertiary, appeared earlier in America. It is also probable, as I have shown in the “Reports” already referred to, that it appeared earlier in the Arctic than in the temperate zone. Hence an Arctic or American flora, really Devonian, may readily be mistaken for Lower Carboniferous by a botanist basing his calculations on the fossils of temperate Europe. Even in America itself, it would appear, from recent discoveries in Virginia and Ohio, that certain Devonian forms lingered longer in those regions than farther to the northeast;[BX] and it would not be surprising if similar plants occurred in later beds in Devonshire or in the south of Europe than in Scotland. Still, these facts, properly understood, do not invalidate the evidence of fossil plants as to geological age, though errors arising from the neglect of them are still current.

[BX] Andrews, “Palæontology of Ohio,” vol. ii.; Meek, “Fossil Plants from Western Virginia,” Philosophical Society, Washington, 1875.

VI.—Geological Relations of some Plant-bearing Beds of Eastern Canada.
(“Report on Erian Plants,” 1871.)

The Gaspé sandstones have been fully described by Sir W. E. Logan, in his “Report on the Geology of Canada,” 1863. He there assigns to them a thickness of seven thousand and thirty-six feet, and shows that they rest conformably on the Upper Silurian limestones of the Lower Helderberg group (Ludlow), and are in their turn overlaid unconformably by the conglomerates which form the base of the Carboniferous rocks of New Brunswick. I shall add here merely a few remarks on points in their physical character connected with the occurrence of plants in them.

Prototaxites (Nematophyton) Logani and other characteristic Lower Erian plants occur in the base of the sandstones at Little Gaspé. This fact, along with the occurrence, as stated in my paper of 1863, of rhizomes of Psilophyton preserving their scalariform structure, in the upper part of the marine Upper Silurian limestones,[BY] proves the flora of the Devonian rocks to have had its beginning at least in the previous geological period, and to characterise the lower as well as the upper beds of the Devonian series. In this connection I may state that, from their marine fossils, as well as their stratigraphical arrangement, Sir W. E. Logan and Mr. Billings regard the lower portions of the Gaspé sandstones as the equivalents of the Oriskany sandstone of New York. On the other hand, the great thickness of this formation, the absence of Lower Devonian fossils from its upper part, and the resemblance of the upper beds to those of the newer members of the Devonian elsewhere, render it probable that the Gaspé sandstones, though deficient in the calcareous members of the system, seen farther to the westward, represent the whole of the Devonian period.