Some Salient Points in the Science of the Earth

by Sir John William Dawson

[104] Reports Geological Survey of Canada.

[105] Fontaine has well described the Mesozoic flora of Virginia, American Journal of Science, January, 1879.

This latter established itself in Greenland, and probably all around the Arctic circle, in the mild period of the earliest Eocene, and as the climate of the northern hemisphere became gradually reduced from that time till the end of the Pliocene, it marched on over both continents to the southward, chased behind by the modern arctic flora, and eventually by the frost and snow of the Glacial age. This history may admit of correction in details; but, so far as present knowledge extends, it is in the main not far from the truth.

Perhaps the first great question which it raises is that as to the causes of the alternations of warm and cold climates in the north, apparently demanded by the vicissitudes of the vegetable kingdom. Here we may set aside the idea that in former times plants were suited to endure greater cold than at present. It is true that some of the fossil Greenland plants are of unknown genera, and many are new species to us; but we are on the whole safe in affirming that they must have required conditions similar to those necessary to their modern representatives, except within such limits as we now find to hold in similar cases among existing plants. Still we know that at the present time many species found in the equable climate of England will not live in Canada, though species to all appearance similar in structure are natives of the latter. There is also some reason to suppose that species, when new, may have greater hardiness and adaptability than when in old age, and verging toward extinction. In any case, these facts can account for but a small part of the phenomena, which require to be explained by physical changes affecting the earth as a whole, or at least the northern hemisphere. Many theoretical views have been suggested on this subject, which will be found discussed elsewhere, and perhaps the most practical way to deal with them here will be to refer to the actual conditions known to have prevailed in connection with the introduction and distribution of the principal floras which have succeeded each other in geological history.

If we can assume that all the carbon now sealed up in limestones and in coal was originally floating in the atmosphere as carbon dioxide, then we would have a cause which might seriously have affected the earlier land floras—that, for instance, which may have existed in the Eozoic age, and those well known to us in the Palæozoic. Such an excess of carbonic acid would have required some difference of constitution in the plants themselves; it would have afforded them a super-abundance of wood-forming nutriment, and it would have acted as an obstacle to the radiation of heat from the earth, almost equal to the glass roof of a greenhouse, thus constituting a great corrective of changes of temperature. Under such circumstances we might expect a peculiar and exuberant vegetation in the earlier geological ages, though this would not apply to the later in any appreciable degree. In addition to this we know that the geographical arrangements of our continents were suited to the production of a great uniformity of climate. Taking the American continent as the simpler, we know that in this period there existed in the interior plateau between the rudimentary eastern and western mountains a great inland sea, so sheltered from the north that its waters contained hundreds of species of corals, growing with a luxuriance unsurpassed in the modern tropics. On the shores and islands of such a sea we do not wonder that there should have been tree-ferns and gigantic lycopods. In the succeeding Carboniferous, vast areas, both on the margins and in the interior of the continent, were occupied with swampy flats and lagoons, the atmosphere of which must have been loaded with vapour, and rich in compounds of carbon, though the temperature may have been lower than in the Devonian. There still remained, however, more especially in the west, a remnant of the old inland sea, which must have greatly aided in carrying a warm temperature to the north.

If now we pass to the succeeding Jurassic age, we find a more meagre and less widely distributed flora, corresponding to less favourable geographical and climatal conditions, while in the Cretaceous and Eocene ages a return to the old condition of a warm Mediterranean in continuation of the Gulf of Mexico gave those facilities for vegetable growth, which carried plants of the temperate zone as far north as Greenland.

It thus appears that those changes of physical geography and of the ocean currents to which reference is so often made in these papers, apply to the question of the distribution of plants in geological time.

These same causes may help us to deal with the peculiarities of the great Glacial age, which may have been rendered exceptionally severe by the combination of several of the continental and oceanic causes of refrigeration. We must not imagine, however, that the views of those extreme glacialists, who suppose continental ice caps reaching half way to the equator, are borne out by facts. In truth, the ice accumulating round the pole must have been surrounded by water, and there must have been tree-clad islands in the midst of the icy seas, even in the time of greatest refrigeration. This is proved by the fact that in the lower Leda clay of Eastern Canada, which belongs to the time of greatest submergence, and whose fossil shells show sea water almost at the freezing point, there are leaves of poplars and other plants which must have been drifted from neighbouring shores. Similar remains occur in clays of similar origin in the basin of the great lakes and in the West, and are not Arctic plants, but members of the North Temperate flora.[106] These have been called "interglacial," but there is no evidence to prove that they are not truly glacial. Thus, while the arctic flora must have continued to exist within the Arctic circle in the Glacial age, we have evidence that those of the cold temperate and subarctic zones continued to exist pretty far north. At the same time the warm temperate flora would be driven to the south, except where sustained in insular spots warmed by the equatorial currents. It would return northward on the re-elevation of the land and the return of warmth.

[106] Pleistocene Plants of Canada, Dawson and Penhallow, Bull, Geol. Socy., America, 1890. In Europe the Arctic flora extended, relatively to present climate, farther south.

If, however, our modern flora is thus one that has returned from the south, this would account for its poverty in species as compared with those of the early Tertiary. Groups of plants descending from the north have been rich and varied. Returning from the south they are like the shattered remains of a beaten army. This, at least, has been the case with such retreating floras as those of the Lower Carboniferous, the Permian, and the Jurassic, and possibly that of the Lower Eocene of Europe.