Some Salient Points in the Science of the Earth

by Sir John William Dawson

[108] Report on 49th Parallel, 1875.

Other plants equally illustrate the decadence of important types of vegetable life. In the beautiful family of the Magnolias there exists in America a most remarkable and elegant tree, whose trunk attains sometimes a diameter of 7 feet and a height of 80 or 90 feet. Its broad deep green leaves are singularly truncate at the end, as if artificially cut off, and in spring it puts forth a wealth of large and brilliant orange and yellow flowers, from which it obtains the name of Tulip tree. It is the Liriodendron tulipifera of botanists, and the sole species of its genus. This Tulip tree has a history. All through the Tertiary beds we find leaves referable to the genus, and belonging not to one species only, but to several, and as we go back into the Cretaceous, the species seem to become more numerous. Many of them have smaller leaves than the modern species, others larger, and some have forms even more quaint than that of the existing Tulip tree. The oldest that I have seen in Canada is one from the Upper Cretaceous of Port McNeil in the north of Vancouver Island, which is as large as that of the modern species, and very similar in form. Thus this beautiful vegetable type culminated long geological ages ago, and was represented by many species, no doubt occupying a prominent place in the forests of the northern hemisphere. To-day only a single species exists, in our warmer regions, to keep up the memory of this almost perished genus; but that species is one of our most beautiful trees.

The history of the Sequoias or giant Cypresses, of which two species now exist in limited areas in California, is still more striking. These giant trees, monsters of the vegetable kingdom, are, strange to say, very limited in their geographical range. The greater of the two, Sequoia gigantea, the giant tree par excellence, seems limited to a few groves in California. At first sight this strikes us as anomalous, especially as we find that the tree will grow somewhat widely both in Europe and America when its seeds are sown in suitable soil. The mystery is solved when we learn that the two existing species are but survivors of a genus once diffused over the whole northern hemisphere, and represented by many species, constituting, in the Later Cretaceous and Eocene ages, vast and dark forests extending over enormous areas of our continents, and forming much of the material of the thick and widely distributed Lignite beds of North-western America. Thus the genus has had its time of expansion and prevalence, and is now probably verging on extinction, not because there are not suitable habitats, but either because it is now old and moribund, or because other and newer forms have now a preference in the existing conditions of existence.

The Plane trees, the Sassafras, the curious Ginkgo tree or fern-leaved yew of Japan, are cases of similar decadence of genera once represented by many species, while other trees, like the Willows and Poplars, the Maples, the Birches, the Oaks and the Pines, though of old date, are still as abundant as they ever were, and some genera would seem even to have increased in number of species, though on the whole the flora of our modern woods is much less rich than those of the Miocene and Eocene, or even than that of the Later Cretaceous. The early Tertiary periods were, as we know, times of exuberant and gigantic animal life on the land, and it is in connection with this that the vegetable world seems to have attained its greatest variety and luxuriance. Even that early post-glacial age in which primitive man seems first to have spread himself over our continents was one richer both in animal and plant life than the present. The geographical changes which closed this period and inaugurated the modern era seem to have reduced not only the area of the continents but the variety of land life in a very remarkable manner. Thus our last lesson from the genesis and migrations of plants is the humbling one that the present world is by no means the best possible in so far as richness of vegetable and animal life is concerned.

Reference has been made to the utility of fossil plants as evidence of climate; but the subject deserves more detailed notice. I have often pondered on the nature of the climate evidenced by the floras of the Devonian and Carboniferous; but the problem is a difficult one, not only because of the peculiar character of the plants themselves, so unlike those of our time, but because of the probably different meteorological conditions of the period. It is easy to see that a flora of tree-ferns, great lycopods and pines is more akin to that of oceanic islands in warm latitudes than anything else that we know. But the Devonian and Carboniferous plants did not flourish in oceanic islands, but for the most part on continental areas of considerable dimensions, though probably more flat and less elevated than those of the present day. They also grew, from Arctic latitudes, almost, if not altogether, to the equator; and though there are generic differences in the plants of these periods in the southern hemisphere, yet these do not affect the general facies. There are, for example, characteristic Lepidodendroids in the Devonian and Carboniferous of Brazil, Australia, and South Africa. If now we consider the plants a little more in detail, coniferous and taxine trees grow now in very different latitudes and climates. There is therefore nothing so very remarkable in their occurrence. The great group of Cordaites may have been equally hardy; but it is noteworthy that their geographical distribution is more limited. In Europe, for example, they are more characteristic in France than in Great Britain. Ferns and Lycopods and Mare's-tails are also cosmopolitan, but the larger species belong to the warmer climates, and nowhere at present do they become so woody and so complex in structure as they were in the older geological periods. At the present day, however, they love moisture rather than aridity, and uniformity of temperature rather than extreme light and heat. The natural inference would be that in these older periods geographical and other conditions must have conspired to produce a uniform and moist climate over a large portion of the continents. The geographical conditions of the Carboniferous age, and the distribution of animal life on the sea and land, confirm the conclusion based on the flora. Further, if, as seems probable, there was a larger proportion of carbon dioxide in the atmosphere than at present, this would not only directly affect the growth of plants, but would impede radiation, and so prevent escape of heat by that means, while the moisture exhaled from inland seas and lagoons and vastly extended swamps, would tend in the same direction.

It would, however, be a mistake to infer that there were not local differences of climate. I have elsewhere[109] advocated the theory that the great ridge of boulders, the New Glasgow conglomerate, which forms one margin of the coal field of Picton, in Nova Scotia, is an ice-formed ridge separating the area of accumulation of the great thirty-six feet seam from an outer area in which aqueous conditions prevailed, and little coal was formed. In this case, an ice-laden sea, carrying boulders on its floes and fields of ice, must have been a few miles distant from forests of Lepidodendra, Cordaites, and Sigillariæ, and the climate must have been anything but warm, at least at certain seasons. Nor have we a right to infer that the growth of the coal-plants was rapid. Stems, with woody axes and a thick bark, containing much fibrous and thick-walled cellular tissue, are not to be compared with modern succulent plants, especially when we consider the sparse and rigid foliage of many of them. Our conclusion should, therefore, be that geographical conditions and the abundance of carbon dioxide in the atmosphere favoured a moist climate and uniform temperature, and that the flora was suited to these conditions.

[109] "Acadian Geology," Carboniferous of Picton.

As to the early Mesozoic flora, I have already suggested that it must have been an invader from the south, for which the intervening Permian age had made way by destroying the Palæozoic flora. This was probably effected by great earth-movements changing geographical conditions. But in the Mesozoic the old conditions to some extent returned, and the Carboniferous plants being extinct, their places were taken by pines, lycopods, and ferns, whose previous home had been in the insular regions of the tropics, and which, as climatal conditions improved, pushed their way to the Arctic circle. But, being derivatives of warm regions, their vitality and capacity for variation were not great, and they only locally and in favourable conditions became great coal producers. The new flora of the Later Cretaceous and the Tertiary, as previously stated, originated in the Arctic, and marched southward.

These newer Cretaceous plants presented from the first the generic aspects of modern vegetation, and so enable us much better to gauge their climatal conditions. In general, they do not indicate tropical heat in the far north, but only that of the warm temperate zone; but this in some portions of the period certainly extends to the middle of Greenland, unless, without any evidence, we suppose that the Cretaceous and lower Tertiary plants differed in hardiness of constitution from their modern representatives. They prove, however, considerable oscillations of climate. Gardner, Nathorst and Reid have shown this in Europe, and that it extends from the almost tropical flora of the lower Eocene to the Arctic flora of the Pleistocene. In America, owing, as Grey has suggested, to its great north and south extension, the changes were more regular and gradual. In the warmer periods of the Cretaceous, the flora as far north as 55° was similar to that of Georgia and Northern Florida at the present day, while in the cooler period of the Laramie (Lower Eocene, or more probably Paleocene) it was not unlike that of the Middle States. In the Pleistocene, the flora indicates a boreal temperature in the Glacial age. Thus there are no very extreme contrasts, but the evident fact of a warm temperate or subtropical climate extending very far north at the same times when Greenland had a temperate climate. As I have elsewhere shown,[110] discoveries in various parts of North America are beginning to indicate the precise geographical conditions accompanying the warmer and colder climates.

[110] Trans. Royal Society of Canada, 1890-1.