Some Salient Points in the Science of the Earth

by Sir John William Dawson

"The objection to calling the Stigmariæ roots and their processes rootlets, appears to me a finical application of modern botanical usages to times for which they do not hold. We might equally object to the application of the term roots to those which spring from the earthed-up stems of Calamites, radiating as they do from nodes which, in the air, would produce branchlets. Grand' Eury's figures show abundant instances of this. We might also object to the exogenous stems described by Williamson, which belong to cryptogamous plants; and, unlike anything modern, are made up exclusively of scalariform tissue. If the articulation and regular arrangement of those gigantic root hairs, the rootlets, or 'leaves' of Stigmaria, are to be regarded as depriving them of the name which clearly describes their function, we may call them underground branches, though, by so doing, we set at nought both their function and their mode of growth."

Dr. Williamson, in a recent paper, expresses the same view in the following terms[120]:—"At that period (the Carboniferous age) no Angiosperms existed on the earth, and even the Gymnosperms were very far from reaching their modern development. Under these circumstances the Cryptogams chiefly became the giant forest trees of that remote age. To become such, they required an organization very different in some respects from that of their degraded living representatives. Hence we must not appeal to these degenerate types for illustrations and explanations of structures no longer existing. Still less must we turn to what we find in the Angiosperms, that wholly distinct race which has taken the place of the primæval Cryptogams in our woods. The primeval giants of the swampy forests had doubtless a morphology assigned to them, adapted to the physical conditions by which they were surrounded; but if even their dwarfed and otherwise modified descendants fail to throw light upon morphological details once so common, still less must we expect to obtain that light from the living and wholly different flowering plants."

[120] Natural Science, July, 1892.

With the remarkable trees above referred to, there coëxisted a vast multitude of ferns, some arborescent, others herbaceous, tall, reed-like plants, the Calamites, allied to modern Mare's-tails, a very remarkable family of plants allied to modern Cycads and Pines; the Cordaites, which seem to have grown plentifully in certain parts of the coal areas—probably the drier parts, so that their remains sometimes constitute the greater part of small seams of coal. There were also true pine-like trees, though these would seem to have grown most abundantly on the higher levels. Nor was strictly aquatic vegetation wanting. We find, both in the preceding Devonian and the Carboniferous, that the little aquatic plants now known as Rhizocarps, and structurally allied to the Ferns—such plants as the floating Salvinia, and the Pillworts of our swamps, were vastly abundant, and they may have filled and choked up with their exuberant growth many of the lakes and slow streams of the period, furnishing layers of cannel and "macrospore" coal, and earthly bitumen or Torbanite.

We have hitherto confined our attention to the great Carboniferous period, so called, as emphatically the age of coal; but this mineral, and allied forms of carbon, were produced both before and after. Even in that old Laurentian age, which includes the oldest rocks that we know, formed when the first land had just risen out of the waters, there are thick beds of graphite, or plumbago, chemically the same with anthracite coal, and which must have been produced by the agency of plants, whether terrestrial or aquatic. We may suppose that the plants of this remote age were of very humble type as much lower than those of the coal formation as these are lower than those of the present day; but if so, then, on the analogy of the Carboniferous, they would be high and complex representatives of those low types. But there is another and more startling possibility; that the Laurentian may have been a period when vegetable life culminated on the earth, and existed in its most complete and grandest forms in advance of the time when it was brought into subordination to the higher life of the animal. In the meantime, the Laurentian rocks are in a state of so extreme metamorphism that they have afforded no certain indication of the forms or structures of the vegetation of the period.

We find indications of plant life through all the Palæozoic groups succeeding the Laurentian; but it is not till we reach the Devonian, the system immediately preceding the Carboniferous, that we find an abundance of forms not essentially different from those of the Carboniferous, though similar in details. Only a few and very small beds of coal were accumulated in this age; but there was an immense abundance of bituminous shale enriched with the macrospores of Rhizocarps. The Ohio black shale, which is said to extend its outcrop across that state with a breadth of ten to twenty miles, and a thickness of 550 feet, is filled with macrospores of Protosalvinia, as is its continuation in Canada.

Above the great coal formation the Permian and Jurassic contain beds of coal, though of limited extent, and formed in the case of the two latter of very different plants from those of the Carboniferous. In the Cretaceous and Tertiary ages, after the abundant introduction of species of forest trees still living, coal making seems to have obtained a new impulse, so that in China and the western part of America there are coals of great extent and value, all made of plants of genera still existing. In the Cretaceous coal of Vancouver Island there are remains of such modern trees as the Poplars, Magnolias, Palmettos, Sequoias, and a great variety of other genera still living in America. Out of the remains of these, under favouring conditions, quite as good coal as that of the coal formation has been made, although the plants are so different. There is, indeed, reason to believe that those now rare trees, the Sequoias, represented at the present time only by the big trees of California, and their companion, the redwood, were then spread universally over the northern hemisphere, and formed dense forests on swampy flats which led to the accumulation of coal beds in which the trunks and leaves of the Sequoias formed main ingredients, so that Sequoia and its allies in this later age take the place of the Sigillariæ of the coal formation. Last of all, coal accumulation is still going on in the Everglades of Florida, the dismal swamp of Virginia, and the peat-bogs of the more northern regions. So the vegetable kingdom has, throughout its long history, been continually depriving the atmosphere of its carbon dioxide, and accumulating this in beds of coal. In the earlier ages indeed, this would seem to us to have been its main use.

To the modern naturalist, vegetable life, with regard to its uses, is the great accumulator of pabulum for the sustenance of the higher forms of vital energy manifested in the animal. In the Palæozoic this consideration sinks in importance. In the Coal period we know few land animals, and these not vegetable feeders, with the exception of some insects, millipedes, and snails. But the Carboniferous forests did not live in vain, if their only use was to store up the light and heat of those old summers in the form of coal, and to remove the excess of carbonic acid from the atmosphere. In the Devonian period even these utilities fail, for coal does not seem to have been accumulated to any great extent, though the abundant petroleum of the Devonian is, no doubt, due to the agency of aquatic vegetation. In addition to scorpions, a few insects are the only known tenants of the Devonian land, and these are of kinds whose lame probably lived in water, and were not dependent on land plants. We may have much yet to learn of the animal life of the Devonian; but for the present, the great plan of vegetable nature goes beyond our measures of utility; and there remains only what is perhaps the most wonderful and suggestive correlation of all, namely, that our minds are able to trace in these perished organisms structures similar to those of modern plants, and thus to reproduce in imagination the forms and habits of growth of living things which so long preceded us on the earth.

In another way Huxley has put the utilitarian aspect of the case so admirably, that I cannot refrain from quoting his clever apotheosis of nature in connection with the production of coal.

"Nature is never in a hurry, and seems to have had always before her eyes the adage, 'Keep a thing long enough, and you will find a use for it.' She has kept her beds of coal for millions of years without being able to find a use for them; she has sent them beneath the sea, and the sea beasts could make nothing of them; she had raised them up into dry land, and laid the black veins bare, and still for ages and ages there was no living thing on the face of the earth that could see any sort of value in them; and it was only the other day, so to speak, that she turned a new creature out of her workshop, who, by degrees, acquired sufficient wits to make a fire, and then to discover that the black rock would burn.