Climate and Time in Their Geological Relations / A Theory of Secular Changes of the Earth's Climate

by James Croll

Conditions necessary for the Preservation of the Coal Plants.—But in order to the formation of coal, it is not simply necessary to have a condition of climate suitable for the growth, but also for the preservation, of a luxuriant vegetation. The very existence of coal is as much due to the latter circumstance as to the former; nay more, as we shall yet see, the fact that a greater amount of coal belongs to the Carboniferous period than to any other, was evidently due not so much to a more extensive vegetable growth during that age, suited to form coal, as to the fact that that flora has been better preserved. Now, as will be presently shown, we have not merely in the warm periods of a glacial epoch a condition of climate best suited for the growth of coal plants, but we have also in the cold periods of such an epoch the condition most favourable for the preservation of those plants.

One circumstance necessary for the preservation of plants is that they should have been covered over by a thick deposit of sand, mud, or clay, and for this end it is necessary that the area upon which the plants grew should have become submerged. It is evident that unless the area had become submerged, the plants could not have been covered over with a thick deposit; and, even supposing they had been covered over, they could not have escaped destruction from subaërial denudation unless the whole had been under water. Another condition favourable, if not essential, to the preservation of the plants, is that they should have been submerged in a cold and not in a warm sea. Assuming that the coal plants grew during a warm period of a glacial epoch, we have in the cold period which succeeded all the above conditions necessarily secured.

It is now generally admitted that the coal trees grew near broad estuaries and on immense flat plains but little elevated above sea-level. But that the Lepidodendra, Sigillariæ, and other trees, of which our coal is almost wholly composed, grew on dry ground, elevated above sea-level, and not in swamps and shallow water, as was at one time supposed, has been conclusively established by the researches of Principal Dawson and others. After the growth of many generations of trees, the plain is eventually submerged under the sea, and the whole, through course of time, becomes covered over with thick deposits of sand, gravel, and other sediments carried down by streams from the adjoining land. After this the submerged plain becomes again elevated above the sea-level, and forms the site of a second forest, which, after continuing to flourish for long centuries, is in turn destroyed by submergence, and, like the former, becomes covered over with deposits from the land. This alternate process of submergence and emergence goes on till we have a succession of buried forests one above another, with immense stratified deposits between. These buried forests ultimately become converted into beds of coal. This, I presume, is a fair representation of the generally admitted way in which our coal-beds had their origin. It is also worthy of notice that the stratified beds between the coal-seams are of marine and not of lacustrine origin. On this point I may quote the opinion of Professor Hull, a well-known authority on the subject: “Whilst admitting,” he says, “the occasional presence of lacustrine strata associated with the coal-measures, I think we may conclude that the whole formation has been essentially of marine and estuarine origin.”[242]

Coal-beds necessarily imply Oscillations of Sea-level.—It may also be observed that each coal-seam indicates both an elevation and a depression of the land. If, for example, there are six coal-seams, one above the other, this proves that the land must have been, at least, six times below and six times above sea-level. This repeated oscillation of the land has been regarded as a somewhat puzzling and singular circumstance. But if we assume coal to be an inter-glacial formation, this difficulty not only disappears, but all the various circumstances which we have been detailing are readily explained. We have to begin with a warm inter-glacial period, with a climate specially suited for the growth of the coal trees. During this period, as has been shown in the chapter on Submergence, the sea would be standing at a lower level than at present, laying bare large tracts of sea-bottom, on which would flourish the coal vegetation. This condition of things would continue for a period of 8,000 or 10,000 years, allowing the growth of many generations of trees. When the warm period came to a close, and the cold and glacial condition set in, the climate became unsuited for the growth of the coal plants. The sea would begin to rise, and the old sea-bottoms on which, during so long a period, the forests grew, would be submerged and become covered by sedimentary deposits brought down from the land. These forests becoming submerged in a cold sea, and buried under an immense mass of sediment, were then now protected from destruction, and in a position to become converted into coal. The cold continuing for a period of 10,000 years, or thereby, would be succeeded by another warm period, during which the submerged areas became again a land-surface, on which a second forest flourished for another 10,000 years, which in turn became submerged and buried under drift on the approach of the second cold period. This alternate process of submergence and emergence of the land, corresponding to the rise and fall of sea-level during the cold and warm periods, would continue so long as the eccentricity of the earth’s orbit remained at a high value, till we might have, perhaps, five or six submerged forests, one above the other, and separated by great thicknesses of stratified deposits, these submerged forests being the coal-beds of the present day.

Fig. 10.

It is probable that the forests of the Coal period would extend inland over the country, but only such portions as were slightly elevated above sea-level would be submerged and covered over by sediment and thus be preserved, and ultimately become coal-seams. The process will be better understood from the following diagram. Let A B represent the surface of the ground prior to a glacial epoch, and to the formation of the beds of coal and stratified deposits represented in the diagram. Let S S′ be the normal sea-level. Suppose the eccentricity of the earth’s orbit begins to increase, and the winter solstice approaches the perihelion, we have then a moderately warm period. The sea-level sinks to 1, and forests of sigillariæ and other coal trees cover the country from the sea-shore at 1, stretching away inland in the direction of B. In course of time the winter solstice moves round to aphelion and a cold period follows. The sea begins to rise and continues rising till it reaches 1′. Denudation and the severity of the climate destroy every vestige of the forest from 1′ backwards into the interior; but the portion 1 1′ being submerged and covered over by sediment brought down from the land is preserved. The eccentricity continuing to increase in extent, the second inter-glacial period is more warm and equable than the first, and the sea this time sinks to 2. A second forest now covers the country down to the sea-shore at 2. This second warm period is followed by the second cold period, more severe than the first, and the sea-level rises to 2′. Denudation and severity of climate now destroy every remnant of the forest, from 2′ inland, but of course the submerged portion of 2 2′, like the former portion 1 1′, is preserved. During the third warm period (the eccentricity being still on the increase) the sea-level sinks to 3, and the country for the third time is covered by forests, which extend down to 3. This third warm period is followed by a cold glacial period more severe than the preceding, and the sea-level rises to 3′, and the submerged portion of the forest from 3 to 3′ becomes covered with drift,—the rest as before being destroyed by denudation and the severity of the climate. We shall assume that the eccentricity has now reached a maximum, and that during the fourth inter-glacial period the sea-level sinks only to 4, the level to which it sank during the second inter-glacial period. The country is now covered for the fourth time by forests. The cold period which succeeds not being so severe as the last, the sea rises only to 4′, which, of course, marks the limit of the fourth forest. The eccentricity continuing to diminish, the fifth forest is only submerged up to 5′, and the sixth and last one up to 6′. The epoch of cold and warm periods being now at a close, the sea-level remains stationary at its old normal position S S′. Here we have six buried forests, the one above the other, which, through course of ages, become transformed into coal-beds.

It does not, however, necessarily follow that each separate coal-seam represents a warm period. It is quite possible that two or more seams separated from each other by thin partings or a few feet of sedimentary strata might have been formed during one warm period; for during a warm period minor oscillations of sea-level sufficient to submerge the land to some depth might quite readily have taken place from the melting of polar ice, as was shown in the chapter on Submergence.

It may be noticed that in order to make the section more distinct, its thickness has been greatly exaggerated. It will also be observed that beds 4, 5, and 6 extend considerably to the left of what is represented in the section.

But it is not to be supposed that the whole phenomena of the coal-fields can be explained without supposing a subsidence of the land. The great depth to which the coal-beds have been sunk, in many cases, must be attributed to a subsidence of the level. A series of beds formed during a glacial epoch, may, owing to a subsidence of the land, be sunk to a great depth, and become covered over with thousands of feet of sediment; and then on the occurrence of another glacial epoch, a new series of coal-beds may be formed on the surface. Thus the upper series may be separated from the lower by thousands of feet of sedimentary rock. There is another consequence resulting from the sinking of the land, which must be taken into account. Had there been no sinking of the land during the Carboniferous age, the quantity of coal-beds now remaining would be far less than it actually is, for it is in a great measure owing to their being sunk to a great depth that they have escaped destruction by the enormous amount of denudation which has taken place since that remote age. It therefore follows that only a very small fraction of the submerged forests of the Coal period do actually now exist in the form of coal. Generally it would only be those areas which happened to be sunk to a considerable depth, by a subsidence of the land, that would escape destruction from denudation. But no doubt the areas which would thus be preserved bear but a small proportion to those destroyed.