The Story of the Earth and Man

by Sir John William Dawson

Such a process may seem difficult of comprehension. To understand it we must bear in mind some of its conditions. First, the amount of this wrinkling was extremely small relatively to the mass of the earth. In the diagram on page 162 it is greatly exaggerated, yet is seen to be quite insignificant, however gigantic in comparison with microscopic weaklings like ourselves. Secondly, it was probably extremely slow. Beds of solid rock cannot be suddenly bent into great folds without breaking, and the abruptness of some of the folds may be seen from our figure, copied from Rogers (page 162), of some of the foldings of the Appalachian Mountains. Thirdly, the older rocks below the Carboniferous and the Devonian must have been in a softened and plastic state, and so capable of filling up the vacancies left by the bending of the hard crust above. In evidence of this, we have in the Lower Permian immense volcanic ejections—lavas and other molten rocks spewed out to the surface from the softened and molten masses below. Fourthly, the basin of the Atlantic must have been sufficiently strong to resist the immense lateral pressure, so that the yielding was all concentrated on the weaker parts of the crust near the old fractures at the margins of the great continents. In these places also, as we have seen in previous papers, the greatest thickness of deposits had been formed; so that there was great downward pressure, and probably, also, greater softening of the lower part of the crust. Fifthly, as suggested in a previous chapter, the folding of the earth’s crust may have resulted from the continued shrinkage of its interior in consequence of cooling, leading after long intervals to collapse of the surface. Astronomers have, however, suggested another cause. The earth bulges at the equator, and is flattened at the poles in consequence of, or in connection with, the swiftness of its rotation; but it has been shown that the rotation of the earth is being very gradually lessened by the attraction of the moon.[T] Pierce has recently brought forward the idea[U] that this diminution of rotation, by causing the crust to subside in the equatorial regions and expand in the polar, might produce the movements observed; and which, according to Lesley, have amounted in the whole course of geological time to about two per cent, of the diameter of our globe. We thus have two causes, either of which seems sufficient to produce the effect.

[T] Sir William Thomson, who quotes Adams and Delaunay.

[U] “Nature,” February, 1871.

Viewed in this way, the great disturbances at the close of the Palæozoic period constitute one of the most instructive examples in the whole history of the earth of that process of collapse to which the crust was subject after long intervals, and of which no equally great instance occurs except at the close of the Laurentian and the close of the Mesozoic. The mineral peculiarities of the Permian are also accounted for by the above considerations. Let us now notice some of these. In nearly all parts of the world the Permian presents thick beds of red sandstone and conglomerate as marked ingredients. These, as we have already seen, are indications of rapid deposition accompanying changes of level. In the Permian, as elsewhere, these beds are accompanied by volcanic rocks, indicating the subterranean causes of the disturbances. Again, these rocks are chiefly abundant in those regions, like Western Europe, where the physical changes were at a maximum. Another remarkable feature of the Permian rocks is the occurrence of great beds of magnesian limestone, or dolomite. In England, the thick yellow magnesian limestone, the outcrop of which crosses in nearly a straight line through Durham, Yorkshire, and Nottingham, marks the edge of a great Permian sea extending far to the eastward. In the marls and sandstones of the Permian period there is also much gypsum. Now, chemistry shows us that magnesian limestones and gypsums are likely to be deposited where sea water, which always contains salts of magnesia, is evaporating in limited or circumscribed areas into which carbonate of lime and carbonate of soda are being carried by streams from the land or springs from below;[V] and it is also to be observed that solutions of sulphuric acid, and probably also of sulphate of magnesia, are characteristic products of igneous activity. Hence we find in various geological periods magnesian limestones occurring as a deposit in limited shallow sea basins, and also in connection with volcanic breccias. Now these were obviously the new Permian conditions of what had once been the wide flat areas of the Carboniferous period. Still further, we find in Europe, as characteristic of this period, beds impregnated with metallic salts, especially of copper. Of this kind are very markedly the copper slates of Thuringia. Such beds are not, any more than magnesian limestones, limited to this age; but they are eminently characteristic of it. To produce them it is required that water should bring forth from the earth’s crust large quantities of metallic salts, and that these should come into contact with vegetable matters in limited submerged areas, so that sulphates of the metals should be deoxidized into sulphides. A somewhat different chemical process, as already explained, was very active in the coal period, and was connected with the production of its iron ores; but, in the Permian, profound and extensive fractures opened up the way to the deep seats of copper and other metals, to enrich the copper slate and its associated beds. It is also to be observed that the alkaline springs and waters which contain carbonate of soda, very frequently hold various metallic salts; so that where, owing to the action of such waters, magnesian limestone is being deposited, we may expect also to find various metallic ores.

[V] Hunt, “Silliman’s Journal,” 1859 and 1863.

Let us sum up shortly this history. We have foldings of the earth’s crust, causing volcanic action and producing limited and shallow sea-basins, and at the same time causing the evolution of alkaline and metalliferous springs. The union of these mechanical and chemical causes explains at once the conglomerates, the red sandstones, the trap rocks, the magnesian limestones, the gypsum, and the metalliferous beds of the Permian. The same considerations explain the occurrence of similar deposits in various other ages of the earth’s history; though, perhaps, in none of these were they so general over the Northern Hemisphere as in the Permian.

From the size of the stones in some of the Permian conglomerates, and their scratched surfaces, it has been supposed that there were in this period, on the margins of the continents, mountains sufficiently high to have snow-clad summits, and to send down glaciers, bearing rocks and stones to the sea, on which may have floated, as now in the North Atlantic, huge icebergs.[W] This would be quite in accordance with the great elevation of land which we know actually occurred; and the existence of snow-clad mountains along with volcanoes would be a union of fire and frost of which we still have examples in some parts of the earth’s surface, and this in proximity to forms of vegetable life very similar to those which we know existed in the Permian.

[W] Ramsay has ably illustrated this in the Permian conglomerates of England.

With the exception of a few small and worthless beds in Russia, the Permian is not known to contain any coal. The great swamps of the coal period had disappeared. In part they were raised up into rugged mountains. In part they were sunken into shallow sea areas. Thus, while there was much dry land, there was little opportunity for coal production, or for the existence of those rank forests which had accumulated so much vegetable matter in the Carboniferous age. In like manner the fauna of the Permian waters is poor. According to Murchison, the Permian limestones of Europe have afforded little more than one-third as many species of fossils as the older Carboniferous. The fossils themselves also have a stunted and depauperated aspect, indicating conditions of existence unfavourable to them. This is curiously seen in contrasting Davidson’s beautiful illustrations of the British Lamp-shells of the Permian and Carboniferous periods. Another illustrative fact is the exceptionally small size of the fossils even in limestones of the Carboniferous period when these are associated with gypsum, red sandstones, and magnesian minerals; as, for instance, those of some parts of Nova Scotia. In truth, the peculiar chemical conditions conducive to the production of magnesian limestones and gypsum are not favourable to animal life, though no doubt compatible with its existence. Hence the rich fauna of the Carboniferous seas died out in the Permian, and was not renewed; and the Atlantic areas of the period are unknown to us. They were, however, probably very deep and abrupt in slope, and not rich in life. This would be especially the case if they were desolated by cold ice-laden currents.

During the Permian period there was in each of our continental areas a somewhat extensive inland sea. That of Western America was a northward extension of the Gulf of Mexico. That of Eastern Europe was a northward extension of the Euxine and Caspian. In both, the deposits formed were very similar—magnesian limestones, sandstones, conglomerates, marls, and gypsums. In both, these alternate in such a way as to show that there were frequent oscillations of level, producing alternately shallow and deep waters. In both, the animal remains are of similar species, in many instances even identical. But in the areas intervening between these sea basins and the Atlantic the conditions were somewhat different. In Europe the land was interrupted by considerable water areas, not lakes, but inland sea basins; sometimes probably connected with the open sea, sometimes isolated. In these were, deposited the magnesian limestone and its associated beds in England, and the Zechstein and Rotheliegende with their associates in Germany. In America the case was different. In all that immense area which extends from the Atlantic to the plains east of the Mississippi, we know no Permian rocks, unless a portion of those reckoned as Upper Carboniferous, or Permo-carboniferous in Northern Nova Scotia, and Prince Edward Island, should be included in this group. If such existed, they may possibly be covered up in some places by more modern deposits, or may have been swept away by denudation in the intervening ages; but even in these cases we should expect to find some visible remains of them. Their entire absence would seem to indicate that a vast, and in many parts rugged and elevated, continent represented North America in the Permian period. Yet if so, that great continent is an absolute blank to us. We know nothing of the animals or plants which may have lived on it, nor do we even know with certainty that it had active volcanoes, or snow-clad mountains sending down glaciers.