This great depression must have been of very long continuance, since in Western Europe it sufficed for the production of nearly 1,000 feet in thickness of chalk, a rock which, being composed almost entirely of microscopic shells, is, as we shall see in the sequel, necessarily of extremely slow growth. If we regard the Cretaceous group as one of our great ages or cycles, it seems to be incomplete. The sandstones and clays known as the Greensand and Gault constitute its lower or shallow-water member. The chalk is its middle or deep-sea member, but the upper shallow-water member is missing, or only very locally and imperfectly developed. And the oldest of the succeeding Tertiary deposits, which indicate much less continuous marine conditions, rest on the chalk, as if the great and deep sea of the Cretaceous age had been suddenly upheaved into land. This abrupt termination of the last cycle of the Mesozoic is obviously the reason of the otherwise inexplicable fact that the prevalent life of the period ceases at the top of the chalk, and is exchanged immediately and without any transition for the very different fauna of the Tertiary. This further accords with the fact that the Cretaceous subsidence ended in another great crumpling of the crust, like that which distinguished the Permian. By this the Mesozoic time was terminated and the Cainozoic inaugurated; while the Rocky Mountains, the Andes, the Alps, and the Himalayas, rose to importance as great mountain ranges, and the continents were again braced up to retain a condition of comparative equilibrium during that later period of the earth’s chronology to which we ourselves belong.
LIFE ON LAND IN THE MESOZOIC PERIOD.
In the foreground are a Pine, Cycads, and a Pandanus; also small Mammals, an herbivorous Dinosaur, and a Labyrinthodont. In the distance are other Dinosaurs and Crocodiles. In the air are birds (Archæopterux) and Pterodactyls.
Was the length of the Mesozoic time equal to that of the Palæozoic? Measured by recurring cycles it was. In the latter period we find five great cycles, from the Lower Silurian to the Permian inclusive. So in the Mesozoic we have five also, from the Trias to the Cretaceous inclusive. We have a right to reckon these cycles as ages or great years of the earth; and so reckoning them, the Mesozoic time may have been as long as the Palæozoic. But if we take another criterion the result will be different. The thickness of the deposits in the Palæozoic as compared with the Mesozoic, where these are severally best developed, may be estimated as at least four or five to one; so that if we suppose the beds to have been formed with equal rapidity in the two great periods, then the older of the two was between four and five times as long as the latter, which would indeed be only a little greater than one of the separate ages of the Palæozoic. Either, therefore, the deposits took place with greater rapidity in the Palæozoic, or that period was by much the longer of the two. This it will be observed, is only another aspect of the great laws of geological sequence referred to in our last paper.
Let us look into this question a little more minutely. If the several pulsations of our continents depended upon any regularly recurring astronomical or terrestrial change, then they must represent, at least approximately, equal portions of time, and this, if proved, would settle the question in favour of an equal duration of these two great eras of the earth’s history. But as we cannot yet prove this, we may consider what light we can derive from the nature of the rocks produced. These may be roughly classified as of two kinds: First, the beds of sediment, sand, clay, etc., accumulated by the slow chemical decay of rocks and the mechanical agency of water. Secondly, the beds formed by accumulation of the harder and less perishable parts of living beings, of which the limestones are the chief. With reference to the first of these kinds of deposit, the action of the atmosphere and rains on rocks in the earlier times might have been somewhat more powerful if there was more carbonic acid in the atmosphere, that substance being the most efficient agent in the chemical decay of rocks. It might have been somewhat more powerful if there was a greater rainfall. It must, on the other hand, have been lessened by the apparently more equable temperature which then prevailed. These differences might perhaps nearly balance one another. Then the rocks of the older time were quite as intractable as those of the newer, and they were probably neither so high nor so extensive. Further, the dips and emergences of the great continental plateaus were equally numerous in the two great periods, though they were probably, with the exception of the latest one of each, more complete in the older period. In so far, then, as deposition of sediment is concerned, these considerations would scarcely lead us to infer that it was more rapid in the Palæozoic. But the Palæozoic sediments may be estimated in the aggregate at about 50,000 feet in thickness, while those of the Mesozoic scarcely reach 8,000. We might, therefore, infer that the Palæozoic period was perhaps five or six times as long as the Mesozoic.
If we take the second class of rocks, the limestones, and suppose these to have been accumulated by the slow growth of corals, shells, etc., in the sea, we might, at first sight, suppose that Palæozoic animals would not grow or accumulate limestone faster than their Mesozoic successors. We must, however, consider here the probability that the older oceans contained more lime in solution than those which now exist, and that the equable temperature and extensive submerged plateaus gave very favourable conditions for the lower animals of the sea, so that it would perhaps be fair to allow a somewhat more rapid rate of growth of limestone for the Palæozoic. Now the actual proportions of limestone may be roughly stated at 13,000 feet in the Palæozoic, and 3,000 feet in the Mesozoic, which would give a proportion of about four and a quarter to one; and as a foot of limestone may be supposed on the average to require five times as long for its formation as a foot of sediment, this would give an even greater absolute excess in favour of the Palæozoic on the evidence of the limestones an excess probably far too great to be accounted for by any more favourable conditions for the secretion of carbonate of lime by marine animals.