THE MESOZOIC AGES.
Physically, the transition from the Permian to the Trias is easy. In the domain of life a great gulf lies between; and the geologist whose mind is filled with the forms of the Palæozoic period, on rising into the next succeeding beds, feels himself a sort of Rip Van Winkle, who has slept a hundred years and awakes in a new world. The geography of our continents seems indeed to have changed little from the time of the Permian to that next succeeding group which all geologists recognise as the beginning of the Mesozoic or Middle Age of the world’s history, the Triassic period. Where best developed, as in Germany, it gives us the usual threefold series, conglomerates and sandstones below, a shelly limestone in the middle, and sandstones and marls above. Curiously enough, the Germans, recognising this tripartite character here more distinctly than in their other formations, named this the Trias or triple group, a name which it still retains, though as we have seen it is by no means the earliest of the triple groups of strata. In England, where the middle limestone is absent, it is a “New Red Sandstone,” and the same name may be appropriately extended to Eastern America, where bright red sandstones are a characteristic feature. In the Trias, as in the Permian, the continents of the northern hemisphere presented large land areas, and there were lagoons and landlocked seas in which gypsum, magnesian limestones, and rock salt were thrown down, a very eminent example of which is afforded by the great salt deposits of Cheshire. There were also tremendous outbursts of igneous activity along the margins of the continents, more especially in Eastern America. But with all this there was a rich land flora and a wonderful exuberance of new animal life on the land; and in places there were even swamps in which pure and valuable beds of coal, comparable with those of the old coal formation, were deposited.
The triple division of the Trias as a cycle of the earth’s history, and its local imperfection, are well seen in the European development of the group, thus:—
| German Series. | French Series. | English Series. |
| Keuper, Sandstone and Shale | Marnes Irisées | Saliferous and gypseous Shales and Sandstones. |
| Muschelkalk, Limestone and Dolomite | Calcaire Coquillier | Wanting. |
| Bunter, Sandstone and Conglomerate | Grès bigarré | Sandstone and Conglomerate. |
The Trias is succeeded by a great and complex system of formations, usually known as the Jurassic, from its admirable development and exposure in the range of the Jura; but which the English geologists often name the “Oolitic,” from the occurrence in it of beds of Oolite or roe-stone. This rock, of which the beautiful cream-coloured limestone of Bath is an illustration, consists of an infinity of little spheres, like seeds or the roe of a fish. Under the microscope these are seen to present concentric layers, each with a radiating fibrous: structure, and often to have a minute grain of sand or fragment of shell in the centre. They are, in short, miniature concretions, produced by the aggregation of the calcareous matter around centres, by a process of molecular attraction to which fine sediments, and especially those containing much lime, are very prone. This style of limestone is very abundant in the Jurassic system, but it is not confined to it. I have seen very perfect Oolites in the Silurian and the Carboniferous. The Jurassic series, as developed in England, may be divided into three triplets or cycles of beds, in the following way:
| Upper Jurassic | Purbeck Beds. Portland Limestone. Portland Sand. | |
| Middle Jurassic | Kimmeridge Clay, etc. Coral Rag, Limestone. Lower Calcareous Grit, Oxford Clay, etc. | |
| Lower Jurassic[AD] | Cornbrash and Forest Marble. Great and inferior Oolite, Limestone. Lias Clays and Limestones. |
[AD] This last group is very complex, and might perhaps admit of sub division, locally at least, into subordinate cycles.
These rocks occupy a large space in England, as the names above given will serve to show; and they are also largely distributed over the continent of Europe and Asia which had evidently three great and long-continued dips under water, indicated by the three great limestones. In America the case was different. The Jurassic has not been distinctly recognised in any part of the eastern coast of that continent, which then perhaps extended farther into the Atlantic than it does at present; so that no marine beds were formed on its eastern border. But in the west, along the base of the Rocky Mountains and also in the Arctic area, there were Jurassic seas of large extent, swarming with characteristic animals. At the close of the Jurassic period our continents seem to have been even more extensive than at present. In England and the neighbouring parts of the continent of Europe, according to Lyell, the fresh-water and estuarine beds known as the Wealden have been traced 320 miles from west to east, and 200 miles from north-west to south-east, and their thickness in one part of this area is estimated at no less than 2,000 feet. Such a deposit is comparable in extent with the deltas of such great rivers as the Niger or even the Mississippi, and implies the existence of a continent much more extensive and more uniform in drainage than Europe as it at present exists. Lyell even speculates on the possible existence of an Atlantic continent west of Europe. America also at this time had, as already stated, attained to even more than its present extension eastwards. Thus this later Jurassic period was the culmination of the Mesozoic, the period of its most perfect continental development, corresponding in this to the Carboniferous in the Palæozoic.
The next or closing period of this great Mesozoic time brought a wondrous change. In the Cretaceous period, so called from the vast deposits of chalk by which it is characterized, the continents sunk as they had never sunk before, so that vast spaces of the great continental plateaus were brought down, for the first time since the Laurentian, to the condition of abyssal depths, tenanted by such creatures as live in the deepest recesses of our modern oceans. This great depression affected Europe more severely than America; the depression of the latter being not only less, but somewhat later in date. In Europe, at the period of greatest submergence, the hills of Scandinavia and of Britain, and the Urals, perhaps alone stood out of the sea. The Alps and their related mountains, and even the Himalayas, were not yet born, for they have on their high summits deep-sea beds of the Cretaceous and even of later date. In America, the Appalachians and the old Laurentian ranges remained above water; but the Rocky Mountains and the Andes were in great part submerged, and a great Cretaceous sea extended from the Appalachians westward to the Pacific, and southward to the Gulf of Mexico, opening probably to the North into the Arctic Ocean.