5. The Corallian stage, so named from the corals with which it abounds, is one of the most distinctive in the Jurassic system. It is traceable across the greater part of England, over the continent of Europe from Normandy to the Mediterranean, through the east of France, and along the whole length of the Jura Mountains and the flank of the Swabian Alps. While it was being formed, the greater part of Europe lay beneath a shallow sea, the floor of which was clustered over with reefs of coral.

6. The Kimmeridgian group or stage is typically displayed at Kimmeridge on the coast of Dorsetshire, whence its name. It there consists of dark shales, some of which are so highly bituminous as to burn readily, and which will probably be eventually of commercial value as a source for the distillation of mineral oil. This group of strata has yielded a larger number of reptilian genera and species than any other in the Mesozoic systems of Britain plesiosaurs, ichthyosaurs, pterosaurs, deinosaurs, turtles, and crocodiles. It is well developed in France and Germany.

7. The Portlandian stage, so called from the Isle of Portland where it is well seen, consists of a lower set of sandy beds (Portland Sand), and a higher and thicker series of limestones and calcareous freestones, some of the beds containing abundant nodules and layers of flint. These rocks are prolonged into France near Boulogne-sur-Mer.

8. The Purbeckian group or stage is best seen in the Isle of Purbeck, hence its name. It lies on an upraised surface of Portlandian beds, showing that after the deposition of these beds there was some disturbance of the sea-bed, portions of which were uplifted partly into land and partly into shallow brackish and fresh waters. The Purbeck beds are subdivided into three substages the lowest consisting of fresh-water limestones, with layers of ancient soil ("dirt-beds "), in which the stumps of cycadaceous trees still stand in the positions in which they grew (Fig. 171); the middle sub-stage contains oysters and other marine shells which prove that owing to subsidence the area sank under the sea; while in the higher subdivision fresh-water fossils reappear. Among the more interesting organisms yielded by the Purbeck beds are the remains of numerous insects and of the marsupials already referred to, which chiefly occur as lower jaws in a stratum about 5 inches thick. When the bodies of dead animals float out to sea the first bones likely to drop out of the decomposing carcases are the lower jaws; hence the greater frequency of these bones in the fossil state. Strata belonging to the Purbeckian stage and

including red and green marls, with dolomite and gypsum, are found in north-western Germany, showing in that region also the elevation of the floor of the Jurassic sea into detached basins.

In India, a mass of strata 6300 feet thick is found in Cutch, and from its fossils is believed to represent the European Jurassic system from the Bajocian up to the top of the Portlandian stage. In Australia and New Zealand, recognisable Jurassic fossils have also been found, showing the extension of the Jurassic system even to the Antipodes. In North America, Jurassic rocks are not largely developed; but in Colorado they have yielded an abundant series of organic remains, including fishes, tortoises, pterosaurs, deinosaurs, crocodiles, and marsupials.

CHAPTER XXIV

CRETACEOUS

THE CRETACEOUS system received its name in Western Europe, because in England and in Northern France its most conspicuous member is a thick mass of white chalk (Latin, Creta). It covers a far more extensive area of the surface of this continent than any of the preceding systems. Its western extremity reaches to the north of Ireland and the Western Islands of Scotland. It covers a large part of the east and south of England, stretching thence into France, where it forms a broad band, encircling the Tertiary basin of Paris. It sweeps across Belgium into Westphalia, underlies the vast plain of Northern Germany and Denmark, whence it is prolonged into Southern Russia, where it overspreads many thousands of square miles. It flanks most of the principal mountain-chains of Europe-the Pyrenees, Alps, Apennines, and Carpathians. It spreads far and wide over the basin of the Mediterranean Sea, extending across vast tracts of Northern Africa, and from the Adriatic athwart Greece and Turkey into Asia Minor, whence it is prolonged through the Asiatic continent.

As most of the rocks of the system are of marine origin, we at once perceive how entirely different the Cretaceous geography must have been from that of the present day, and to what a great extent the existing land of the Old World lay then below the sea. But in tracing out the distribution of the rocks, geologists have found that the Cretaceous sea did not sweep continuously across Europe. On the contrary, as they have ascertained, the old northern land still rose over the site of Northern Britain and Scandinavia, while to the south of it a wide depression extended across the area of Southern Britain, Northern France, Belgium, and the North German plain, eastwards to Bohemia and Silesia. This vast northern basin was the theatre of a remarkable succession

of geological revolutions. While its eastern portions, during the earlier part of the Cretaceous period, were submerged under the sea, its western tracts were the site of the delta of a great river, probably descending from the land that still lay massed towards the north. During the later ages of the period, the whole of this area formed a broad and long gulf or inlet, the southern margin of which seems to have been defined by the ridge of old rocks that runs from the headlands of Brittany through Central France, the Black Forest, and the high grounds of Bohemia. South of that ridge lay the open ocean which extended all over Southern Europe and the north of Africa, and spread eastwards into Asia.

Bearing in mind this peculiar disposition of sea and land, we can understand why the development of the Cretaceous system, alike in regard to its deposits and its fossils, should be so different in the area of the northern basin from that of the southern regions. In the one case we meet with the local and changing accumulations of a comparatively shallow and somewhat isolated portion of the sea-bed, wherein are mingled abundant traces of the proximity of land. In the other we are presented with evidence of a wide open sea, where the same kinds of deposits and the same forms of marine life extend with little change over vast distances. Obviously, it is not the local type of the northern basin, but the more general and widespread type of Southern Europe that should be taken for the distinctive characteristics of the Cretaceous system. But the northern basin was the first to be systematically explored, and is still the best known, and hence its features have not unnaturally usurped the place of importance which ought properly to be assigned to the other wider area.

Regarding the period as a whole, let us first consider the general character of its distinguishing flora and fauna, and then pass on to trace the history of the period as revealed by the succession of strata. The plants of the Cretaceous system show that the vegetable kingdom had now made a most important advance in organisation. In the lower half of the system the fossil plants yet found are on the whole like those of the Jurassic rocks—that is, they include some of the same genera of ferns, cycads, and conifers which these rocks contained. But already the ancestors of our common trees and flowering plants must have made their appearance, for in the upper half of the system their remains occur in abundance. This earliest dicotyledonous flora numbered among its members species of maple, alder, aralia, poplar, myrica, oak, fig, walnut, beech, plane, sassafras, laurel,

cinnamon, ivy, dogwood, magnolia, gum-tree, ilex, buckthorn, cassia, credneria, and others. The modern aspect of this assemblage of plants is in striking contrast to the more antique look of all the older floras. There were likewise species of pine (Pinus), Californian pine (Sequoia), juniper, and other conifers, various

FIG. 184. Cretaceous Plants. (a) Quercus rinkiana (3); (b) Cinnamomum sezannense (3); (c) Ficus atavina (3); (d) Sassafras recurvata (3); (e) Juglans arctica ().

cycads, forms of screw-pine (Pandanus), palms (Sabal), and numerous ferns (Gleichenia, Asplenium, etc.) This flora spread over the land, surrounding the northern Cretaceous basin, and extended northwards even as far as North Greenland, from which nearly 200 species of Cretaceous plants have been obtained. The inference may be deduced that the climate of the globe must then have been much warmer than at present. The luxuriant vegetation disinterred from the Cretaceous rocks of North Greenland