coal-seams occur, which have been worked to some extent. On the continent, this division is characteristically marine; it reaches its greatest development in Provence, where it is 950 feet thick. It runs through the Jura Mountains, where it is made up of more than 300 feet of strata, chiefly limestone. In Germany the strata from the top of the Lias to the base of the Callovian group-that is, the two stages of Bajocian and Bathonian-are classed together as the Middle or Brown Jura, or Dogger.

The Bathonian stage is named from Bath, where its subdivisions are admirably exposed. At its base is a local argillaceous band known as Fuller's Earth, because long used for fulling cloth. The chief member of the stage in the south-west of England is the Great or Bath Oolite, a succession of limestones, often oolitic, with clays and sands. The Stonesfield Slate is the name locally given to some thin-bedded limestones and sands forming the lower part of the Great Oolite, and of high geological interest from having supplied among their fossils remains of land-plants, numerous insects, bones of enaliosaurs and deinosaurs, and of small marsupials. The Great Oolite abounds in corals, and contains numerous genera of mollusca, fishes, and reptiles. The Cornbrash (so named from its friable (brashy) character, and forming good soil for corn) is one of the most persistent bands in the English Jurassic system, retaining its characters all the way from the south-western counties to near the Humber.

The Oxfordian stage, sometimes called the Middle or Oxford Oolite, consists of a lower zone of calcareous sandstone known as the Kellaways rock or Callovian, from the name of a place in Wiltshire, and of a thick upper stiff blue and brown clay, called, from the locality where it is well developed, the Oxford Clay, and containing numerous ammonites, belemnites, and oysters, but no corals. In Germany, the strata from the base of the Callovian to the top of the Purbeckian group are known as the Malm or White Jura.

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.

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.

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.

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 sub-stages: the lowest consisting of fresh-water limestones, with layers of ancient soil (" dirtbeds "), 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 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 XXIII.

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 across 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 extend 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 basin was filled by a broad and long gulf or inlet, the southern margin of which seems to have been defined by the ridge of old rocks that run 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 prox