Lower.

Neocomian

In Southern England a fluviatile (partly marine succession of sands and clays (Wealden), surmounted by sands, clays, and limestones (Lower Greensand); in Northern England a series of clays and limestones, with marine fossils (upper part of Speeton clay); limestones and marls of Neuchâtel; compact crystalline limestones in Provence.

It will be remembered that towards the close of the Jurassic period the floor of the sea in the western part of the European area was gently raised, some of the younger Jurassic marine limestones being ridged up into islets or low land, with lakes or estuaries in which the Purbeck beds were deposited. This terrestrial condition of the geography was maintained and extended in the same region during the early part of the Cretaceous period. The geological history of Europe as revealed by the various subdivisions in the foregoing Table may be briefly given.

Neocomian (from Neocomum, the old Latin name of Neuchâtel in Switzerland). This stage in the south of England and thence eastwards across Hanover consists of a mass of sand and clay sometimes 1800 feet thick, representing the delta of a river. Only a portion of this delta remains, but as it extends in an east and west direction for at least 200, and from north to south for perhaps 100 miles, its total area may have been 20,000 square miles, indicating a large river comparable with the Quorra of the present day. This stream not improbably descended from the north or north-west. It carried down the drifted vegetation of the land with occasional carcases of the iguanodons and other terrestrial or amphibious creatures of the time. From their great development in the Weald of Sussex these delta-deposits have been called Wealden. Beyond the area overspread by the sand and mud of the delta, the ordinary marine sediments accumulated, with their characteristic organic remains. We find these sediments in Yorkshire (upper part of Speeton clay), which must then have lain beyond the estuary of the river. They stretch thence eastwards through North-Western Germany, and are found at the base of the Cretaceous system through France into Switzerland. The Lower Greensand which overlies the Wealden group in the south of England contains marine fossils, and points to the submergence of the delta.

Albian (from the department of the Aube in France). In England this stage nearly corresponds to the band of dark, stiff, blue clay known as the Gault. Extending over the Wealden sands and clays the Gault (100 to 200 feet or more in thickness), with its abundant marine fossils, shows how thoroughly the Wealden delta was now submerged beneath the sea.

Cenomanian (from Coenomanum, the old Latin name of the town of Mans in the department of Sarthe, France). This stage comprises a group of impure chalky, glauconitic, and sandy deposits lying at the base of the Chalk in England and the north of France. Certain sandy portions of this group have been called the Upper Greensand. The Glauconitic (or Chloritic) Marl is an impure, dull white, or yellowish chalk, with grains of glauconite and phosphatic nodules. The Chalk-Marl is an impure band of chalk sometimes overlain by a zone of Grey Chalk which forms the base of the true Chalk-without-flints. these deposits indicate the accumulations of a shallow sea, probably not far from land. Traced eastwards into Germany, they undergo great changes in their lithological characters, passing at last in Saxony and Bohemia into sandstones and clays full of remains of terrestrial vegetation,

All

and even including some thin seams of coal. It is in these beds that the oldest dicotyledonous plants in Europe have been found. It is evident that land existed in the heart of Germany during this stage of the Cretaceous period. In Southern France, on the other hand, the corresponding strata are massive hippurite-limestones which sweep through the great Mediterranean basin, and show how large an area of Southern Europe then lay under the sea.

Turonian (from Touraine). This stage includes the lower part of the Chalk, above the Grey Chalk. The thick mass of white crumbly limestone known as the Chalk has been referred to as the most conspicuous member of the Cretaceous system in the west of Europe. It has long been grouped into two parts, a lower band of "Chalkwithout-flints," and an upper band of "Chalk-with-flints." The former corresponds, on the whole, with the Turonian stage. The Chalk is a remarkably pure limestone composed chiefly of crumbled foraminifera, urchins, molluscs, and other marine organisms. It must have been laid down in a sea singularly free from fine sediment; but there is no evidence that this sea was one of great depth. On the contrary, though the Chalk itself resembles the Globigerina ooze of the deeper parts of the Atlantic Ocean, the characters of its foraminifera and other organic remains indicate comparatively shallow-water conditions. The basin in which it was laid down shallowed eastwards, where, from the evidence of sandstones, coal-seams, and plants, there was land at the time; while, probably, towards the west there was connection with the open sea. The total thickness of the Chalk, including the Cenomanian, Turonian, and Senonian stages, exceeds 1200 feet.

Senonian (from Sens, in the department of Yonne).

This stage corresponds generally with the original English subdivision of Upper Chalk, or Chalk-with-flints. Its most conspicuous feature is the presence of the layers of nodules or irregular lumps of black flint which mark the stratification of the Chalk. The origin of these concretions has been the subject of much discussion among geologists, and it cannot be said to have been even yet satisfactorily solved. Some marine plants (diatoms) and animals (radiolarians, sponges, etc.) secrete silica from sea-water, and build it up into their framework. But the flints are not mere siliceous organisms, though organic remains may often be observed enclosed within them. They are amorphous lumps of dark silica, containing a little organic matter. By some process, not yet well understood, these aggregations of silica have gathered usually round organic nuclei, such as sponges, urchins, shells, etc. The decomposition of organic matter on the sea-floor may have been the principal cause in determining the abstraction and deposition of silica. Not infrequently an organism, such as a brachiopod or echinus, originally composed of carbonate of lime, has been completely transformed into flint.

The Chalk is well exposed along the sea-cliffs of the east and south of England. It forms the promontories of Flamborough Head, Dover, Beachy Head, and the Needles in the Isle of Wight. The white cliffs of Kent are repeated on the opposite coast of France, where the Chalk with all its lithological and palæontological characters reappears, and whence it extends through Northern France into Belgium.

Danian (from Denmark). This stage has not been recognised in England. Its component chalky strata occur in scattered patches over Northern France, Belgium, and Denmark, to the south of Sweden.

The Cretaceous hippurite-limestones of Southern Europe and the basin of the Mediterranean are prolonged through Asia Minor into Persia, where they cover a vast area. They have been found likewise on the flanks of the Himalaya Mountains, so that the open Cretaceous sea must have stretched right across the heart of the Old World. In the Indian Deccan a great extent of country, estimated at 200,000 square miles, lies buried under horizontal or nearly horizontal sheets of lava, which have a united thickness of from 4000 to 5000 feet, and were erupted during the Cretaceous period. These eruptions, from the presence of interstratified layers containing remains of fresh-water shells, land-plants, and insects, are believed to have taken place on land and not under the sea.

Cretaceous rocks cover an enormous area in North America. They attain no great thickness in the Eastern States, but they thicken southwards, until in Texas they present massive limestones indicative of deeper and clearer water than elsewhere in this region. They attain gigantic proportions in Colorado, Utah, and Wyoming, whence they are prolonged northwards into the British territories. With a maximum thickness of 11,000 to 13,000 feet, they have yielded a remarkably abundant and varied series of organic remains. In their upper parts (Laramie group) they contain a large assemblage of land-plants, half of which were allied to still living American trees, and in some places these plants are aggregated into valuable seams of coal. The numerous reptilian and bird remains found in these strata have been already noticed.

Rocks assigned to the Cretaceous system cover a wide region of Queensland, and also attain a considerable thickness in New Zealand.