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Rh distinctive assemblage of species. By these means the Cretaceous rocks of the world have now been correlated zone with zone, with a degree of exactitude proportional to the palaeontological information gained in the several areas of occurrence.

The Cretaceous system falls naturally into two divisions, an upper and a lower, in all but a few limited regions. In the table on page 288 the names of the principal stages are enumerated; these are capable of world-wide application. The sub-stages are of more local value, and too much importance must not be attached to them for the correlation of distant deposits. The general table is designed to show the relative position in the system of some of the more important and better-known formations; but it must be remembered that the Cretaceous rocks of Europe can now be classified in considerable detail by their fossils, the most accurate group for this purpose being the cephalopods. The smaller table was compiled by T. C. Chamberlin and R. D. Salisbury to show the main subdivisions of the North American Cretaceous rocks. The correlation of the minor subdivisions of Europe and America are only approximate.

Relation of the Cretaceous Strata to the Systems above and below.—In central and northern Europe the boundary between the Cretaceous and Tertiary strata is sharply defined by a fairly general unconformity, except in the Danian and Montian beds, where there is a certain commingling of Tertiary with Cretaceous fossils. The relations with the underlying Jurassic rocks are not so clearly defined, partly because the earliest Cretaceous rocks are obscured by too great a thickness of younger strata, and partly because the lowest observable rocks of the system are not the oldest, but are higher members of the system that have overlapped on to much older rocks. However, in the south of England, in the Alpine area, and in part of N.W. Germany the passage from Jurassic to Cretaceous is so gradual that there is some divergence of opinion as to the best position for the line of separation. In the Alpine region this passage is formed by marine beds, in the other two by brackish-water deposits. In a like manner the Potomac beds of N. America grade downwards into the Jurassic; while in the Laramie formation an upward passage is observed into the Eocene deposits. There is a very general unconformity and break between the Lower and Upper Cretaceous; this has led Chamberlin and Salisbury to suggest that the Lower Cretaceous should be regarded as a separate period with the title “Comanchean.”

Physiographical Conditions and Types of Deposit.—With the opening of the Cretaceous in Europe there commenced a period of marine transgression; in the central and western European region this took place from the S. towards the N., slow at first and local in effect, but becoming more decided at the beginning of the upper division. During the earlier portion of the period, S. England, Belgium and Hanover were covered by a great series of estuarine sands and clays, termed the Wealden formation (q.v.), the delta of a large river or rivers flowing probably from the N.W. Meanwhile, in the rest of Europe alternations of marine and estuarine deposits were being laid down; but over the Alpine region lay the open sea, where there flourished coral reefs and great banks of clam-like molluscs. The sea gradually encroached upon the estuarine Wealden area, and at the time of the Aptian deposits uniform marine conditions prevailed from western Europe through Russia into Asia. This extension of the sea is illustrated in England by the overlap of the Gault over the Lower Greens and on to the older rocks, and by similar occurrences in N. France and Germany.

Almost throughout the Upper Cretaceous period the marine invasion continued, varied here and there by slight movements in the opposite sense which did not, however, interfere with the quiet general advance of the sea. This marine extension made itself felt over the old central plateau of France, the N. of Great Britain, the Spanish peninsula, the Armorican peninsula, and also in the Bavarian Jura and Bohemia; it affected the northern part of Africa and East Africa; in N. America the sea spread over the entire length of the Rocky Mountain region; and in Brazil, eastern Asia and western Australia, Upper Cretaceous deposits are found resting directly upon much older rocks. Indeed, at this time there happened one of the greatest changes in the distribution of land and water that have been recorded in geological history.

We have seen that in early Cretaceous times marine limestones were being formed in southern Europe, while estuarine sands and muds were being laid down in the Anglo-German delta, and that beds of intermediate character were being made in parts of N. France and Germany. During later Cretaceous times this striking difference between the northern and southern facies was maintained, notwithstanding the fact that the later deposits were of marine origin in both regions. In the northern region the gradual deepening and accompanying extension of the sea caused the sandy deposits to become finer grained in N.W. Europe. The sandy beds and clays then gave way to marly deposits, and in these early stages glauconitic grains are very characteristically present both in the sand and in the marls. In their turn these marly deposits in the Anglo-Parisian basin were succeeded gradually and somewhat intermittently by the purer, soft limestone of the chalk sea, and by limestones, similar in character, in N. France, extra-Alpine Germany, S. Scandinavia, Denmark and Russia. Meanwhile, the S. European deposits maintained the characters already indicated; limestones (not chalk) prevailed, except in certain Alpine and Carpathian tracts where detrital sandstones were being laid down.

The great difference between the lithological characters of the northern and southern deposits is accompanied by an equally striking difference between their respective organic contents. In the north, the genera Inoceramus and Belemnitella are particularly abundant. In the south, the remarkable, large, clam-like, aberrant pelecypods, the Hippuritidae, Rudistes, Caprotina, &c., attained an extraordinary development; they form great lenticular banks, like the clam banks of warm seas, or like our modern oyster-beds; they appear in successive species in the different stages of the Cretaceous system of the south, and can be used for marking palaeontological horizons as the cephalopods are used elsewhere. Certain genera of ammonites, Haploceras, Lytoceras, Phylloceras, rare in the north, are common in the south; and the southern facies is further characterized by the peculiar group of swollen belemnites (Dumontia), by the gasteropods Actionella, Nerinea, &c., and by reef-building corals. The southern facies is far more widespread and typical of the period than is the chalk; it not only covers all southern Europe, but spreads eastwards far into Asia and round the Mediterranean basin into Africa. It is found again in Texas, Alabama, Mexico, the West Indies and Colombia; though limestones of the chalk type are found in Texas, New Zealand, and locally in one or two other places. The marine deposits are organically formed limestones, in which foraminifera and large bivalve mollusca play a leading part, marls and sandstones; dolomite and oolitic and pisolitic limestones are also known.