and dried again. Where the argillaceous matter predominates much, a tenacious clay marl of the ordinary characters, and bluish-gray colour, is the result. And when the sand is in excess, a finegrained grayish sandstone of loose texture is produced, which forms a connecting link with the green sand below. The more argillaceous form contains 30 per cent. of carbonate of lime; the more cretaceous 82, with 18 silex and alumina (chiefly the former), and a trace of iron. Beds of chert are occasionally found in these strata as at Reigate, and flinty nodules in some parts of Cambridgeshire. The upper cretaceous beds, near the junction with the chalk, contain organic remains of nearly the same description as those of the lower chalk, viz. nautilus, inoceramus, echini, alcyonia, and sponges. The lower and more argillaceous strata are distinguished by a great variety of singular fossils, especially multilocular shells. We have several species of ammonites, nautilus, hamites, scaphites, turrilites, belemnites, dentalium, vermicularia, cerithium, euomphalus, patella, terebratula, arca, nucula, pecten, inoceramus, a variety of spatangus, madrepores, and pentacrinite. Remains of vertebral animals are not common; but of wood, numerous fragments are found with the ligneous fibre. The breadth of this tract is generally from one to two miles; and its thickness from 300 to 400 feet. The strata are conformable in inclination to the superjacent chalk, and therefore commonly horizontal, but in the disturbed ranges of the isle of Wight and Purbeck, they are thrown into an upright position. See Plates IV & V.

THE GREAT CHALK FORMATION.

S XI. THE CHALK FORMATION.

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This stratum forms by its extent and distinctive characters one of the most remarkable mineral features of England. It would seem as if an interval of time had elapsed between the completion of the chalk beds, and the deposit of others upon it; for the surface of the chalk at its boundary with the superincumbent layers, bears marks of having undergone, during that period, a partial destruction after it was consolidated. There is spread over it, a stratum of debris, consisting chiefly of flints washed out of its mass. Moreover, the surface is irregularly worn into numerous cavities, of which many are deep, and filled up with the same debris. At the junction of the chalk with the sand and gravel of the plastic elay formation, deep indentations are observed on its surface, which are sections of long furrows and cavities apparently produced by the action of agitated water on the chalk before it was protected by the covering of clay.

The enormous quantity of chalk-flint pebbles completely rolled and rounded which are found in the plastic clay to the south of London, show that the chalk itself must have been consolidated before that partial wasting of its upper strata by water. To this hydraulic action, MM. Cuvier and Brogniart ascribe the irregular ridges and furrows on the surface of the French chalk, and the Meudon breccia. The immense scale on which this destruction was carried on may be inferred from the vast extent of the English pebble beds. That a long period of time probably intervened between the deposition of the chalk, and of its clay coating, is

rendered probable also by the total difference of the organic remains found in the two strata. By such wasting causes the chalk must have been reduced to much narrower bounds than it occupied at first. On the north of Northamptonshire, on the borders of Rutland and Leicestershire, and in the vale of Shipstone in Warwickshire, accumulations of chalk flints, mixed with rounded fragments of hard chalk, occur in such quantities as to warrant the idea of this formation having once covered these districts, now 50 miles distant from its nearest line. The transition from the chalk to the more recent deposits appears to have been sudden, not gradual. In a few cases indeed a bed of intermediate character, a cretaceous marl, lies between them; which tends to show that where the series of deposits was permitted to proceed quietly, such a gradation might have gone on.

The band of chalk which stretches across the eastern and southern counties of England from Yorkshire to Dorsetshire is to be regarded merely as the western edge of a most extensive tract of this formation. Stretching from the Thames to the Don the chalk occupies the interior area of the great central basin of Europe. This concavity is bounded on the north by the primitive mountain districts of Russian Finland, Sweden, Norway and Scotland; on the west by the transition and primitive chains of Cumberland, Wales, Devonshire and Brittany; on the south, by the primitive mountains branching from the Cevennes in the centre of France, the Alps, with the various grand groups of Germany, as the Black Forest, the Rhingau, and

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the Vosges, the Bohemian, Thuringian, Saxon, Silesian and Carpathian mountains; on the east, by the Ural chain and its branches. The chalk does not rest on the mountains themselves; but within the area which they circumscribe at a certain distance from them, an interior area may be traced, over which the substratum of chalk is believed to extend.

Through the northern coasts of France it occupies an extent exactly corresponding to its line on the southern shores of England. At the north of the Seine, its outer edge, reposing on green sand, turns south, and runs on to Blois, where the deposits above the chalk overlie and conceal its southern extremity. It re-appears at Montargis, and bending again northward, forms a sort of promontory, passing east of Troyes, Rheims, and Valenciennes, having the green sand, oolites and lias cropping out further eastward. At Valenciennes, most of these formations are wanting, and the chalk, with a few beds of green sand (there called Turtia), repose horizontally on the truncated edges of the coal basin, extending along the banks of the Meuse to Liege and Aix. Here the coal is therefore worked beneath the chalk. Hence the chalk may be considered as ranging beneath the sandy and diluvial tracts of North Germany towards Berlin. The whole of this territory is a vast sandy heath, covered with a deep detritus of diluvial gravel, in the midst of which occur enormous rounded blocks of granite, traceable to the opposite shores of the Baltic, exhibiting one of the most sublime phenomena of geology. Nine-tenths of this gravel are

chalk flints; and Mr. Conybeare had the sagacity to discover about half-a-mile from Luneburg, on the way to Hamburg, a chalk pit which had escaped the attention of former observers. It contains the usual alternation of flints, along with good specimens of the inoceramus, echinites, and most of the characteristic chalk fossils. On entering Poland, the chalk lays aside the cloak which had for a space concealed it, and re-appears in a line of hills nearly parallel to the Carpathians. It is finely displayed at Cracow, where it reposes on green sand, and exhibits flints in abundance, with the customary organic remains. Passing thence by Lemberg, it appears to stretch into Russia. Dr. Clarke noticed hills of chalk on the Don. The town of Bielogorod (white city), takes its name from white chalky hills in the neighbourhood. Chalk with its usual flints and fossils, was observed by Engelhardt, even in the Crimea. Chalky cliffs are seen on the northern coast of the Island of Rugen, and on the opposite shore of the Baltic in Pomerania and Mecklenburg. A chalk tract occurs near Malmo in Sweden. The chalk is supposed to traverse Holstein, to the mouth of the Elbe, and thence crosses the German Ocean to Flamborough-head in Yorkshire. Thus is completed its great circuit.

This extensive chalk tract presents a remarkable uniformity of geological character; and proves the general action of the causes which produced mineral deposits. The organic remains also, to the amount of, at least, eight-tenths, are common to all the localities in which chalk has been recognised; as in England, France, the Netherlands, Germany,