rhinoceros, and crested with horns, -such a creature must have been the Iguanadon! Nor were the inhabitants of the waters much less wonderful; witness the Plesiosaurus, which only required wings to be a flying dragon." The name of Mantell will ever be associated with the Wealden beds, as that of Cuvier is with the Paris basin, Murchison with Silurian strata, and Lyell with the Tertiary deposits.

CHAPTER IX.

CRETACEOUS SYSTEM.

HE scheme of deposits included in this system, ranging from a thousand to twelve hundred feet in thickness, receives its name from the common mineral, chalk (creta), the most prominent member of the group. This is one of the best known and clearly defined formations of the globe, resting upon wealden strata in Kent and Sussex, upon oolitic rocks in other places, and, where these are wanting, upon the lias. In England, the cretaceous system extends over a considerable part of the south-eastern and eastern counties, overlooks the waters of the Channel at Brighton, and the German Ocean at Flamborough Head in Yorkshire, though it is not continuous through the interval between these two points. From the latter headland, it travels inland in a large curve to the Humber, reappears on the Lincolnshire side of the river, and proceeds in a straight broad South of the estuary of the Wash, it ocband to Spilsby, where its course is interrupted.

curs in a detached curved tract in Norfolk. At Thetford, in that county, the chalk becomes continuous, from thence ranging southward through the counties of Cambridge, Bedford, Oxford, and Wilts, to the sea in Dorset, and, proceeding eastward from Wiltshire through the south-eastern counties, it extends to the coast of Kent at Dover. The cretaceous system makes no appearance in Wales or Scotland, but it occurs in the north of Ireland. On the continent, it is largely developed in France, bordering the Channel from Boulogne to Havre, and surrounding in a broad ring the tertiary basin of Paris. The members of the formation appear in various parts of Germany, in the Alps, the Carpathians, and Pyrenees. In North America, it is supposed to occur abundantly on the Missouri to the foot of the Rocky Mountains. But in Ireland, the chalk occurs under circumstances which nowhere belong to it in England, for there, in the county of Antrim, it has been broken through and overlain, by that magnificent outburst of basalt which built up the Giants' Causeway. Here, the metamorphic change induced by the action of intense heat is manifest, in the conversion of the chalk into a crystalline marble, while the lias has been changed into a kind of flinty slate, and the coal shales and red sandstone have been variously hardened. In some foreign localities cretaceous strata have likewise been

invaded by igneous rocks, at Weinbohla on the Danube, and in the Pyrenees, by granitic varieties.

The members of the cretaceous system, and the order of succession in the strata, are as follows:

Chalk.

Gault.

Green sand.

Upper chalk. -a white and soft mass, with chert nodules at regular intervals, and layers of

flints.

Lower chalk-a harder and variously tinted mass, either without or with fewer flints.
Chalk marl-highly calcareous, bluish, laminated beds of clay.

Upper green sand-a mass of sands, occasionally indurated to chalky or cherty sandstone,
of green, grey, or white colour, with nodules of chert.

Soft bluish marly clay, with green grains, Tetsworth clay, Folkestone clay.
Lower green sand-a considerable mass of green or ferruginous sands, with layers of
chert, local beds of gault, rocks of cherty or chalky limestone, and deposits of ochre and
fuller's earth.

This is a detail of the system as it occurs in the south of England. In other cretaceous localities some of the preceding beds are wanting. The north of England has no upper green sand, Yorkshire none of the lower, which is largely developed in Lincolnshire. The Carpathian Mountains have no chalk, but green sand in abundance; and this is the case also with the Alps.

Green sand. This deposit, the base of the cretaceous system, divided into upper and lower beds, separated by a seam of soft blue clay, derives its name from its arenaceous composition and green hues, though yellow tints are frequently exhibited. The sand presents various degrees of fineness, and is employed as a manure in some parts of the United States with considerable success. The colouring matter has been analysed with much care by several distinguished chemists, with the following results :

If the fertilising power of the sand depended alone upon the potassa, the English and Massachusetts deposits would obviously be of no value, but to its virtue as a manure the oxide of iron and the other ingredients may contribute. The green sand contains numerous fossils, zoophytes, sponges, echinites, and shells, which appear in the terrain néocomien of the French geologists, a deposit appertaining to the same formation. It has thus been styled from the site where it is largely developed, in the neighbourhood of Neuf

Hippurites, a name derived from that of the common herb hippuris, the mare's tail, the stem of which the shells are supposed to resemble, and Spherulites, shells of a globular form, as the title indicates. In the cretaceous system of England, as it appears in the midland counties, the green sand occupies comparatively low tracts; but in Wilts it constitutes a secondary range of hills, presents high insulated masses on the confines of Dorset, forms the summit of the extensive table land of Blackdown in Devon, and appears at the elevation of nearly a thousand feet in Leith Hill in Surrey, thus rivalling in height the chalk hills of the North and South Downs. The formation is variously a loose sand, and formed into sandstone by a calcareous cement.

Gault. This is a provincial name for beds of chalky clay overlying the green sand, varying in colour from a light grey to a dark blue, holding irregular balls of argillaceous ironstone and iron pyrites. The craie tufeau of the continent corresponds with our English gault, but no where are the strata very persistent in their occurrence, or of much importance. Marine shells are abundant, generally distinguished by their brilliant pearly lustre, of which the engraving shows an example.

Chalk. This mineral, occupying the upper part of the system, so useful in the arts and in agriculture, is a carbonate of lime, composed Terebratula digona. of nearly 44 parts of carbonic acid, and 56 of lime. It seldom, however, occurs pure, but has earthy admixtures. Some of the foreign chalks contain as much as from eight to ten per cent. of carbonate of magnesia, and are characterised by minute black spots, like gunpowder grains, imbedded in them. The two divisions of the chalk formation, the upper and lower, differ to some extent in their character. The upper is in general friable and soft, so as to be readily scratched with the nail; the lower is harder, and is sometimes used as a building-stone. The colour of the upper chalk is often a clear snowy white, while that of the lower passes into a dusky grey, and even a red. The upper beds contain numerous nodules of siliceous or flinty matter, arranged in layers, which are either entirely absent, or not so common in the lower. In general, the appearance of stratification in the chalk is not distinct, arising from the soft yielding nature of the mass, admitting of the beds passing indiscriminately into each other. The red colour of the lower part of the Yorkshire chalk is clearly seen in the cliffs of the coast between Flamborough and Filey; and this hue characterises the scaglia in the neighbourhood of Genoa, considered to be a mode of chalk.

Chalk with numerous flints, that with few flints, and that without them, may be seen well exposed in the range of rocks reaching from Shakespeare's Cliff, near Dover, along the coast to the South Foreland, and round that point to St. Margaret's Bay. The occurrence of these siliceous aggregations, so different from the surrounding mass, is

the most remarkable feature of the deposit. They exhibit every variety of shape, and differ in magnitude from the circumference of an inch to that of a yard. They are commonly separated from each other, being entirely enclosed by the chalk, and occur in layers horizontally disposed in the rock; but in one instance a continuous stratum of flint rises from the beach near St. Margaret's Bay, about an inch and a half thick, and may be readily traced from thence for a distance of two miles. But this is an exception to a rule which very generally obtains. The flints in chalk are almost universally detached nodules, with rugged edges, arranged in horizontal strata. These strata are separated from each other by varying distances, and differ also in their own breadth. Upon examination, the flints are frequently found to be siliceous accumulations around organic substances, parts of shells, sponges, and other marine forms, into the most minute pores of which the siliceous matter has penetrated. In many cases the organic structure has been admirably preserved; but where it has perished, its place has either been left vacant, or filled up with a sparry incrustation. Mr. Brande makes a remark, with a view to throw light upon the enigmatical appearance of the flints, that if finely powdered silica be mixed with other earthy bodies, and the whole diffused through water, the grains of silica have, under certain circumstances, a tendency to aggregate into small nodules. Now grains of quartz are discoverable in chalk, and some conceive it to be a probable explanation of the case, that silex, which occurs in a state of solution in the hot springs of Iceland, was largely present in the waters which deposited the upper chalk, and was aggregated by the elective affinity of its own particles, forming, under control of some peculiar attraction, upon the now enclosed organic substances as so many central nuclei. We may be tempted to inquire respecting the prevalence of silica in the seas of the period; but such inquiries are endless, and would be equally pertinent and unanswerable, concerning the material of the chalk and the magnesia so largely prevalent during the deposition of the magnesian limestone.

Large masses of limestone are evidently owing to the organic efforts of secreting animals, being entirely composed of comminuted shells and corallines; and a similar organic origin is now generally ascribed to the chalk, for, upon being submitted to microscopical examination, its particles exhibit those appearances of structure which identify them either as excessively minute shells and corals, or portions of them. But if we admit the vital origin of chalk, and of all the enormous masses of carbonate of lime which compose nearly one-eighth part of the superficial crust of the globe, this is obviously no solution of the problem, for we have every reason to suppose that the inhabitants of the ocean secreted the material of their structures from the waters of the deep. The question, therefore, remains to be answered, whence the sea obtained its immense amount of calcareous matter, which appears to have more remarkably characterised it at certain eras than at others. The chief difficulty in the case is, as Dr. Buckland states, that it could not have resulted, like sand and clay, from the mechanical detritus of rocks of the granitic series, because the quantity of lime these rocks contain bears no proportion to its large amount among the derivative strata. Perhaps the true theory may be, that, as lime appears to some extent in lava, basalt, and various kinds of trap-rocks, the ocean obtained its carbonate of lime from springs charged with carbonic acid gas percolating through those igneous masses, and received, at some periods, a preponderating supply from the prevalence of igneous discharges.

Chalk, with an abundance of flints, presents itself in the cliffs of this formation in the north, and there, on the Yorkshire coast, exposed to the full violence of the German Ocean, the rocks bear witness to the excavating power of the waves, the abrading influence of the stormy wind and rain, and their own comparative fragility. The general contour of the chalk developed in this district has been noticed, that of a

curve or bow, having one end at Flamborough Head, and the other on the Humber, the middle part bending inland. The breadth of the extremities of the bow, both at the ocean and the river, is comparatively small; but that of the central region amounts to about 15 miles, and the length from end to end to about sixty, without including the sinuosities of the outline. The chalk first appears on the coast, above the level of the sea, a little to the north of Bridlington Quay. From thence it gradually rises towards Flamborough and Speeton, forming the cliffs, which rise precipitously 300 feet, but reaching a greater elevation away from the shore. Proceeding westward towards the wolds, the chalk becomes still more lofty, and attains its highest elevation in Wilton beacon, ascertained to be 809 feet above the level of the sea. From thence, towards the Humber, the hills diminish; yet at Hunsley beacon, six miles from the river, the elevation is still 531 feet. The most striking part of the district is the Flamborough headland, and its neighbourhood. The chalk here shows a tendency to split in a perpendicular direction, and the cliffs in consequence exhibit in various places somewhat of a columnar aspect. The rough waves of the ocean beating into the lower part of the perpendicular fissures have hollowed out niches, grottoes, and large caves, some of them highly romantic, harder or more protected masses of chalk remaining as pillars to support the undermined rock. In the loftier Speeton cliffs, a few miles further on the coast, an analogous effect appears at their summit, produced by the beating of the rains, to that caused by the dashing of the waves at the base of those at Flamborough. Deep sinuosities and breaks have been worn by the rains in the upper part of these rocks, the firmer chalk remaining between them, presenting to a spectator from the beach the appearance of the walls of some stupendous castle, surmounted by a range of pinnacles. A less boisterous sea, and a sloping mound, keep the base comparatively uninjured. The Speeton cliffs are interesting, for here the lower coloured chalk is exposed beneath the upper, the former exhibiting a brick-red or chocolate colour, with green, blue, and grey tinges, which strikingly contrast with the milk-white aspect of the latter, brilliant when lighted up by the beams of the rising or the setting sun.

The fossils of the chalk, and of the cretaceous system in general, are eminently the remains of oceanic life, very few examples of land organisms occurring in any of its formations. Plants are rare, and are nearly all referable to marine types, fuci, and other sea-weeds, a circumstance which is understood as indicating that the sea was very little disturbed by inundations from the land during the deposition of the strata, otherwise the remains of ferns and other forms of terrestrial vegetation would have been im

bedded. Among the spoils of the ocean we are presented with sponges, zoophytes, star-fishes, many shells, and echinites. Of the latter, a most abundant family, a representation of the most common species, Spatangus cor-anguinum, or the heart-shaped echinite, is given in the annexed cut. Remains of vertebrated fishes have been exhumed by Dr. Mantell, but no mammalia have been discovered. "There appears," says Professor Phillips, "no sufficient evidence in the fossils of this system to justify any positive inference as to the character of the climate then prevailing in the northern zones."

An addition to the cretaceous system, its uppermost member, entirely wanting in the British series, occurs on the continent, remarkable for bringing to light a marine reptile, the remains of which have since been found in the chalk of England. This is a friable, shelly, and sandy limestone, containing layers of flints, of which St. Peter's Mountain, in the neighbourhood of Maestricht, is composed. Here the celebrated Hoffman, in the