Part 31

The vegetable kingdom of this period forms an introduction to the vegetation of the present time. Placed at the close of the Secondary epoch, this vegetation prepares us for transition, as it were, to the vegetation of the Tertiary epoch, which, as we shall see, has a great affinity with that of our own times.

The landscapes of the ancient world have hitherto shown us some species of plants of forms strange and little known, which are now extinct. But during the period whose history we are tracing, the vegetable kingdom begins to fashion itself in a less mysterious manner; Palms appear, and among the regular species we recognise some which differ little from those of the tropics of our days. The dicotyledons increase slightly in number amid Ferns and Cycads, which have lost much of their importance in numbers and size; we observe an obvious increase in the dicotyledons of our own temperate climate, such as the alder, the wych-elm, the maple, and the walnut, &c.

“As we retire from the times of the primitive creation,” says Lecoq, “and slowly approach those of our own epoch, the sediments seem to withdraw themselves from the polar regions and restrict themselves to the temperate or equatorial zones. The great beds of sand and limestone, which constitute the Cretaceous formation, announce a state of things very different from that of the preceding ages. The seasons are no longer marked by indications of central heat; zones of latitude already show signs of their existence.

“Hitherto two classes of vegetation predominated: the cellular Cryptogams at first, the dicotyledonous Gymnosperms afterwards; and in the epoch which we have reached--the transition epoch of vegetation--the two classes which have reigned heretofore become enfeebled, and a third, the dicotyledonous Angiosperms, timidly take possession of the earth--they consist at first of a small number of species, and occupy only a small part of the soil, of which they afterwards take their full share; and in the succeeding periods, as in our own times, we shall see that their reign is firmly established; during the Cretaceous period, in short, we witness the appearance of the first dicotyledonous Angiosperms. Some arborescent Ferns still maintain their position, and the elegant Protopteris Singeri, Preissl., and P. Buvigneri, Brongn., still unfold their light fronds to the winds of this period. Some Pecopteri, differing from the Wealden species, live along with them. Some Zamites, Cycads, and Zamiostrobi announce that in the Cretaceous period the temperature was still high. New Palms show themselves, and, among others, Flabellaria chamæropifolia is especially remarkable for the majestic crown at its summit.

“The Conifers have endured better than the Cycadeæ; they formed then, as now, great forests, where Damarites, Cunninghamias, Araucarias, Eleoxylons, Abietites, and Pinites remind us of numerous forms still existing, but dispersed all over the earth.

“From this epoch date the Comptonias, attributed to the Myricaceæ; Almites Friesii, Nils., which we consider as one of the Betulaceæ; Carpinites arenaceus, Gœp., which is one of the Cupuliferæ; the Salicites, which are represented to us by the arborescent willows; the Acerinæ would have their Acerites cretaceæ, Nils., and the Juglanditæ, the Juglandites elegans, Gœp. But the most interesting botanical event of this period is the appearance of the Credneria, with its triple-veined leaves, of which no less than eight species have been found and described, but whose place in the systems of classification still remains uncertain. The Crednerias, like the Salicites, were certainly trees, as were most of the species of this remote epoch.”

In the following illustration are represented two of the Palms belonging to the Cretaceous period, restored from the imprints and fragments of the fossil remains left by the trunk and branches in the rocks of the period (Fig. 130.)

But if the vegetation of the Cretaceous period exhibits sensible signs of approximation to that of our present era, we cannot say the same of the animal creation. The time has not yet come when Mammals analogous to those of our epoch gave animation to the forests, plains, and shores of the ancient world; even the Marsupial Mammals, which made their appearance in the Liassic and Oolitic formations, no longer exist, so far as is known, and no others of the class have taken their place. No climbing Opossum, with its young ones, appears among the leaves of the Zamites. The earth appears to be still tenanted by Reptiles, which alone break the solitudes of the woods and the silence of the valleys. The Reptiles, which seem to have swarmed in the seas of the Jurassic period, partook of the crocodilian organisation, and those of this period seem to bear more resemblance to the Lizards of our day. In this period the remains of certain forms indicate that they stood on higher legs; they no longer creep on the earth, and this is apparently the only approximation which seems to connect them more closely with higher forms.

It is not without surprise that we advert to the immense development, the extraordinary dimensions which the Saurian family attained at this epoch. These animals which, in our days, rarely exceed a yard or so in length, attained in the Cretaceous period as much as twenty. The marine lizard, which we notice under the name of Mosasaurus, was then the scourge of the seas, playing the part of the Ichthyosauri of the Jurassic period; for, from the age of the Lias to that of the Chalk, the Ichthyosauri, the Plesiosauri, and the Teleosauri were, judging from their organisation, the tyrants of the waters. They appear to have become extinct at the close of the Cretaceous period, and to give place to the Mosasaurus, to whom fell the formidable task of keeping within proper limits the exuberant production of the various tribes of Fishes and Crustaceans which inhabited the seas. This creature was first discovered in the celebrated rocks of St. Peter’s Mount at Maestricht, on the banks of the Meuse. The skull alone was about four feet in length, while the entire skeleton of Iguanodon Mantelli, discovered by Dr. Mantell in the Wealden strata, has since been met with in the Hastings beds of Tilgate Forest, measuring, as Professor Owen estimates, between fifty and sixty feet in length. These enormous Saurians disappear in their turn, to be replaced in the seas of the Tertiary epoch by the Cetaceans; and henceforth animal life begins to assume, more and more, the appearance it presents in the actually existing creation.

Seeing the great extent of the seas of the Cretaceous period, Fishes were necessarily numerous. The pike, salmon, and dory tribes, analogous to those of our days, lived in the seas of this period; they fled before the sharks and voracious dog-fishes, which now appeared in great numbers, after just showing themselves in the Oolitic period.

The sea was still full of Polyps, Sea-urchins, Crustaceans of various kinds, and many genera of Mollusca different from those of the Jurassic period; alongside of gigantic Lizards are whole piles of animalculæ--those Foraminifera whose remains are scattered in infinite profusion in the Chalk, over an enormous area and of immense thickness. The calcareous remains of these little beings, incalculable in number, have indeed covered, in all probability, a great part of the terrestrial surface. It will give a sufficient idea of the importance of the Cretaceous period in connection with these organisms to state that, in the rocks of the period, 268 genera of animals, hitherto unknown, and more than 5,000 species of special living beings have been found; the thickness of the rocks formed during the period being enormous. Where is the geologist who will venture to estimate the time occupied in creating and destroying the animated masses of which this formation is at once both the cemetery and the monument? For the purposes of description it will be convenient to divide the Cretaceous series into lower and upper, according to their relative ages and their peculiar fossils.

THE LOWER CRETACEOUS PERIOD.

English equivalents. French classification.

Lower Greensand, upper part. Étage Aptien st. Lower Greensand, lower part. „ Néocomien supérieur. Weald clay and Hastings sands. „ Néocomien inférieur.

The Lower Wealden or Hastings Sand consists of sand, sandstone, and calciferous grit, clay, and shale, the argillaceous strata predominating. This part of the Wealden consists, in descending order, of:--

Feet. Tunbridge Wells sand--Sandstone and loam 150 Wadhurst clay--Blue and brown shale and clay, with a little calc grit 100 Ashdown sands--Hard sand, with beds of calc grit 160 Ashburnham sands--Mottled, white, and red clay and sandstone 330

The Hastings sand has a hard bed of white sand in its upper part, whose steep natural cliffs produce the picturesque scenery of the “High rocks” of Hastings in Sussex.

Calcareous sandstone and grit, in which Dr. Mantell found the remains of the Iguanodon and Hylæosaurus, form an upper member of the Tunbridge Wells Sand. The formation extends over Hanover and Westphalia; the Wealden of these countries, according to Dr. Dunker and Von Meyer, corresponding in their fossils and mineral characters with those of the English series. So that “we can scarcely hesitate,” says Lyell, “to refer the whole to one great delta.”

Lyell’s “Elements of Geology,” p. 349.

The overlying Weald clay crops out from beneath the Lower Greensand in various parts of Kent and Sussex, and again in the Isle of Wight, and in the Isle of Purbeck, where it reappears at the base of the chalk.

The upper division (or the Weald clay) is, as we have said, of purely fresh-water origin, and is supposed to have been the estuary of some vast river which, like the African Quorra, may have formed a delta some hundreds of miles broad, as suggested by Dr. Dunker and Von Meyer.

The Lower Greensand is known, also, as the Néocomien, after Neocomium, the Latin name of the city of Neufchatel, in Switzerland, where this formation is largely developed, and where, also, it was first recognised and established as a distinct formation. Dr. Fitton, in his excellent monograph of the Lower Cretaceous formations, gives the following descending succession of rocks as observable in many parts of Kent:--

Feet.

1. Sand, white, yellowish, or brown, with concretions of limestone and chert 70 2. Sand, with green matter 70 to 100 3. Calcareous stone, called Kentish rag 60 to 80

These divisions, which are traceable more or less from the southern part of the Isle of Wight to Hythe in Kent, present considerable variations. At Atherfield, where sixty-three distinct strata, measuring 843 feet, have been noticed, the limestone is wholly wanting, and some fossils range through the whole series, while others are confined to particular divisions; but Prof. E. Forbes states, that when the same conditions are repeated in overlying strata the same species reappear; but that changes of depth, or of the mineral nature of the sea-bottom, the presence or absence of lime or of peroxide of iron, the occurrence of a muddy, sandy, or gravelly bottom, are marked by the absence of certain species, and the predominance of others.

Ibid, p. 340.

Among the marine fauna of the Néocomian series the following are the principal. Among the Acephala, one of the largest and most abundant shells of the lower Néocomian, as displayed in the Atherfield section, is the large Perna Mulleti (Fig. 131).

a, exterior; b, part of the upper hinge.]