amongst the palms. There are 167 species of Angiospermous Dicotyledons, of which about one-half are still represented by living species. The order Proteinæ has three genera-Proteoides, Embothrium, and Aristolochites; and amongst the Laurinæ are Laurus, Persea, Sassafras, Cinnamomum, Oreodaphne; whilst Magnolia and Lyriodendron are amongst the Polycarpiæ. This flora should be carefully noticed, in order not to be deceived by the statement that Dicotyledons of the above-mentioned genera are necessarily of Tertiary age. The flora would at the present time be normal in a climate like that of the South of Europe of from 35° to 40° N. lat. Probably one-half of the Dicotyledons are allied to recent American forms.

The development of Reptilian life in America during the Cretaceous age was extraordinary. There were very few species of Ichthyosaurus in the American area, whilst they were so plentiful in the European Cretaceous strata, and they appear to have been replaced by the Mosasaurs. The order Plesiosauria was well represented, but mainly by species of genera related to Pliosaurus.

The Mosasauria ruled supreme in the American Cretaceous seas, and Marsh says that some were 60 feet long and others 10 or 12 feet in length. They were swimming lizards with four paddles. Crocodilia, some with biconcave and others with procoelian vertebræ, prevailed during the same age. Amongst the Pterosauria was the genus Pteranodon, some species having a spread of wings of from 10 to 25 feet; they replaced the Pterodactyls of Europe. The American forms had no teeth. Chelonians existed, but the most remarkable Reptilia found, were dwellers on the land, of the group Dinosauria. These represented the Iguanodon of the Lower Cretaceous of Europe, and the Dinosaurs noticed by Seeley in the Maestricht chalk. The Upper Cretaceous Dinosaurs of America are a Hadrosaurus of the marine beds and several genera and species, Agathaumas being found amidst the relics of Dicotyledonous leaves in the Lignitic series.

CHAPTER XIX.

LOWER CRETACEOUS OR NEOCOMIAN FORMATION.

The Lower Cretaceous strata-Upper Neocomian-Folkestone and Hythe beds-Atherfield clay-Similarity of conditions favouring reappearance of species after short intervals-Upper Speeton clay-Middle Neocomian -Tealby series-Middle Speeton clay-Lower Neocomian-Lower Speeton clay-Wealden formation-Freshwater character of the WealdenWeald clay-Hastings sand-Punfield beds of Purbeck, DorsetshireFossil shells and fish of the Wealden-Area of the Wealden-Flora of the Wealden of England, Belgium, and Greenland-Tithonian strata.

We now come to the Lower Cretaceous formation, which was formerly called Lower Greensand, and for which it will be useful, for reasons before explained (p. 242), to use the term 'Neocomian.'

UPPER NEOCOMIAN.

Folkestone and Hythe beds. --The sands which crop out beneath the Gault in Wiltshire, Surrey, and Sussex are sometimes in the uppermost part pure white, at others of a yellow and ferruginous colour, and some of the beds contain much green matter. At Hythe they contain layers of calcareous matter and chert, and at Maidstone and other parts of Kent the limestone called Kentish Rag is intercalated. This somewhat clayey and calcareous stone forms strata two feet thick, alternating with quartzose sand. The total thickness of these Folkestone, Sandgate, and Hythe beds is less than 300 feet, and the Hythe beds are seen to rest immediately on a grey clay, to which we shall presently allude as the Atherfield clay. Among the fossils of the Hythe beds we may mention Nautilus plicatus (fig. 277), Ancyloceras (Scaphites) gigas (fig. 278), which has been aptly described as an Ammonite more or less uncoiled; Trigonia caudata (fig. 280), Gervillia anceps (fig. 279), a bivalve genus allied to Avicula, and Terebratula sella (fig. 281). In ferruginous beds of the same age in Wiltshire is found the remarkable shell called Diceras Lonsdalii (fig. 282), which abounds in the Upper and Middle Neocomian of Southern Europe.

Fig. 277.

Atherfield clay.-We mentioned before that the Hythe series rests on a grey clay. This clay is only of slight thickness in Kent and Surrey, but acquires great dimensions at AtherFig. 278.

field, in the Isle of Wight. The difference, indeed, in mineral character and thickness of the Upper Neocomian formation near Folkestone, and the corresponding beds in the south of the Isle

of Wight, about 100 miles distant, is truly remarkable. In the latter place we find no limestone answering to the Kentish Rag, and the entire thickness from the bottom of the Atherfield clay

to the top of the Neocomian, instead of being less than 300 feet as in Kent, is given by the late Professor E. Forbes as 843 feet, which he divides into sixty-three strata, forming three groups. The uppermost of these consists of ferruginous sands; the second of sands and clay; and the third or lowest of a brown clay, abounding in fossils.

Pebbles of quartzose sandstone, jasper, and flinty slate, together with grains of chlorite and mica, occur; and fragments and waterworn fossils of the Oolitic rocks speak plainly, as Mr. Godwin-Austen has shown, of the nature of the pre-existing formations, by the wearing down of which the Neocomian beds were formed. The land, consisting of such rocks, was doubtless submerged before the origin of the White Chalk, a deposit which was formed in a more open and probably deeper sea, and in clearer waters.

Among the shells of the Atherfield clay the most abundant

is the large Perna Mulleti, of which a reduced figure is here given (fig. 283).

Similarity of conditions causing reappearance of species.— Some species of mollusca and other fossils range through the whole Cretaceous series, while others are confined to particular subdivisions, and Edward Forbes laid down a law which has since been found of very general application in regard to estimating the chronological relations of consecutive strata. Whenever similar conditions, he says, are repeated, the same species reappear, provided too great a lapse of time has not intervened; whereas if the length of the interval has been geologically great, the same genera will reappear represented by distinct species. 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, or a sandy, or a gravelly bottom, are marked by the banishment of certain species and the predominance of others. But these differences of conditions, being mineral, chemical, and local in their nature, have

no necessary connection with the extinction, throughout a large area, of certain animals or plants. When the forms proper to loose sand or soft clay, or to perfectly clear water, or to a sea of moderate or great depth, recur with all the same species, we may infer that the interval of time has been, geologically speaking, small, however dense the mass of matter accumulated. If the genera remain the same, and the species are changed, we have entered upon a new period; and no similarity of climate, or of geographical and local conditions, can then recall the old species which a long series of destructive causes in the animate and inanimate world has gradually annihilated. A species or a variety which has become extinct, never reappears.

Speeton clay.-On the coast beneath the White Chalk of Flamborough Head, in Yorkshire, an argillaceous formation crops out, called the Speeton clay. It is several hundred feet in thickness, and its paleontological relations have been ably worked out by Professor Judd,' and he has shown that it is separable into three divisions, the uppermost of which, 150 feet thick, and containing 87 species of mollusca, decidedly belongs to the Atherfield clay and associated strata of Hythe and

Fig. 284.

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Folkestone, already described. It is characterised by the Perna Mulleti (fig. 283) and Terebratula sella (fig. 281), and by Ammonites Deshayesii (fig. 284), a well-known Hythe and Atherfield fossil. Remains of skeletons of the genera Plesiosaurus and Teleosaurus have been obtained from this clay. At the base of this upper division of the Speeton clay there occurs a layer of large Septaria, formerly worked for the manufacture of cement. This bed is crowded with fossils, especially Ammonites, some of which are of great size.

MIDDLE NEOCOMIAN.

Tealby series. -At Tealby, a village in the Lincolnshire Wolds, there are, beneath the White Chalk, some non-fossiliferous ferruginous sands about twenty feet thick, beneath which are beds of clay and limestone about fifty feet thick, with an interesting suite of fossils, among which are Pecten cinctus (fig. 285), from 9 to 12 inches in diameter, Ancyloceras Duvallii (fig. 286), and some 40 other shells, many of them common to the

1 Judd, Speeton Clay, Quart. Geol. Journ. vol. xxiv. p. 218.