of which the stems, fruits, and leaves are found in abundance; the shells of the gryphaa, so peculiarly plentiful in the lias; the coiled-up ammonites of innumerable species; the pterodactyle, or flying lizard; the fresh-water and marine turtles; and above all, the ichthyosaurus, plesiosaurus, and other sauroid reptiles, whose marvellous forms and variety have suggested for the oolite the not inappropriate title of "the age of reptiles." Still higher in. the scale of being than these are the remains of birds fitted for aerial flight, archeopteryx, and the marsupial mammals, amphitherium, phascolotherium, spalacotherium, plagiaulax, &c., and the small-hoofed mammal, stereognathus, described and figured by Mr Charlesworth and Professor Owen. Building-stone, pavingstone, limestone, marble, alum, coal, ironstone, jet, and lithographic slate, are the principal economic products of the system.

XIV.

THE CHALK OR CRETACEOUS SYSTEM, COMPRISING THE

GREENSAND AND CHALK GROUPS.

126. IMMEDIATELY above the fresh-water beds of the Wealden in the south of England occur a set of well-defined marine sands, dark marl-clays, and thick beds of chalk. These strata, which seldom exceed in the aggregate 1000 or 1500 feet in thickness, constitute the Cretaceous system-chalk (L. creta) being the most prominent and remarkable feature in the formation. Though neither of great thickness nor widely developed as to area, the Chalk is in many respects one of the most remarkable rock-systems in the British Islands, and has consequently long attracted the research of geologists. As the uppermost member of the younger secondaries, it closes the record of Mesozoic life; and of the innumerable species which composed the flora and fauna of the secondary epochs, comparatively few have been detected in tertiary or post-tertiary strata. Lithologically, it is composed of cretaceous, argillaceous, and arenaceous rocks—the first predominating in the upper, and the two last in the lower portion of the system. The strata, as occurring in the south of England, are usually grouped as follows: UPPER CHALK.-Generally soft white chalk, containing numerous flint and chert nodules more or less arranged in layers.

CHALK.

GREENSAND.

LOWER CHALK.-Harder and less white than the upper, and generally with fewer flints. (Reddish in the north of England, and with abundance of flints.)

CHALK MARL.-A greyish earthy or yellowish marly chalk, sometimes indurated.

UPPER GREENSAND.-Beds of silicious sand, occasionally indurated to chalky or cherty sandstone, of a green or greyish white, with nodules of chert.

GAULT.-A provincial name for a bluish tenacious clay, sometimes marly, with indurated argillaceous concretions and layers of greensand.

LOWER GREENSAND.-Beds of green or ferruginous sands, with layers of chert and indurated sandstones, local beds of gault, rocks of chalky or cherty limestone (Kentish rag), and fuller's earth.

127. The preceding synopsis affords a sufficient outline of the composition and succession of the cretaceous system. Of course, considerable local differences occur, and it is sometimes difficult to determine the equivalents of the beds as typically developed in Kent and adjoining counties. Thus, the lower chalk of Yorkshire and of Havre in France contains abundant flint nodules; in Devon and Dorset a gritty bed with numerous fossils occurs towards the base of the chalk; in Lincoln and York a stratum of red chalk is thought to represent the gault of the southern counties; and the Kentish ragstone, which is largely quarried near Maidstone, is wholly unrepresented in the Isle of Wight. When we come to compare the Continental strata with those of England, still wider differences prevail; and in North America the rocks which are charged with cretaceous fossils are often mere sands and clays, sometimes even shingly, and only in certain districts associated with the beds of yellow coralline and silicious limestones. The lower greensand is sometimes termed the "Neocomian group (Neocomiensis, rock of Neufchâtel), this portion of the system being thought to be more typically developed in the neighbourhood of Neufchâtel in Switzerland; but recent facts scarcely support this view, and for all practical purposes the terms Chalk, Gault, and Greensand are sufficiently distinctive.

1, Siphonia; 2, Ventriculites; 3. Manon: 4, Scyphia; 5, Textularia; 6, Lituola;
7, Orbitoides; 8, Rotalia.

128. The organic remains found in the cretaceous system are, with a few exceptions, eminently marine, comprising fucoids,

sponges, corals, star-fishes, molluscs, crustacea, fishes, and reptiles. As might be expected, FOSSIL PLANTS are comparatively rare in the British chalk-rocks, and these for the most part drifted and imperfect fragments; but among the cretaceous beds of other regions, seams of lignite and coal are by no means unfrequent. The marine species are apparently allied to the algæ, confervæ, &c., and are termed chondrites and confervites. The terrestrial types are fragments of tree-ferns, cones of coniferous trees, cycadites and zamites, and are known by such names as pinites, strobilites (strobilus, a fir-cone), carpolithes (carpos, a fruit), and zamiostrobus. Of the ANIMAL remains, which are in general beautifully preserved and to be seen in almost every collection, we can only notice one or two examples under each order or family. Of spongiform bodies we have the common and characteristic choanites, spongia, scyphia, and ventriculites. Of zoophytes and polyzoa there are numerous astrea, alveolites, and orbitolites, flustra and retepora. Of echinoderms or sea-urchins there are many species in every state of perfection, as cidaris, galerites,

1, Marsupites; 2 Goniaster; 3, Hemipneustes; 4, Ananchytes; 5, Galerites.

spatangus, and micraster. Of foraminiferous shells, which compose in great part the chalk strata, there are rotalia, dentalina, textularia, &c. Of annelids, abundant serpularia and vermicularia; and of crustaceans, species of lobsters, astacus, and of crabs, dagurus. The remains of testacea or shell-fish are extremely

numerous, and in such a state of perfection that the conchologist can at once assign them a place in his classification. Of the char

1, Pecten; 2, Terebratula; 3, Gervillia; 4, Ostrea; 5, Plagiostoma; 6, Inoceramus 7, Radiolites; 8, Hippurites; 9, Cinulia.

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