acteristic bivalves may be noticed arca, cardium, trigonia, gryphæa, inoceramus, ostrea, pecten, and terebratula, with the curious massive shells hippurites and diceras. Among the univalves or gasteropods, cerithium, rostellaria, dentalium, and natica, are typical and characteristic. The chambered shells also appear in vast profusion, and in highly curious forms, as compared with those of the earlier formation. Of these the coiled-up ammonite, the dart-like belemnite (the "thunderbolts" of the English peasant), the hook-shaped hamite (hamus, a hook), the boat-shaped scaphite (scapha, a skiff), the rod-like baculite (baculus, a staff), and the nautilus, are the most frequent and typical.

129. The vertebrate remains are those of fishes and reptiles, with occasional remains of birds and mammalia. Of the fishes the majority are still placoid and ganoid; but the ctenoid and cycloid orders, to which almost all existing fishes belong, are here

1

2

3

AAA

5

1, Corax pristodontus; 2, Lamna crassidens; 3, Otodus obliquus; 4, Lamna elegans;

5, Notidanus microdon.

for the first time found in the rocky strata.

Of the placoids the

teeth and spines are as usual the only remains; the former being

most abundantly represented by acrodus, ptychodus (wrinkletooth), otodus, corax, lamna, &c., and the latter by chimæra.

Of the ganoids, lepidotus, gyrodus (twisted-tooth), and pycnodus (thick-tooth), are the most typical. Of the ctenoid or comb-scaled order several species of beryx (closely allied to the perch) have been detected; and of the cycloideans the saurocephalus and osmeroides are those most frequently found in collections. The sauroid reptiles seem identical with, or at least closely allied to, those of the wealden, and are represented by pterodactylus, plesiosaurus, mososaurus, iguanodon, and chelonia. Bird-bones, termed cimoliornis (Gr. kimolia, white chalk-marl, and ornis, a bird), have been described by Professor Owen, who has also surmised that certain mammalian remains are those of quadrumana or monkeys. 130. Regarding the geographical distribution of the chalk, though the several areas may be partial or limited, strata containing the peculiar fossils of the system have been discovered in many countries. As already mentioned, it is fully developed in the south and south-east of England; it is found in the north of Ireland; and from the frequent occurrence of flint nodules (as in the Buchan district of Aberdeenshire) it is supposed to have covered at one time the oolites of the north of Scotland. It is spread over wide areas in France and Germany, and occurs in connection with the Alps, Carpathians, and Pyrenees. Chalk fossils have been collected in the south of India; equivalents of the gault and greensands have been investigated in the states of New Jersey, Texas, and Alabama; in the north-western prairies, and Vancouver Island; and strata apparently of the same age have been noticed in Colombia in South America.

131. Though exhibiting faults and fractures, no igneous rocks have been found associated with the chalk of England. In the north of Ireland the strata are indurated, disrupted, and overlaid by basalt and other traps, as remarkably displayed at the Giant's Causeway; and in the Pyrenees and Alps the system partakes more or less of all those upheavals, by traps and secondary granites, which are so characteristic of these lofty ranges. Where unbroken by igneous eruptions, the physical aspect of chalk districts is readily distinguished by the rounded outlines of their hills and valleys, as typically exhibited in the "wolds" and "downs" of Kent and Sussex. These downs are described as 66 covered with a sweet short herbage, forming excellent sheep-pasture, generally bare of trees, and singlarly dry even in the valleys, which for miles wind and receive complicated branches, all descending in a regular slope, yet are frequently left entirely dry; and, what is more singular, contain no channel, and but little other circumstantial proof of the action of water, by which they were certainly excavated."

132. Combining all the features of the system-its composition, fossils, and geographical distribution we are warranted in regarding the chalk as a truly marine deposit, filling up limited sea-areas which were thronged with oceanic life, and which received at intervals the drift of rivers that flowed through countries enjoying a high and genial temperature. The cycas and zamia are plants which betoken a warm climate; and though vegetable drift seldom appears among the chalk strata of Europe in such profusion as to form more than scattered patches of lignite (as in the lower measures near Rochelle), yet must this circumstance be ascribed more to the unfavourable position of the sea of deposit for the reception of such drift, than to the scantiness of vegetation on the dry land. Among the cretaceous strata of the Saskatchewan prairies and Vancouver Island, there occur workable seams of coal-thus showing that the chalk period, like every other, had its areas of exuberant and coal-forming vegetation. Again, the corals and huge sauroid reptiles betoken more of tropical than of temperate conditions, a circumstance that seems further established by the presence of remains apparently allied to the monkeys. Respecting the conditions of the waters in which the chalk, so unlike ordinary limestones, was deposited, and within whose mass flints were subsequently aggregated, geologists are by no means agreed, though the calcareous ooze now forming over so vast an area of the North Atlantic seems to point the way to a satisfactory explanation. This much, however, is certain, that the chalk is a mechanical deposit, from waters loaded with calcareous particles, and abounding in minute foraminiferous shells which constitute a large portion of the mass, and not, as at one time supposed, a precipitate from chemical solution. The abundance of enclosed sponges, corals, shells, and fragments of vegetables, also confirms this view, and compels us to seek for the enclosed layers and nodules of flint an origin similar to that of nodules of ironstone and chert in shale. Flints are composed almost entirely of pure silex, with a trace of iron, clay, and lime; they are usually aggregated round some nucleus of sponge, shell, sea-urchin, or other organism; and there is no difficulty in conceiving the silex to have been originally in solution in the waters of deposit, and subsequently segregated in layers and nodules as we now behold it.

133. Industrially, the chief products of the system in Britain are chalk and flint. Chalk, as an almost pure carbonate of lime, is calcined like ordinary limestones, and employed by the bricklayer, plasterer, cement-maker, and farmer; and levigated, it furnishes the well-known "whiting" of the painter. Flint calcined

and ground is used in the manufacture of china, porcelain, and flint-glass; and before the invention of percussion-caps, was in universal use for gun-flints. In the south of England flints are employed as road-material; and the larger nodules are sometimes taken for the building of walls and fences. Beds of fuller's earth occur in the lower series; and some of the indurated strata, like the "Kentish rag" and Chalk-marl of Cambridgeshire, furnish local supplies of building-stone. From the Gault and Lower Greensand are also obtained the coprolitic nodules, now ground down and used as a manure, on account of their containing a large percentage of phosphate of lime. More recently, several of the workable coal-seams of Vancouver Island and British North America have been discovered to belong to the cretaceous epoch.

RECAPITULATION.

The cretaceous system-so called from the chalk-beds which form its most notable feature-is the last or uppermost of the secondary formations. All its types of life are strictly Mesozoic, and of the numerous species found in the Trias, Oolite, and Chalk, comparatively few have been detected in Tertiary strata. It is an error, however, to suppose that there is any sharp line of demarcation either between Paleozoic and Mesozoic, or Mesozoic and Cainozoic. As one rock-system runs into another system, so does one great Life-period pass into another period, and it is only when viewed as a whole that we can note the differences that exist between their respective plants and animals. Our "groups," and systems," and "periods" are mere provisional expedients; useful as such, but leading to error when invested with any other significance. As typically developed in the south of England, the Cretaceous system has been separated into two groups, the Chalk and Greensand, and these comprise, in descending order, the following members :

66

These various members, both in point of composition and fossil remains, bear evidence of deposit in seas of limited area, and of a climate suitable for the growth of cycadaceae on land, and of corals and gigantic saurians and turtles in the waters. Palæontologically, its remains are eminently marine, and comprise numerous species of sponges, corals, star-fishes, sea-urchins, shell-fish, crustacea, fishes, and reptiles. Remains of birds and mammalia have also been detected, but these are too imperfect and obscure to warrant any definite conclusion as to their character and affinities. The chief, and indeed the only, industrial products of the system in England are chalk, flint, phosphate nodules, and some inferior building-stones; but in some foreign localities lignite, coal, and ironstone occur in available abundance.