more especially to the secondary systems as displayed in Western Europe; for it must be observed that in India, Eastern Europe, in Farther Asia, and Southern Africa, as well as in various tracts of North America, the conditions of deposit seem to have been widely different, as there both oolitic and cretaceous coal-fields are by no means uncommon. And these coals, as noticed in a former sketch (Coals and Coal - Formations), must have been formed, like all others, partly from vegetation that grew and accumulated in situ, and partly from drifts and rafts borne down by rivers during seasons of flood and inundation.

There is thus nothing very puzzling in the lithology or mere rock-formations of the secondary ages. In the sandstones of the Trias studded with footprints and rainprints, and reticulated with sun-cracks, we see the sandy deposits of shallow shores; in the shelly limestones, the accumulations of somewhat deeper waters; and in the clayey marls, with their masses of rock-salt and gypsum, the muds of lagoons and sea-creeks alternately submerged and cut off from communication with the outer waters. In the limestones and calcareous clays of the Lias and Oolite, we perceive the coral-growths and organic debris of exterior seas; in their freestones and shell-beds, the drifts of the nearer shore; while in their coals and jets we discover the growths of sea-swamps and deltas, and the drifts of streams and rivers. Again, in the greensands of the Chalk we trace the operations of the more exposed shores ; while in the Chalk itself we perceive the slowly-accumulated calcareous ooze of the quieter waters. For just as in the Atlantic and other sea-beds, the fine calcareous mud resulting from organic debris (the shields of foraminifera, the waste of corals and shells and other exuviæ) is now collecting in great thickness and purity over extensive

areas, so in the older cretaceous ocean similar agencies were at work in accumulating those masses of chalk which now constitute the cliffs and downs of southern England.* There is nothing, we repeat, very difficult of explanation in the mere sediments of the secondary ages, while, as during all other periods, vulcanic eruptions were here and there breaking up their continuity, and occasionally alternating with their strata. In England the secondary rocks have suffered little or no disturbance from igneous agency, and hence their broad and unbroken succession in that country. All to the south-east of a line roughly drawn from the Severn to the Tees is occupied by these formations, and the reader has only to cast his eye over the geological map, to perceive how regularly and continuously they follow each other. In Scotland and Ireland, however, (Skye, Giant's Causeway, &c.), as well as in the Jura and other Continental districts, the secondary rocks are much disturbed and altered by igneous eruptions-a proof that during this, as during all other ages, vulcanicity displayed itself only along certain lines and within certain centres with notable intensity.

As a life-period, the secondary systems, though characterised each by its own peculiar forms, have yet so many features in common, that they may conveniently be regarded as representing one great and unbroken cycle of world-history. As marine deposits they abound in all the lower forms of life-foraminifera, sponges, corals, encrinites, star-fishes, sea-urchins, polyzoa, shell-fish, crustacea, and worms; contain at the same time many orders of insects; are replete with numerous families of fishes and reptiles; and now, for the first time in geological history, give

* For more detailed explanation of this deep-sea calcareous ooze or mud, see page 44, in the sketch entitled "Waste and Reconstruction."

unmistakable evidence of birds and mammalia. Without dwelling on minutiæ, which would be out of place in a popular sketch like the present, we may yet advert to some of the more remarkable features in the life of the secondary ages, such as the great preponderance of nautilus and cuttle-fish like forms among the mollusca; the marvellous variety of reptilian life, which has led to the designation "the age of reptiles;" the first unmistakable appearance of bird-life; and the occurrence of mammals of the lower or pouch-bearing section. There are other noticeable features too, such as the disappearance of the plants peculiar to the coal-measures, and their replacement by other treeferns, by cycads, zamias, pine-trees, and palms; the disappearance of the graptolites, trilobites, eurypterites, and bone-encased fishes that characterised the older systems; the prevalence of homocercal or equal-lobed-tailed fishes unknown in paleozoic times, and similar peculiarities; but these we must subordinate to the more remarkable features above alluded to.

The highest order of molluscan life is the Cephalopod, or those which move about by the arm-like feelers that encircle the head. To this order belong the nautilus and cuttle-fish—the former possessing an external chambered shell, and the latter having no external shell, but supported by an internal bone or osselet. The nautilus is now the only representative of the shelled division; and though a number of genera belong to the shell-less orders, they do not occur in anything like overwhelming numbers. In the secondary seas, however, these cephalopodous mollusca were predominating forms, their shells and internal bones occurring in myriads, and this more especially during the oolitic and cretaceous periods. Everywhere throughout the limestones, shales, and clays, their remains are scattered in profusion-the chambered shells, from the size of the

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smallest coin to the circumference of a carriage-wheel, and the internal bones from the thickness of a quill to the size of a man's arm. There are few things so noticeable in these secondary ages as this exuberance of chambered shells -ammonites, baculites, hamites, scaphites, and turrilites or of these internal bones-belemnites, acanthoteuthis, belemnoteuthis, conoteuthis, and leptoteuthis. Many of the strata are surcharged with their remains; and as the cephalopods are active and predaceous in their habits, the lower life in which they fed must have been still more abundant, while they in turn became the chosen food of the larger fishes and reptiles. Occasionally we hear, as a matter of marvel, of the capture in tropical waters of cuttlefishes eight or ten feet in length; how transcendently more wonderful the thronging of these secondary seas with myriads of the same order still more gigantic in size and more diversified in feature! Strange, too, that the ocean which then swarmed with nautiloid forms should now contain only a single genus! Inexplicable, were there no plan of vital progression to which these extinctions and creations could be systematically conformed.

Even still more remarkable was the exuberance of reptilian life that thronged the seas, the estuaries, the rivers, and river-plains of the secondary ages. Nothing known before or since of reptile-life is at all comparable either in point of variety, size, or numbers. Our museums are replete with their remains, and in conditions so perfect, that almost every feature can be restored in natural proportion and life-like reality. Whale-like and confined to the waters, crocodilian-like and amphibious, mammalian-like

* Ammonites (coiled up like the horn of Jupiter Ammon); baculites (straight or staff-shaped); hamites (hook-shaped); scaphites (boat-shaped); turrilites (tapering and turret-shaped); belemnites (dart-like); acanthoteuthis (thorn-like organ); belemno- (bolt-like); cono- (conical); lepto(slender); and so on.

and treading the land, or bird-like and winging the airthese saurians were undoubtedly the most notable features of the period. It was indeed the "age of reptiles," when every habitat aquatic, terrestrial, and aerial-was occupied; and every function-carnivorous, herbivorous, and insectivorous was performed by these creatures. Frog-like, but bulky as an ox, the labyrinthodon squatted on the muddy river-flats; whale-like, the ichthyosaur paddled through the waters; iguana-like, but huge as an elephant, the iguanodon browsed on the succulent herbage; bloodthirsty and mightier still, the deinosaur crouched for his prey in the forest; while bat-like, under the cliffs and adown the riverbanks, fluttered the insectivorous pterodactyle.* Every place seems to have been usurped and every function performed by these abounding reptilian orders; and this for long ages, while bird-life was gradually coming into force, and mammalian life dawning into existence.

This bird-life makes its first appearance in the Trias, the surfaces of whose flaggy sandstones are thickly imprinted with their footsteps. These ornithichnites, or bird-footprints, occur in all sizes, from the slender impress of feet like those of the snipe and sandpiper to the heavy implant of others many times bulkier than those of the largest ostrich. Waders and runners, they seem to have frequented the sandy shores of creeks and estuaries which were also the favourite haunts of reptiles, and thus many of these Triassic sandstones are crossed and re-crossed, tracked and retracked, by footprints whose nature (reptilian or ornithic) it is yet impossible to determine. How wonderful the

variety and complexity of the vital record! how strange that the impression of a passing foot should remain as evi

*

Labyrinthodon, from the labyrinthine structure of its teeth; ichthyosaur, fish-like saurian; iguanodon, having teeth like the existing iguana; deinosaur, terrible saurian; pterodactyle, finger-winged or flying reptile.