to roll themselves into a sort of defensive ball. The mode of progression is doubtful. The animal may have had soft paddles, which were incapable of preservation, or it may have used the flexible power of its body to produce locomotion. Its eyes form a particularly interesting feature, both for their organization and the use that an eminent geologist, Dr. Buckland, has made of them, in explaining the condition of the seas in ancient geological eras. There are two eyes, each consisting of 400 compartments, or spherical lenses, which are so placed on the surface of a cornea, projecting conically upwards, that they all look outwardly from the animal's head. These, then, are raised, so that the animal, from its position at the bottom of the waters, can see all around without any hinderance from its own protuberant body; but their inward lines of vision do not cross each other, as that would have been an unnecessary waste of power-a striking instance of the combination of fertility and economy that Nature so often loves to present unto us. Wealth, not waste, seems ever her motto. But the trilobite has been a means of important special instruction to the geologist. It told Dr. Buckland that the air, the light, and the sea-waters of the incalculably distant eras when the trilobites flourished in such amazing profusion, were essentially as they are now. For, first, if the deep waters had been turbid, such delicate organs of vision would have been useless; second, had the atmosphere differed from its present condition, the rays of light would have been also affected to a different result, and then we should not have found, as we do find, the eyes of existing crustaceans agreeing with the older crustaceans in question; and, thirdly, as to light itself, it is certain that the mutual relations of light and optical vision were essentially the same then as now, because the essential organizations of the eye in both periods is the same—a happy instance of sound logical and geological deduction.

Already the distinction with which we are familiar, between the vegetable feeders and the Carnivora, or flesh-eaters, existed. The trilobites belonged to the latter class.

Condition of the Earth's Surface.—The chief characteristics of the surface of the crust during the existence of these systems are shown pretty clearly by the differences of the strata. The clay slates must have been compressed from fine clayey soil into their present state by waters of immense depth, but undisturbed by agitation. The sand and gravel of the grauwacke (or grey rock) reveal the effects of rivers, and of the action of the sea upon its shores. The lime of the Silurian rocks tells us of the long labours of the coral builders in raising the beds and reefs of limestone.

Igneous Rocks associated with the system.-Certain igneous rocks, not already mentioned, are generally associated with these aqueous rocks. They are serpentine, porphyry with greenstone, and other varieties of trap; the last we shall speak of in connection with the volcanic rocks. Serpentine derives its name from the contrasts of colour that it often exhibits, and which distantly resemble the skin of some serpents. It usually contains much magnesian earth. The term porphyry is derived from a Greek word, signifying purple, and is as old as the days of Pliny, when it was applied to a reddish rock, containing crystallized felspar, brought from Egypt, and used in ancient sculpture. It is now applied to all unstratified rocks in which detached crystals are imbedded.

Scenery of the Transition Rocks.-The finest examples of the scenery produced by the transition rocks-as those under notice are called by some

geologists, who consider them to occupy a place between the primary and the secondary-may be found in Wales, where, says Professor Phillips, supported by granite, and mixed with igneous masses, the slaty rocks of the English lakes rise to more than three thousand feet in height, and present a variety of outline, and intricacy of combination, which, in connection with clear lakes and considerable waterfalls, leave to Switzerland little superiority. But they also extend generally over the world, sloping away from the sides of its principal mountain ranges.

Slate

Uses. Some of the uses of the rocks of this formation hardly need to be mentioned, they are so well known. The clay slates supply our schools with the popular instrument of instruction in writing and ciphering, and the roofs of our houses with the best of coverings. boxes are also beginning to be used in our conservatories, and the slate itself for a variety of other ornamental purposes. The Silurian rocks contribute their help in the shape of flagstones for our street payements; whilst the limestones furnish various ornamental marbles. But the metals they give us are still more important. Indeed, these rocks generally are (with the exception of the lead and ironstone of the carboniferous system) the richest of all others in this respect, as they include gold, silver, tin, lead, copper, &c., which are found in metallic veins traversing the clay slate.

SECONDARY STRATA.

The Carboniferous System: The Old Red Sandstone, or Devonian Rocks.

Name, &c., of the System. This system includes the Old red sandstone, the Mountain limestone, and the Coal measures; from the latter the group derives its name-coal-bearing.

The Old Red Sandstone.-Upon the hollows and elevations of the undulating bed of the sea, around the ranges of the primary rocks, produced by the later phenomena of the first great geological era, the secondary strata began to be deposited. We have already seen how the matter to be deposited was ever in process of accumulation, from the wear of the substance of the primary rocks, and have spoken sufficiently of the various agencies and influences that operated to cause transport, deposition, and condensation. The entire thickness of the secondary strata, which comprise the Old red sandstone, the Mountain limestone, and the Coal measures, was small as compared with the thickness of the preceding rocks, and extended over much less space. The reasons are obvious; the one sprang from the causes at work through the whole globe, the second from causes that concerned chiefly only those parts of the primary rocks that were gradually exposed to atmospheric and other action. But, if of less depth and extent, they are of far greater variety and number, in the alternations of the lesser strata, and are, as a whole, greatly superior in all that concerns the development and support of organized beings. More and more, too, do they seem to approximate, in the circumstances of their formation, to the existing phenomena of external nature.

Name, &c.-It looks, at first, as though nature had receded, rather than advanced, in the earliest of the secondary strata, the Old red sandstone, a rock called old to distinguish it from another-the New red sandstone,

which is found above the Coal measures, denominated Red on account of the colour it exhibits in Devonshire, where it is most abundant; and hence the other name for the system-the Devonian. In this there is a decided decrease in the number of organic fossils, and what little vegetation had struggled into existence during the preceding systems is scarcely any longer to be found. The reasons appear to be that volcanic action was renewed with additional violence, which not only rendered the atmosphere and raised lands unfit for plants, but disengaged vast quantities of mineral matterthe peroxide of iron-which, being dissolved in the seas, rendered them less fit generally for the support of animal life. It is this iron which gives the peculiar colour to such large portions of the sandstone rocks.

Geographical Developments of the Old Red Sandstone. The chief developments of the system in this country are in Devonshire (where it overlies the Silurian, and flanks the transition hills, as these again flank the primary ones), in Cornwall, Wales, Herefordshire, Shropshire, Worcestershire, Scotland. In Russia it extends over a space as large as Great Britain. The whole of the northern part of Scotland, from Cape Wrath to the north flank of the Grampians (which are granite and gneiss), have been described as consisting of a nucleus of granite, gneiss, and other similarly formed rocks, set, as it were, in a sandstone frame. The flat position of the strata partly causes this great surface extension: the earlier rocks are highly inclined. The thickness of the system in parts extends to ten thousand feet.

General Characteristics.-The sandstone is one of the most clearly developed of all geological systems. Where it exists in flat strata the scenery is uninteresting to the eye of the lover of the picturesque, though the farmer finds there a soil light and fertile; but where the sandstone rises into mountains there is a marvellous change. The hills are less high and abrupt than those previously formed, but are more lofty and varied than those of later date. There is a constant change of view. All the peculiarly charming incidents of a natural landscape-such as gentle undulations, deep glens, and woody recesses—arise, from time to time, to the eye of the traveller.

Composition. The sandstone varies in composition from a fine-grained hard rock, that can be split into pieces for flagstones and tilestones, to a thick mixture or conglomerate of sand and pebbles, many of the latter being as large as a man's hand. Some calcareous beds are found in the system, consisting of an impure concretionary limestone, called by the country people cornstone. The colours include various shades, from red to grey, and from mottled purple and fawn to a creamy yellow. The mottled colour is chiefly observed in the sandy shales-a sort of imperfect sandstone-that belong to this system, and which are found alternating in thin layers with the sandstone. The whole are evidently littoral depositions that is, they were deposited by the sea-shore. Many of the strata present to our eyes as plainly the ripple marks made by the waves of unimaginable centuries ago, as those which the wanderer by the seaside of to-day sees on the sand of the beach, and which are yet wet from the waters of the last tide. The lower, or grey series, in which the traces of the primary mica are to be found, is the sediment of calm waters. The sandstone and conglomerates owe their position and strata to the action of currents and aqueous agitation. The yellow beds were only deposited when once more all was quiet in their vicinity. Vegetable Life. There are no certain evidences of land plants during the Devonian era.

Of marine ones fuci appear to have been the chief. These

must have grown in a higher temperature than exists where they are now found-a proof of the more general diffusion of a tropical climate in these remote geological periods. On this head we shall have more to say when we speak of the Coal measures.

Animal Life.-No one even supposes there were any land animals during the Devonian era; all the things that breathed and moved had their home in the sea. Their general forms were not materially altered from those of the preceding Silurian era, but the species underwent a material change. Out of the eight hundred species comprised in the one era, only about one hundred passed on into the other. But to counterbalance this there was a large development of fishes, and a general advance in the character of existing organizations. Among the species that were thus preserved, the coral builders may be specially named. They are so abundant in Devonshire as to constitute entire strata-the beds of marble for which Babbacombe, Torquay, and Plymouth are famous. New species, of course, appeared-a monster trilobite, for instance, the Brontes, which was four feet long, and had lobster-like claws. The Cephalopods were now again largely represented, but with important changes in form. Fishes must have been plentiful in the Devonian seas. Upwards of a hundred species have already been reckoned. They were all cartilaginous-a striking feature of distinction from existing fishes, among which the bony-skeletoned are numerous, the cartilaginous few. These fishes were the destructives of their time, and kept down the too luxuriant population, as the Mollusca had done before them. Some are supposed to have been full thirty-six feet long. No less than nine genera of sharks have been discovered in the Russian Devonians. If we divide the Devonian fishes into the two orders, one of placoids-that is, having on the external covering irregular enamelled plates, laid edge to edge-and the other of ganoids, which possess regular enamelled scales overlapping each other, we find that one only of the orders, the first, had existed during the Silurian era; hence it is supposed that in that order the life of fishes may have commenced. Our engravings represent some of the more remarkable creatures found in the Old red sandstone. The annexed (the Cephalaspis Lyelli), is from a specimen in the possession of Sir Charles Lyell, after whom, we presume, it is named, and from whose Elements we copy the design. "Buckler-headed," these animals are called, from the THE CEPHALASPIS LYELLI.-a. One of the strange shield that covers the head. scales of the head. bc. Scales from different To look at it, one would suppose the parts of the body. saddler's or cheesemonger's cuttingknife would be a still more suitable term. Strong as this creature was for resistance, he was weak indeed for active movement-the only organs suitable for that purpose being a range of very small fins.

a

These fishes (the Pterichthys) derive their name from their wing-like appendages, which they are supposed to have erected when threatened by an enemy, in the hope, probably, of frightening him off by such an un-fishlike apparition; or, if that failed, of being used as a weapon of defence. The tail is presumed to have been the organ of motion. These winged fish are

as numerous in, and characteristic of, the Old red sandstone, as were the trilobites of the Silurian era.

But trilobites remained and flourished in the

TO

THE PTERICHTHYS.-Upper side, showing mouth. THE BRONTES FLABELLIFER.

Devonian era also. We give an engraving of one (the Brontes flabellifer), in which, it is to be observed, the head is not quite perfect. Its outline should be wider and rounded. The parts missing in the fossil specimen here represented were, perhaps, softer or thinner, and so decayed, or were broken

away.

Igneous Rocks associated with the Devonian.-The same igneous rocks that we have mentioned as being associated with the aqueous masses of the Silurian system, were also associated with the Devonian, with certain additions, such as amygdaloid, a trap rock, in which are imbedded almondshaped minerals-hence the name. The toad-stones" of our peasantry are varieties of amygdaloid, and have obtained their appellation from the marking and colours resembling those of a toad's skin. Granite is no longer found in intimate connection with the latest-formed rocks-a proof that the granite era that is to say, the time when granite was being constantly formed below, and heaved up on high-had passed away.

The

Trap, and its connection with the system. Without at present entering upon the subject of the volcanic rocks to which trap belongs, it is necessary to point out what is meant by the term. This is derived from the Swedish trappa, a stair, and expresses a peculiarity of the trap rocks, that they often rise in large tabular masses, one above another, like steps. When granite ceased to be upheaved, trap appears to have taken its place. And so we find that the rocks that upheaved the Devonians were trappean, or volcanic. tremendous power that could thus raise immense portions of the earth's surface appears to have been quiescent during the time of the deposition of the Old red sandstone, and then, as though the time had come for which it waited, to have burst forth, scattering new mountain ranges over the earth, against the sides of which was to begin once more the work of material progress, in the deposition of yet a new strata-the mountain limestone.

Uses of the Devonian Rocks.-The uses of the Devonian may be thus summed up:-Tilestones for our house-tops, and flagstones for our foot