depth-the coal measures a thousand yards deep-and the accumulation of trees and other vegetable matters, which are carried down towards the sea in such vast quantities through the greater rivers of the world; as, for instance, in the Mississippi, where what are called rafts, formed of tangled trees, roots, and brushwood, are found several feet thick, and several miles in length. The snags, that is, trunks of great trees, buried in the bed of the same rivers, and which frequently project so high as to endanger steamboats, also suggest how great an influence vegetable remains may exercise, under certain circumstances, in changing or modifying the superficial configuration of particular localities. Plants have at the same time, be it observed, a great conservative tendency. Their matted roots bind together the loose sand of sea-shores, and generally everywhere protect the soil from the power which wind and heavy rains, or inundations, would otherwise exercise over it. Animals. We have already had occasion incidentally to notice the wonderful agency of animal life in building up large portions of the actual surface of the earth. We may still further illustrate this point by a few words upon that most interesting of animalculæ, the coral, whose structures have been a never-ceasing theme of admiration with the poet and naturalist. The animal itself is scarcely so big as a pin's head, is of a soft gelatinous structure, and star-shaped; and in order to form a single branch of coral, millions of them must unite their tiny bodies together. Yet this small, almost invisible creature, through its power of secreting lime from the water, will raise solid structures in the sea capable of resisting the wildest attack of the waves, and which shall extend to immense distances. There are groups of coral reefs in the Pacific extending from 1,100 to 1,200 miles in length, and from 350 to 400 in breadth. Sometimes the reefs appear as islets; sometimes as circular belts, inclosing a sort of lagune, or lake, within; but generally in long ridges, averaging in width from 20 to 100 feet thick. The circumstances under which these tiny architects labour make the results the more wonderful. "No periods of repose are granted," says Mr. Darwin, an intelligent observer, "and the long swell caused by the steady action of the trade-wind never ceases. The breakers exceed in violence those of our temperate regions; and it is impossible to behold them without feeling a conviction that rocks of granite or quartz would ultimately yield and be demolished by such irresistible forces. Yet these low, insignificant coral islets stand, and are victorious; for here another power, an antagonist to the former, takes part in the contest. The organic forces separate the atoms of carbonate of lime one by one from the foaming breakers, and unite them into a symmetrical structure; myriads of architects are at work day and night, month after month, and we see their soft and gelatinous bodies, through the agency of the vital laws, conquering the great mechanical power of the waves of the ocean, which neither the art of man nor the inanimate works of nature could successfully resist." The coral rocks are not, of course, raised directly from the ocean's bottom, but on the summits of hills, and probably of volcanic peaks, both of which are, perhaps, far more numerous at the bottom of the ocean than on the land. Thomas Montgomery, the author of the Pelican Island, gives us the following fine passage descriptive of the labours of the coral animalculæ :— "Millions of millions thus, from age to age, With simplest skill and toil unweariable, No moment and no movement unimproved, To swell the heightening, brightening, gradual mound, By which a hand invisible was rearing A new creation in the secret deep. Omnipotence wrought in them, with them, by them; Still dying upwards as their labours closed: Frail were their frames, ephemeral their lives, On examining a piece of coral, its surface is perceived full of little openings, each of which contains one animal: the whole coral pile, therefore, is as a vast house for the family, in which house each individual has its own apartment. Shell-fish, on the contrary, which possess the same power of secreting lime, use it to form a separate shell for each individual, and which has no connection with that of any other individual, except that of mere contiguity. Oysters, muscles, and cockles are thus separate, even while forming together beds of many miles in extent. After what we have seen of the power of the coral animalcula to affect the earth's surface in the ocean depths, we need not be surprised at the statement that these, and the other tenants of the waters belonging to such tribes as the zoophyta, testacea, &c., flourish in such amazing profusion, that their very exuviæ tend markedly to fill up the almost boundless depths where they inhabit. Care necessary in judging of the ancient from the modern effects of the foregoing influences.-Although we everywhere recognize through all nature fixed laws, we also as universally behold varying conditions under which they act. It is not, therefore, to be assumed that what changes we now see going on in modern times are in themselves a sufficient measure of comparison to estimate the force of the laws causing these changes in remoter ones. Light and heat, for instance, are phenomena exhibiting certain regularities of action called laws: we see the results of their action now; we know the source of light and heat was the same in all past time; and we may, therefore, naturally conclude that their effects were very much the same then as they now are. But, as it has been well pointed out, let the sun's rays be but supposed to fall upon the earth in smaller quantity, through the augmentation of the minor axis of the earth's elliptic orbit; let the temperature of the ethereal spaces rise; who does not see that all the effects depending on the external excitant forces would immediately change? Now this very orbit is variable. Also to allow for possible causes, of great moment in their results, but of which no traces remain.-Again, what physical changes may not be wrought by a cause of so trivial a character that, while its effects might modify the entire future of the globe, it should leave not a trace of itself behind! Suppose, for instance, an earthquake were to sink the Isthmus of Darien but a hundred or two feet below its present level, who can estimate the effects upon the Indian, Mediterranean, Atlantic, and Pacific oceans over areas of enormous extent, and affecting the stratified deposits and physical conditions, and consequent variations, in the relative abundance and geographical distribution of organic life? These and similar facts show how humbly the geologist should pursue his researches-yet how earnestly and unremittingly-if he would base his noble and beautiful science on solid foundations. Nature's efforts for an Equilibrium.-And, independently of the various truths involved in the phenomena we have glanced at in the preceding pages, what-it may be asked-is the higher meaning of the whole ?-the objects sought by all this incessant change and conflict? We may answer, in the eloquent words of the Cyclopædist writer already referred to, "The never-ceasing activity of the powers of nature is an inextinguishable, though an unavailing effort to restore an equilibrium which is incessantly disturbed. The Protean changes of the atmosphere; the varying effects which its chemical and mechanical energies occasion among the masses of dead matter and the forms of life; the flowing of the ocean; the subterranean fire, and wide wasting of the earthquake, are all efforts to obtain rest, consequent on a succession of perturbations. In this sense, not the earth only, but all the solar system-and, perhaps, all the extent of the heavenly spaces-conceivable rather than visible by man-is in the condition of instability described in the Pythagorean philosophy, Nihil est toto quod perstet in orbe." MOUNTAINS OF GRANITE AND MICA SLATE, GLEN SANNOX, ISLE OF ARRAN. On the Successional Order in which Strata should be described.-We commence our description of the different rocks, unstratified and stratified, at what we may call the base of the geological structure. In proceeding from the surface, or latest formation, down to the granite, we take the course that naturally offers itself to us, as residents on the surface. We kave been familiar with this first, and have only delved down from it as our material wants and our scientific aspirations suggested. We have, therefore, presented all the geological formations in this order in the table that we gave in our third chapter. On the other hand, when we come to describe these formations, it is obviously unnatural to begin at what was but the last of a series of operations, all connected with each other in the due order of cause and effect. We will, therefore, commence with the unstratified or igneous rocks. Plutonic and Volcanic Rocks.-These are divisible into two great classes: 1, the Granitic, or, as some geologists call them, the Plutonic; and 2, the Volcanic. We shall speak of the latter in a subsequent portion of these papers. Igneous Rocks not always preceding the Aqueous:-It is important that the student of geology should bear in mind that although, as a broad general rule, the igneous preceded the aqueous rocks, it by no means follows that all the igneous rocks are older than all the aqueous ones. The reverse is very often the case. It is highly probable that the same state of things which originally produced granite during the very earliest period of the earth's history continued still to exist, and to be in operation at limited depths in the earth's crust, long after sedimentary rocks had been deposited. Volcanic formations are still constantly rising under our very eyes, and we need not, therefore, speak of their comparatively modern date in many instances. Granite sometimes formed later than the Rocks that lie above it.—As at once a proof and consequence of the production of granite later than some of the rocks that overlie it, we may instance the many known examples of the upward flow of fluid granite into fissures of the latter, just as in our iron furnaces the melted ore is found to penetrate, as veins, into the centre of the sandstone walls. Sir Charles Lyell, indeed, thinks that granite may be still in process of formation, through the melting of the rocks that lie above it when exposed to the earth's intense interior heat, which thus lessens their amount, destroys what organic remains may have existed in them, and reduces the whole into a part of the general mass of the interior of the earth. Such speculations show us, among other legitimate deductions, that incalculable as geological eras of time already are made to appear, by what we see in the crust, even that crust itself can give us but a part-possibly a small one-of the whole truth. So that the history of an individual world seems scarcely less wonderful than that of all the bodies of space, where, when one has arrived at last at something like the comprehension of a universe, he finds that, instead of being at the end of his journey, he is scarcely nearer to it than when he set out, for now he discovers a plurality of universes, if we continue to use the term universe in the same sense in which we have previously understood it. Granite, general description of.-Granite presents itself in many formsnow as stupendous mountain ranges, such as the Alps, the Pyrenees, the Grampians; now as a floor of an undulating form; now as veins bursting up through the strata above, and ramifying in a thousand different shapes; everywhere presenting in its forms proof of its originally igneous state. In what is called granite districts-that is, where granite appears above the surface-the scenery is of a rugged as well as mountainous character, to which the snow-clad peaks often lend a strangely harmonious combination of the soft and the beautiful with the grand and the desolate. Structure of Granite.-In itself granite is one of the most beautiful of rocks, both as regards structure, variety of constituents, and colours. It is GRANITE. composed chiefly of mica, felspar, and quartz, in distinct crystals; but it is also found to contain hornblende, garnet, talc, and numerous other minerals. The crystals of the mica and felspar are often of great beauty, while the quartz commonly fills up the interstices between the two. The colours are extremely various. Felspar is found red, grey, yellow, white, green; mica-black, grey, white, brown, or silvery; quartz is generally clear white or grey; hornblende dark green or black. The individual grains of the component parts of granite can be clearly distinguished; sometimes they are small, as in the Aberdeen granite. Mica is found in laminæ, some inches across; also in small plates. The felspar in graphic granite forms almost one huge crystallized mass. This latter derives its name from the circumstance that the minerals in it are occasionally found arranged in a manner that has been likened to the lines of Arabic writing. The Uses of Granite are chiefly confined to building, and some of the most extraordinary structures of the world have been formed of it; for instance, the Egyptian pyramids. Its extreme hardness renders it of preeminent value for all edifices of a permanent character. The granite used in London for Waterloo Bridge, and for the river wall of the New Houses of Parliament, was brought from Aberdeen, where it forms the ordinary building stone. Of late years the red granite of Peterhead, in Scotland, has been used for vases, chimney-pieces, &c.: it is brought by machinery to a high polish. Mica and tale may be occasionally found in single crystals of a foot or so square, and then become admirably fitted for splitting up into thin transparent plates, that may be used instead of glass. Some believe the Romans used such plates for the garden frames in which they grew early fruits and flowers. Tale will bear a higher heat than glass without injury; and, when used in ships of war, is rendered less liable to break by the explosion of ordnance. The Chinese use decomposed felspar in the manufacture of their best earthenware; and we ourselves have found it of such value for the same purpose, that many thousand tons are annually brought from Cornwall to the English potteries. Primary Rocks: Gneiss and Mica Schist Systems. Formation of Gneiss and Mica Sehist.-As the crust of the earth cooled under the operation of the influences described in Chapter I., it appears to have crystallized into granite. That was the first step in the economy of creation. The next was produced by the combined influences of this gradually lessening heat, and of atmospheric and watery action upon the surface. Hence the strata known under the names of the Gneiss and Mica Schist systems. These are, unquestionably, the oldest watery deposits known, and have probably preceded the period of the existence of life in any shape. They also extend so largely over the world as to approach nearer to universal formations than any of later date. Nowhere do we find any trace of their formation in the present time; they seem to be altogether |