most important modifiers of the earth's crust; and it acts partly by upheaval, partly by accumulation, and partly by fusing and reconstructing rock-matter in the interior, and by bringing it once more to the surface. In this function it is usually accompanied by the earthquake, which fractures and dislocates the solid crust, uplifting one portion and depressing another, submerging one area beneath the waters and elevating another into dry land. Unlike the volcano, the earthquake produces no direct change on the character of the rocks; but it is the great diversifier of the earth's surface, creating new irregularities, and as a consequence interfering with the action of the atmospheric and aqueous forces that operate on that surface, as well as with the distribution of the life by which it is peopled. But beyond the earthquake and volcano, with all their minor accompaniments of hot-springs, mud-springs, and the like, there seems to be another and still more gigantic, though silent, manifestation of vulcanic power. We allude to those gradual uprises and depressions of portions of the earth's surfacethose crust-motions, if we may so speak-by which certain regions, like the shores of Scandinavia, Spitzbergen, and Siberia, are being slowly raised above the sea-level, and other regions, like the western coast of Greenland and that of the Southern States of America, as slowly depressed beneath it. Many countries, and our own islands among the rest, are marked by lines of ancient sea-beach, denoting uprise above the waters; and could we only see beneath the ocean, we believe there are other regions equally marked by terraces of depression. There is no other power save Vulcanism, or internal heat-force, to which we can ascribe these upheavals and depressions of the crust; but be this as it may, there can be no doubt of such oscillations, and that they form one of the appointed means by which the waste and reconstruction of our continents are held in equilibrium, and new distributions of sea and land accomplished.

The effect of these various agencies-the atmospheric, aqueous, organic, chemical, and igneous-is thus not only to mould and modify the exterior, but at the same time to build up the interior structure of our planet. Their function is at once to wear down the old and to reconstruct the new; to scatter abroad in one region and to accumulate in another. And the new rocks reconstructed must necessarily bear the closest relationship to the old from which they were derived. Their hardness and compactness and crystalline texture is merely a matter of time. Given time and the fitting conditions, and the loosest sand will be converted into the most compact sandstone, the softest mud into the hardest slate, and the earthiest chalk into the most crystalline marble. Though here separated for the purposes of elucidation, these agents are ever working hand in hand-the atmospheric with the aqueous, the aqueous with the chemical, and the chemical with the organic and igneous. And it is this complicated working that renders the composition of the solid crust so varied, the aspect of its rocks so different, and the task of unravelling their history at once the trial and the triumph of geology.

Such is a brief sketch-a mere indication, as it were-of the great forces by which the earth's crust is incessantly modified, its rock-matter wasted and reconstructed, and the equilibrium of its terraqueous distribution sustained. Sketchy as the outline has been, the careful reader will have perceived not only the nature of the modifying agents, but the manner in which they operate; and must feel convinced that, small as may be their results over a given area or during a given time, yet comprising the whole globe and allowing for ages, they are sufficient to accomplish any amount of change-to destroy, in fact, the whole of the existing continents, and to reconstruct new ones from the bed of the ocean, and this by degrees, and over and over again, according to the course of Time, which is illimitable

and beyond all computation. He will also have perceived that, many as are the agents at work, and complicated as are their modes of action, yet, on the whole, they may be conveniently arranged into two grand categories—namely, the powers of waste and degradation from without, and the powers of reconstruction and upheaval from within. As surely as the meteoric and aqueous disintegrate and level, so surely does the igneous reconstruct and upheave; as the chemical and vital dissolve and destroy in one area, so they recombine and build up in another. There is nothing so harmonious as this incessant round of mutation, nothing so marvellous as the variety it produces, and yet nothing so certain as the unity of design by which the whole is combined into one intelligible system. Strange as it may seem, even the comfort and development of man is indissolubly bound up with this system of vicissitude. It is not the rugged and flinty hillside that yields him his sustenance. He cannot build his cities on its peaks or plough its precipices. And yet these hills are the great storehouses of future fertility. The rains, the frosts, the streams, and the rivers are perpetually carrying down from their heights the materials to form the fertile valleys-washing out from the old crystalline rocks the inorganic elements indispensable to vegetable luxuriance. And on these mountain-derived plains man has hitherto settled in communities and built his cities. The plains of the Old World, the historic fulfilments of the past-China, Hindoostan, Mesopotamia and Egypt-were borne down from the mountains of Asia and Africa; just as the prairies, the llanos, and pampas of the New World, the hopes of the advancing future, are the gifts of the Andes and the Rocky Cordilleras. How marvellous this system of interdependence between the organic and inorganic-between the mechanical processes of nature and the social development

of man! How admirable this system of unceasing rejuvenescence! the old hills worn and wrinkled and furrowed by decay, and the younger valleys spreading out in their beauty and freshness and fertility!

"This earth, like the body of an animal," said Hutton, "is wasted at the same time that it is repaired. It has a state of growth and augmentation; it has another state, which is that of diminution and decay. This world is thus destroyed in one part, but it is renewed in another; and the operations by which this world is thus constantly renewed are as evident to the scientific eye as are those in which it is necessarily destroyed." And yet how few will take the trouble to comprehend this system of incessant change, contented to live in the belief that this earth has always been as it is—has been so from the beginning, and will continue to be so to the end! Verily they deprive themselves of much rational enjoyment, pay little regard to the system of nature of which they form so prominent a part, and show little reverence for Him who has given them eyes to see, and understanding to understand, so be it they will only learn to exert them. How admirable the system of compensation by which decay in one part is balanced by renovation in another! The substances disintegrated by water are again reconstructed by fire; the matter dissolved by chemical action is collected anew by vital; and what was appropriated for a while by the living organisms is restored again to the mineral world when vitality has ceased its requirements. Everything in this universe is indissolubly woven into a network of interdependence, and not a mesh could be taken away without destroying the beauty and consistency of the whole.

"From nature's chain whatever link you strike,

Tenth, or ten-thousandth, breaks the chain alike!"

And the reason is obvious; for most of the operating forces

D

we have described arise directly from the earth's primal connections as a member of the solar system. Annual revolution and daily rotation, daily and seasonal alternations of heat and cold, currents and counter-currents of wind, evaporation and rainfall, waves and tides, currents and counter-currents of water, are all results of the earth's relation to the sun and her sister-planets. So in like manner are the phenomena of vegetable and animal growth; so, too, are many of those chemical and electro-magnetic activities with which science is but slenderly acquainted; and so also, perhaps, though in a remoter degree, that manifestation of internal vulcanism concerning which philosophy can do little more than merely hazard conjectures. So long, therefore, as the earth's primal relations endure, these secondary forces must operate as a necessary consequence, and thus the rocky crust must continue to undergo a round of waste and reconstruction as ceaseless as the revolutions of the planetary system, and as permanent in its power. How harmonious the system by which this earth, in the midst of all its mutations, is kept ever fresh and young! Day after day, and year after year, the aspects are ceaselessly changing, but the vitality remains the same; cycle after cycle the forces may shift their direction, but their power remains unimpaired. Everything around us being seemingly stable, it may be difficult to realise this conception of incessant change, just as it is impossible to estimate the lapse of time required for its fulfilment; but an effort must be made, and not till the mind has learned to form some idea of the ceaseless mutations to which the earth's crust is subjected, the causes by which these mutations are effected, and the amount of time required for their production, can it be said to comprehend the fundamental truths upon which the science of geology is erected.