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break and tides ebb and flow, there they are to be witnessed, partly as degrading, but partly also as accumulating and re constructing forces. The mere passage of water over rocksurfaces would of itself have little effect; but as it bears along its burden of sand and gravel and shingle, every particle becomes a tool which grinds, and is in turn ground down in the double process of attrition and erosion. Every runnel and rivulet wears for itself a channel, and bears the eroded material down to the river; the river performs the same operation, but on a larger scale, and with marked intensity during floods and freshets, cutting out ravines and gorges, or scooping out broader valleys, and transporting the debris to the lower levels of lakes, estuaries, and the ocean. There the mud and sand and gravel borne from the higher grounds come at last to rest, subside as sediments, and are thus spread out as alternating strata, to be consolidated by pressure, chemical agency, and other means, and ready, when the event happens, to be upraised as the rock-formations of newer lands. In like manner, also, with the waves and tides and currents of the ocean. Eestlessly and for ever eating into and undermining the sea-cliff, the waves encroach upon the land, pound down the hardest material to shingle and gravel and sand, and this with rapidity according to the nature of the opposing cliff, and the manner it is disposed to the impact of the breakers. The effects of waveaction are perceptible along every exposed shore; here in the undermined and falling cliffs, there in caverns and gorges, and in another part in the " needles," "stacks," and outstanding rock-masses that have been severed from the land. What the waves have worn down the tidal ebb and flow disintegrate still more, and scour and carry away to the stiller depths and more sheltered recesses. And the great ocean-currents, too—like the Arctic with its burden of icebergs and rock-debris, or the Gulf Stream with its drifted sea-weeds and animal exuviae—are also incessantly transporting and reassorting. Everything, however, comes at last to rest in the waters, being either piled on shore as sand, gravel, and shingle, deposited as silt in the deeper and stiller waters, or strewn along the courses of the ocean-currents in long reaches of miscellaneous debris, partly of animal, partly of vegetable, and partly of mineral origin. The effect of aqueous agency is thus partly to wear and waste, and partly to accumulate and reconstruct—to wear down the old continents, and to accumulate the abraded materials in the waters for the formation of newer lands.
The Chemical agencies, though less perceptible, are not less general or less incessant in their action than the meteoric or aqueous. Indeed, in a certain sense the meteoric and aqueous act chemically; but we have hitherto alluded chiefly to their mechanical effects, and now direct attention more especially to their chemical. The carbonic acid of the atmosphere eats into the most crystalline marble; its oxygen converts the hardest ironstone into a soft earthy peroxide, ready to be washed away by the first shower that falls. Every spring that issues from the interior of the earth holds in solution, less or more, some mineral or metallic matter, which it either deposits along its course, or carries forward through stream and river to the ocean. Be it lime, or iron, or flint, or salt—and such matters constitute our petrifying, chalybeate, silicious, and saline springs— these matters must have been dissolved or wasted from the interior, as they are now brought to its surface to form new rock-masses, or to enter into newer combinations in the waters of the ocean. And this chemical effect of springs is vastly increased when the waters are hot, whether bursting forth like the Geysers of Iceland, or simmering like the mud and sulphur vents that appear in the neighbourhood of almost every active volcano. Heat, indeed, is the great promoter of chemical change within the earth's crust; and from this cause arise, no doubt, those discharges of naphtha, petroleum, and the like, that result from the slow decomposition of lignites, coals, and other organic masses. It is not to pressure alone, nor to volcanic heat alone, that the solid strata, originally of sand, gravel, mud, and organic debris, owe their hardness and crystalline texture. Chemical infiltrations and combinations are everywhere as operative as these are, and, indeed, in most instances are the main modifiers of mineral texture, colour, and consistency. And the veins and veinstones—the great repositories of the metallic ores—that traverse the older formations, they, too, are the immediate products of chemical segregation, slowly and silently, but ever at work in these secret recesses. On the whole, chemical actions and reactions within the rocky crust of the earth are incessant, either dissolving and displacing, reconstructing into other forms, or aggregating in other and newer compounds.
The Organic agents fall next to be considered, and of these, as of many other departments of nature, it may be remarked that the minute and unobserved are the most active and effective. It is true that the trees of the forest may be imbedded in peat-bogs or drifted into the mud of estuaries, and that the bones of fishes, reptiles, birds, and mammals may be entombed in the sediments of lakes and seas; but, on the whole, these constitute a small proportion of the containing strata, and even where swept by currents into special shoals and bone-beds, as we know they are in certain parts of the ocean, they form but insignificant accumulations compared with those resulting from the myriad-growths of microscopic organisms. In the vegetable world the plants of the peat-moss and swamp-growth claim our first attention, as out of the constituents of the air and water they elaborate their own substance, and year after year bequeath it to the accumulating mass. How thin soever may be the film that the growth and decay of a single year may add to the accumulation, yet in the course of centuries the peat-growth and swamp-growth thicken, till now within all the colder latitudes there are expanses of vast area and many feet in depth entirely composed of decayed vegetation, and ready to be converted into coal, like the analogous accumulations of former epochs. In the animal world, on the other hand, the coral-reef is perhaps the most striking instance of aggregation by the minutest of means. Barely perceptible to the naked eye, the tiny zoophyte, in countless myriads, secretes the lime held in solution by the waters of the ocean, till year after year and century after century the conjoint structure so increases that at last the "reef" stretches away many leagues in length and many fathoms in thickness. As with coral-reefs so with beds of gregarious and drifted shells, and so also with the enveloping limy and flinty shields of microscopic organisms like the foraminifera and diatoms; the former mere specks of animal jelly, the latter mere points of plant-life, yet so increased by the immensity of their numbers and the rapidity of their growth, that large areas of the sea-bed, as well as lakes and estuaries, are thickly strewn with their calcareous and silicious envelopes. These envelopes, though mere microscopic specks, are yet aggregated in such myriads that they are capable of forming extensive beds of limestone on the one hand, and of flinty rock on the other. And such limestones and flint-rocks we find in the solid crust, exhibiting to the eye of the microscopist the beautiful organisms of which they are composed, and proving that in former ages the earth's crust was built up by the same agencies that still continue to remodel and uphold it. A great proportion of the chalk-rocks of England, the nummulitic limestone that stretches from the Alps eastward through Europe and Asia on to the Philippine Islands,
and the mineral known as tripoli or polishing-slate, are ancient strata formed by analogous microscopic organisms; and the same work still goes forward in the calcareous ooze that covers so large an area of the Atlantic sea-bed, and in the silicious mud of most of our existing estuaries.* The lime and the flint dissolved by springs from the crust of the earth, and borne down by streams and rivers to the ocean, is thus secreted by vital agency and once more converted into solid rock-matter. Nothing is lost; it may change its shape and pass out of sight for a while, but in the long-run it will reappear, altered it may be in form, but essentially the same in substance. Every particle of matter obeys a ceaseless round of change. Now in the crystalline and independent gem, at another time as a constituent of the solid rock; now dissolved in the limpid waters, at another time built up in the structures of plants and animals; now scattered abroad as the decaying exuviae of life, and once more collected into compact and rocky strata.
The Igneous agents generally exert themselves with signal force and marked effects, and yet in some instances their most gigantic results are brought about by stages that are almost imperceptible. The most notable instance of their operation is perhaps in the volcano, which in course of ages piles up its alternate discharges of dust and ashes and lava till it assumes the lofty proportions of an isolated mountain like Etna, or stretches away in long ranges like the fiery cones of the Andes. Whether exerting itself on land or rising up from the depths of the ocean, the volcano is one of the
* Of the microscopic organisms—the foraminifers, polycystines, diatoms, and desmids—that stand, as it were, on the confines of Life, the two former belong to the animal kingdom, and the two latter to the vegetable. The foraminifera secrete calcareous matter, the polycystines and diatoms silicious, and the desmids no appreciable quantity of either. The foraminifera and diatoms, along with the coral-polypes, may therefore be regarded as the main microscopic rock-builders.