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geneous masses, and not in layers or bed above bed. And when we go to the volcano or burning-mountain, and observe the discharges of molten lava, which when cooled assume a structure scarcely distinguishable from that of the basalts and greenstones, we are equally entitled to infer that these have originated like lava, and consequently have been formed through and by the agency of fire; hence we regard them as igneous or eruptive if we refer to their origin, and itnstratificd if to their mode of arrangement. There are thus in the crust of the globe only two great categories of rocks—the aqueous or stratified, and the igneous or unstratified; the former produced through and by the agency of water, the latter through and by the agency of fire. The stratified, by the water-worn particles of which they are composed, and their sedimentary arrangement, layer above layer, give evidence of the forces that operate from without; the unstratified, by their crystalline texture and the manner in which they break through and derange the sedimentary strata, of the forces that exert themselves from within.
These two sets of rocks are being formed at the present day—the stratified or sedimentary in every lake, estuary, and sea, and the unstratified or eruptive around every active volcano. And as nature's operations are incessant, such rocks must have been formed during all time—from the current hour back through untold ages. Did the watery forces—rains, rivers, waves, tides, and ocean-currents—alone prevail, the dry land would in course of time be worn and wasted down to one uniform level, over which the ocean might roll in uninterrupted continuity. But just as certainly as waste and degradation are going on from without, so the fiery forces—the volcano and earthquake— are as incessantly operating from within, upheaving new lands and mountains, and conferring on the whole new irregularity and diversity of surface. The earth's crust is thus held in equilibrium between these two opposing forces, fire and water—between waste and degradation on the one hand, and reconstruction and upheaval on the other. In this way former lands have been wasted and worn down, and former estuaries and seas filled with the sediments; old continents and islands submerged beneath the waters, and the sea-bed upheaved into newer lands. The rocks of the earth's crust are the only memorials of these repeated changes; and if geology is earth-history, it must endeavour not only to decipher the changes they record, but to arrange them in chronological sequence and connection.
When we look, then, at the changes now taking place on the earth's crust, and the new rocks that are in process of formation, we behold in them the exact counterparts of what must have taken place during all former periods. Winds, frosts, rains—springs, streams, rivers—waves, tides, and ocean-currents—are ever weathering and wasting the rock-matter of the globe; and the matter worn down is borne by rivers to lakes and estuaries and seas, and there deposited in layers of mud and clay and sand and gravel, or further reassorted by the tides and currents of the ocean. Coral-reefs, shell-beds, and other masses of animal origin, are also accumulating in various parts of the ocean; while peat-bogs, swamp-growths, and forest-growths are adding analogous masses of vegetable origin to the land. Hotsprings and mineral-springs are also carrying matters in solution from the earth's interior, and depositing these along their courses; while volcanoes are ever throwing out from the same interior showers of dust and ashes, and masses of molten lava. The earthquake, also, which is but another manifestation of volcanic agency, is ever breaking up the rocky crust—here raising the sea-bed into dry land, and there submerging the dry land beneath the ocean—here rending and Assuring, and there producing inequalities and varieties of surface. Whatever is worn and wasted from one portion of the crust is laid down in another; there is nothing lost; but the interchanges and variations are interminable. The crust we dwell upon—stable and enduring as we are accustomed to regard it—is thus a thing of incessant change, protean in its superficial aspects, and evershifting in its terraqueous arrangements. The lands which we now inhabit were formerly the bed of the ocean; and the existing ocean-bed formed at one time the surface of the habitable lands. Sea and land are ever shifting, and have ever shifted places; and thus every epoch of our planet has necessarily been characterised by its own physical and vital aspects.
If the earth's crust be thus continually worn away in one district and reconstructed in another, some portions— like the lavas of Etna and the delta of the Ganges—must be comparatively recent; and others—like the Grampian Mountains and the coal-fields of Britain—of vast antiquity. In the former instances, the lavas and mud-islands are forming beneath our observation; in the latter, the formative processes have ceased, and no perceptible change has occurred for ages. To arrange the rock-formations of the earth into chronological order is one of the first duties of geology, for without this sequence there could be no history, and a connected history of the changes this crust has undergone is the great object of all geological investigation. By a rockformation is meant the strata that have been deposited in any lake, estuary, or sea-area. The layers of mud, clay, marl, sand, and gravel which have filled up any ancient lake constitute a lacustrine formation; the sediments that are similarly deposited in estuaries an estuarine formation; and those deposited in seas, and subsequently upraised into dry land, a marine formation. In course of time, by pressure, chemical action, and other means, sands become sandstones, gravels conglomerates, clayey muds shales, calcareous muds limestones, muds largely impregnated with iron ironstones, and vegetable masses coals; and it is in this way that the sediments of former lakes and estuaries and seas have become the rocky strata that now constitute the crust of the globe. As might be expected, there will often be every degree of admixture among these strata, just as there is every degree of admixture and impurity among the sediments of existing seas and estuaries. There will be sandstones argillaceous, and sandstones calcareous; shales bituminous, shales calcareous, and shales ferruginous; limestones argillaceous, and limestones silicious; coals so pure as to bur n away without leaving scarcely a trace of ashes, and others so stony as to be altogether unfit for fuel. The solid crust is indeed mainly made up of mixed rocks—that is, of arenaceous (sandy), argillaceous (clayey), calcareous (limy), silicious (flinty), bituminous (coaly), ferruginous (ironimpregnated), and other similar compounds; but whether these rocks be sedimentary sandstones, grits, conglomerates, shales, limestones, and ironstones, or fire-formed lavas, greenstones, basalts, and granites, the great object of geology is to distinguish between the older and newer, to arrange them in chronological order, and so arrive, if possible, at a knowledge of the geographical conditions which accompanied their formation. Nor is this endeavour in the least chimerical or uncertain; for as was well remarked by Humboldt, now nearly half a century ago— "The superposition and relative age of rocks are facts susceptible of being established immediately, like the structure of the organs of a vegetable, like the proportions of elements in chemical analysis, or like the elevation of a mountain above the level of the sea. True Geognosy makes known the outer crust of the globe, such as it exists at the present day. It is a science as capable of certainty as any of the physical descriptive sciences can be."
As a general rule the earliest formations will be the deepest-seated, or, in other words, the strata that lie beneath must be older than those that lie above them. Generally speaking, too, the older rocks will be harder and more crystalline in texture than the younger. There may be isolated exceptions to this, but the great fact holds good, that the older formations are really the more crystalline, and that this characteristic becomes less and less marked as we ascend to the recent and superficial layers of clay, sand, and gravel. Again, all the stratified rocks are less or more fossiliferous—that is, contain the petrified remains of plants and animals—and these fossils, as they are called, lead to pretty correct inferences not only as to the relative ages of formations, but as to the conditions under which they were deposited. And they do it in this way. Every lake, or estuary, or sea, imbeds in its sediments the remains of plants and animals that have either been drifted from the land by rivers or have lived and grown in the waters of deposit. As these remains get imbedded in the sands, clays, and calcareous muds, and excluded from the action of the air, they gradually undergo a change, become impregnated with mineral solutions, and in course of time are petrified, or converted into stony matter like the strata that contain them. At the present day the plants and animals entombed in the delta of the Mississippi differ widely from those entombed in the delta of the Ganges; and were these deltas subsequently converted into rockformations, those plants and animals would afford evidence of the kind of life and climate that prevailed in their respective areas. It is in this manner that the fossils found in the earth's crust bespeak the conditions under which they lived—aquatic or terrestrial, fresh-water or marine,