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convulsions; and, fourthly, an accurate series of tabulations and maps to assist us in our deductions. All this is little more than merely beginning to be done. Chemistry and physics have only recently been directed to the subject; and Seismology or Seismography (the science of earth-shocks) is a study as it were but of yesterday.* Till substantial progress has been made in this direction, and sufficient facts for generalisation collected, it were idle to speculate, and waste of time to surmise.
Such is a brief, and we trust not unintelligible, sketch of that Vulcanicity or internal heat-force which is incessantly reacting upon the rocky crust we inhabit. Than the earthquake, volcano, and great crust-pulsation, we have no higher manifestations of natural force; no phenomena before whose power man's weakness becomes more apparent. There are, no doubt, other terrific agencies in nature—the ocean when lashed into fury by storms, the flooded and headlong river, the hurricane, and the thunderstorm. Man, however, learns to brave and battle with these. The hardy islander dares the ocean-storm in his little skiff; civilised nations build their piers and breakwaters that their fleets and navies may ride behind them in defiance of the storm. Man dams and diverts the river-current, restrains it within bounds, or even turns it to account as the moving power of his machinery. By strength and weight of material he can resist the fiercest sweep of the wind-blast; or if need be, can yoke it to his wheels submissive and serviceable. He even toys with the thunder, and brings the lightning down from the storm-cloud. But before the shock of the earthquake and the throes of the volcano, man—savage or civilised— shrinks altogether abject and helpless. With them, how
* See the 'Seismology/ and 'Seismographic Maps,'of the Messrs Mallet, of Dublin. 1861.
ever frequently they may occur, he never becomes familiar. The earth, with which all his ideas of stability are associated, rocks and reels beneath him—his proudest cities become an instantaneous mass of ruin and rubbish; himself falls prostrate, or if he flees, he flees only to accelerate his fate. The volcano casts forth its scorching showers of scoriae and ashes, his pastures and vineyards are utterly consumed, and his homesteads and villages—like Pompeii and Herculaneum —are buried, so that for centuries their very places are unknown. Or the red river of lava spreads slowly and irresistibly down the mountain-side, crushing and consuming the forest-growth like stubble, damming and diverting rivercourses, engulfing villas and towns, and converting the fair face of nature into a wilderness of blistery slag and "the blackness of desolation." *
Opposed to these terrific agencies, man stands utterly impotent; and alas for his ideas of this Scheme of Creation, if he canuot learn to associate them with its necessary order and mechanism! Exposed to incessant powers of waste and degradation from without, the earth's crust is only maintained in habitable equilibrium and variety of surface by this power of vulcanism acting as incessantly from within. As it now manifests itself by the three or four hundred orifices known to geographers, so in all time past have its manifestations been equally apparent—now in this region, now in that—now feebly, now with greater intensity—fitful to all appearance, but obeying, we may rest assured, some fixed and determinable law, could we only grasp the multifarious conditions associated with its expres
* In reflecting on man's impotency in connection with the manifestations of vulcanism, it must not be forgotten that it is within his power, by Artesian borings, to bring up water of high temperature from the interior of the earth, and thus employ it, as he already does natural hotsprings, for sanitary purposes, or for the heating of his dwellings and conservatories.
sion. As at present, so during all the geological periods, we find that it has been gradually elaborating hills and mountain-ranges, upheaving and consolidating the sediments of the sea-bottom, giving diversity of surface to areas of fiat and uniform deposit, raising substances of utility from inaccessible to accessible depths in the earth's crust, and reticulating that stony fabric with lines and walls of injected matter, which under the names of "dykes" and "faults" restrain the percolation of its internal waters, and bring them to the surface in thousands of living and refreshing streams. The great diversity of animal and vegetable life which now adorns this earth is clearly associated with its diversity of surface; and this diversity of surface — this arrangement into hill and dale, into table-land and plain, into island and continent, with all its varying accompaniments of wind and ocean-currents and climate—is the direct result of the earth's incessant vulcanicity. Unless then, when considering the earthquake, volcano, and crust-motion, we can learn to regard them as necessary portions of the orderly mechanism of our planet, we are viewing them in the feeblest light, and much as the cowering savage, who sees in them only disorder and destruction, and the wrath of his offended deities. And even where science cannot pierce the veil, we may rest assured, in this as in other arrangements of nature, that nothing is jarring, but that all, however unintelligible to our finite comprehensions, is in perfect harmony, and indispensable to the maintenance of this wondrous world-plan—
"All discord, harmony not understood;
METAMOEPHISM, OE THE TEANSFOEMATIONS OF EOCK-MATTEE.
METAMORPHISM, DEFINITION OF THE TERM — ALL ROCK-MATTER INCESSANTLY UNDERGOING INTERNAL CHANGE THROUGH PRESSURE, ATTRACTION OF COHESION, CHEMICAL ACTION, HEAT, MAGNETISM, CRYSTALLISATION, AND OTHER SIMILAR FORCES — ILLUSTRATIONS OF THESE RESPECTIVE FORCES AND THEIR MODES OF ACTION—THEIR EFFECTS MOST PERCEPTIBLE IN THE OLDER ROCKS—THE SO-CALLED METAMORPHIC ROCK-SYSTEM—APPARENT OBLITERATION OF ITS FOSSILS—GRADUAL DETECTION OF THESE IN PECULIAR LOCALITIES—RESOLUTION OF THE SYSTEM INTO MINOR SECTIONS AND CHRONOLOGICAL ORDER—INCONCEIVABLE AMOUNT OF TIME IMPLIED IN THE DEPOSITION AND SUBSEQUENT TRANSFORMATIONS OF THE METAMORPHIC SYSTEM — HOW TO DEAL WITH IT AS A PART OF WORLD-HISTORY — ITS CONVENIENCE AS A PROVISIONAL DESIGNATION.
It has been already explained that all the substances composing the earth's crust, from the most recent and superficial soils, sands, muds, and gravels, down to the oldest and deepest slates and schists, are known as rocks and rock-formations. It has also been shown that all these rock-matters are undergoing incessant changes, being weathered and worn from the old hills, borne onwards by streams and rivers, laid down as sediments in lakes and seas and estuaries, and again upheaved by vulcanic agency as the solid strata of newer continents. In this long and ceaseless round they are disintegrated and dissolved, reconstructed and consolidated, and it is to the varied processes of reconstruction and consolidation that we would here direct attention. Every substance, the moment it is laid down as sediment or discharged from the crater of a volcano, begins to suffer change and transformation. The attraction of cohesion, pressure, the percolation of chemical solutions, heat, magnetic currents, crystallisation, and the like, are all more or less altering its internal texture, and in the long-run its external aspect or structure. In course of time, and under the operation of these forces, the softest mud becomes compacted into shale, sand into sandstone, gravel into conglomerate, peat-moss into coal, and coral-growths into limestone; and by a farther transformation shales may become glistening clay-slates, sandstones quartzites, coals anthracites, and limestones sparkling and crystalline marbles. This kind of transformation, or Metamorphism, as it is technically termed, forms one of the most wonderful as it does one of the most difficult chapters in geology, and it is to place it in as simple and intelligible a light as the nature of the subject will permit that we attempt the present Sketch. Of course it is not to be expected that what is often a matter of doubt and difficulty to the professed geologist can be made easy to the comprehension of the casual inquirer; but an indication of the processes of metamorphism can be given, and such indication may lead to a more satisfactory conception of the character and formation of rocks in general.
One of the most obvious agents in the re-formation of disintegrated rock-matter is simple mechanical pressure. As layer after layer of sedimentary matter (mud, clay, sand, gravel, &c.) accumulates in seas and estuaries, the lower and earlier are necessarily pressed upon by all those above them, and thus they are gradually consolidated—muds and clays into shales, sands into sandstones, and gravels into conglomerates. This pressure at great depths must be enormous; and as it is incessant in its action, we can readily perceive how rock-substances may be altered in their structure and