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of metalnorphism, so it is among them that the geologist often meets with his greatest doubts and difficulties. Defining them as the metamorphic, they were at one time regarded as preceding the fossiliferous strata, and as marking a period of the earth's history when life did not exist, and when, of course, its reliquice were not expected to be found in the rocky strata. As research extended, fossils were here and there discovered, even in these metamorphic strata; and hence the necessity of arranging them (see Sketch No. 5) according to this new evidence into " systems " and " formations," as had been done with the younger and more fossiliferous strata. But beyond the deepest in which traces of life have been detected, there still lie vast masses of crystalline and granite-looking schists unresolved, and apparently unresolvable, and to such the designation "metamorphic" is still specially applicable, and may ever remain so. In these we find no legible record of world-history: nothing beyond the great facts that they are stratified rocks, and must have been laid down in seas and estuaries like all other sediments, and that millions of ages must have elapsed during their slow conversion from silts and sands and gravels to crystalline schists and granitoid masses. But though this be the present state of geologic knowledge, we dare not, looking at the progress of the past, presume that further discoveries will not be effected. These old rocks may yet tell their tale of life, just as the Laurentian and Cambrian have done it before them; for, carry our researches backward as we may, we perceive no traces of a beginning, any more than in the existing operations of nature we see indications of an end.
Once more: let it be clearly understood that metamorphism is simply internal mineral change; that under pressure, heat, chemical action, and other kindred forces, all rock-matter in the earth's crust is incessantly undergoing
such transformations; and that while this crust is alternately wasted and reconstructed by the agents described in Sketch No. 2, it is mainly from metamorphism its rocks receive these peculiarities of texture and structure that confer upon them their distinguishing characteristics and economic importance.
THE PEIMAEY PEEIODS.
OLDER AND YOUNGER PORTIONS OF THE EARTH'S CRUST—THESE REPRESENTED BY PERIODS OF TIME OR SYSTEMS OF STRATA— TECHNICAL ARRANGEMENT OF THESE PERIODS AND SYSTEMS—THE PRIMARY OR EARLIEST YET KNOWN, EMBRACING THE LAURENTIAN, CAMBRIAN, AND SILURIAN—COMPOSITION AND DISTRIBUTION OF THESE RESPECTIVE SYSTEMS—THEIR CHARACTERISTIC ROCKS AND FOSSILS — GENERAL PAUCITY OF LIFE — ADVANCE DURING THE SILURIAN—CYCLE OF CRYPTOGAMIC PLANTS AND INVERTEBRATE ANIMALS — GEOGRAPHICAL ASPECTS OF THE PRIMARY PERIODS —INTEREST ATTACHED TO THEM AS THE DAWN OF WORLDHISTORY.
Nothing can be clearer than this: if the higher and more exposed portions of the earth be continuously wasted and worn down, and the rock-material so wasted be as continuously deposited in the receptacles of lakes and estuaries, the older deposits (or formations, if you prefer to call them) will be the deeper, and the newer will be arranged in order of succession above them. Or put it in this way: if the sediments of lakes and seas be gradually converted into solid strata, and now and again upheaved into dry land by vulcanic forces, the oldest, generally speaking, will be the most consolidated and altered. Or again, if the sediments of lakes and seas contain less or more of the remains of plants and animals that have been either drifted from the land or entombed in the areas where they flourished, then the latest enclosed remains will be the least altered, and most resemble those still living in the district. These propositions are so evident that it would be little else than waste of time to attempt a more detailed explanation.
We have, then, in the crust of the earth some formations very recent and others very ancient—so recent that the formative processes are still in action, and so ancient that it requires all the appliances of modern science to say how they were formed, and to determine the nature of the fossil organisms they contain. The great plain of China and the delta of the Mississippi, for instance, are of recent growth, and indeed still in course of accumulation; and by a parity of reasoning we may readily perceive that the prairies of North America and the pampas of South America, though somewhat older, are still of comparatively recent formation. Everything connected with these accumulations—mineral composition or imbedded organisms—has an air of recentness about it, and is readily intelligible; but it is different with the sandstones and limestones that lie beneath, whose structure has been altered, and whose fossils have been mineralised and rendered obscure. These sandstones and shales and limestones were no doubt at one time loose sands and clays and calcareous muds; but pressure, chemical action, and other metamorphic forces, continued through long ages, have converted them into solid strata, and this conversion has been intensified, for the most part, according to their relative dates. As with the stratified or sedimentary formations, so also with the igneous. Such hills as Hecla, Etna, and Teneriffe readily reveal their history; but the Alps, the Pyrenees, and the Grampians, whose internal fires have been long since extinguished, whose rocks have become more crystalline, whose heights have been repeatedly beneath the waters, and from which all the loose volcanic matters have been washed and worn away, present a very
different aspect, and require all the ingenuity of science to interpret their history.
This relative antiquity of rock-deposits may be still more clearly shown by the formations of our own islands. The carse-lands of the Tay and Forth and the fens of Lincolnshire are recent alluvia, and younger than the blue clays and gravels over which they are spread; the stratified sands and clays and gravels in the neighbourhood of London are younger than the chalk and greensands that lie beneath; and these chalk-hills and greensands of Kent and Surrey are newer than the calcareous sandstones and limestones of Portland on which they repose. Again, the oolites or roestones of Portland are younger than the underlying red sandstones and marls of Cheshire, and these newer than the coals and ironstones of Lancashire that are spread out beneath. The coals of Lancashire, Northumberland, and Fife are not so old as the underlying red sandstones and flagstones of Forfarshire; and these again are much younger than the still deeper slates and crystalline schists of the Scottish Highlands. Nor is it superposition alone that proves this relative antiquity. Mineral structure and texture become more intensified with age; fossil organisms become more obscure, and the further we descend in time the wider the divergence from the genera and species that now people the globe. Everything in the earth's crust speaks to this relative antiquity—to this old, older, oldest; and it is the object of the present Sketch to describe the earlier of these formations, and to depict, as far as Geology can, the aspects of the periods when they were gradually accumulating in the primeval waters.
Before entering on this description, however, it may render matters more intelligible to state, and what indeed has been already detailed in No. 1 of these Sketches, that