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The belief in world-wide, or nearly world-wide, catastrophes disappeared. Geologists came to see that the fashioning of the earth■s surface had been going on for a long time, and is still in progress. The law of evolution, they have found, holds true for the crust of the globe just as it does for the myriad tribes of plants and animals that clothe and people it. It is no longer doubted that the existing configuration of the land has resulted from the action of forces that are still in operation, and by observation and reasoning the history of the various phases in the evolution of surface-features can be unfolded. No doubt the evidence is sometimes hard to read in all its details, but its general bearing can be readily apprehended. The salient facts, the principal data, are conspicuous enough, and the mode of their interpretation is in a manner self-evident.
In setting out upon our present inquiry, however, it is obvious that we ought, in the first place, to know something about rocks and the mode of their arrangement. We must make some acquaintance with the composition and the structure or architecture of the earth-■s crust before we can form any reasonable conclusion as to the origin of its surface-features. Now, so far as that crust is accessible to observation, it is found to be built up of two kinds of rock, one set being of igneous origin, while the other appears to consist mainly of the products of water action. These last are typically represented by such rocks as conglomerate, sandstone, and shale, which are only more or less ancient sediments, formed and accumulated in the same way as the gravel, sand, and mud of existing rivers, lakes, and seas. Another common rock of aqueous origin is limestone, of which there are countless varieties—some formed in lakes, like the shellmarls of our own day; others representing the calcareous ooze and coral-reefs of ancient seas ; while yet others are obviously chemical precipitates from water surcharged with carbonate of lime. Now and again, also, we meet with rocks of terrestrial origin, such, for example, as many beds and seams of peat, lignite, and coal, which are simply the vegetable ddbris of old land-surfaces. To these land-formed beds we may add certain sandstones of wind-blown origin— indurated sand-dunes, in short.
The igneous rocks consist partly of lavas and fragmental materials which have been ejected at the surface, as in modern volcanoes, and partly of formerly molten masses which have cooled and consolidated below ground. The former, therefore, are spoken of as volcanic, the latter as plntonic or hypogene rocks. As it is useful to have some general name for the rocks which owe their origin to the action of epigene agents (i. e., the atmosphere, terrestrial water, ice, the sea, and life), we may term these derivative, since they have been built up chiefly out of the relics of pre-existing rocks and the de'brisoi plants and animals. Byand-by we shall learn that igneous and derivative rocks have in certain regions been subjected to many remarkable changes, and are in consequence so altered that it is often hard to detect their original character. These altered masses form what are called the metamorphic rocks. They are typically represented by such rocks as gneiss, mica-schist, clay-slate, etc.
The derivative rocks, with which in many regions igneous rocks are associated, occupy by far the larger portion of the land-surface, entering abundantly into the composition of low grounds and mountains alike. Most of these derivatives are sedimentary accumulations, and very many are charged with the remains of animals and plants. By noting the order in which such stratified deposits occur, and by comparing and correlating their fossils, geologists have been able to group them into a series of successive systems, the oldest being that which occurs at the bottom of the series.1 The united thickness of the several systems probably exceeds twenty miles, but it must not be supposed that all these occur together in any one region. Many broad acres of the earth■s surface are occupied by the rocks belonging to one system only. In other countries two or more systems may be present. Again, each individual system is of very variable thickness—swelling out here, thinning off there: in some lands being represented by strata many thousands of feet in thickness, in others dwindling down to a few yards. In short, we may picture to ourselves each system as consisting of a series of larger and smaller lenticular sheets, irregularly distributed over the earth's surface. The various systems thus frequently overlap, the younger stealing over the surface of the older so as often to bury these out of sight.
1 See Appendix for Table of Geological Systems.
The metamorphic rocks do not appear at the surface over such extensive areas as those just referred to. Nevertheless, they are widely distributed, and now and again overspread continuously vast regions. The enormous tract that extends from the Great Lakes of North America to the shores of the Arctic Ocean is almost entirely occupied by them. Another immense area of crystalline schistose rocks is met with in Brazil. The Highlands of Scotland, the Scandinavian Peninsula, and North Finland are in like manner largely composed of them, and the same is the case with many parts of Africa, Asia, and Australia. It is further noteworthy that similar rocks form the backbones of most of the great mountain chains of the globe. As already indicated, metamorphic rocks are of various origin, some of them being primarily of igneous and others of aqueous formation. Those which form the nuclei of the youngest mountain chains are sometimes of relatively recent age, while those occupying such broad tracts as Brazil, the Canadian uplands, etc., are of vast antiquity. Crystalline schistose rocks, with associated granites and other igneous rocks, seem everywhere to underlie the sedimentary fossiliferous formations. Very often the latter are separated by a broadly marked line of demarcation from the schists, granites, etc., upon which they repose. In other cases the sedimentary rocks become gradually altered as they are traced downwards, until eventually they themselves assume the aspect of crystalline schists, penetrated here and there by granitoid igneous rocks.
The origin of those ancient crystalline schists has been much discussed, but does not concern us here. Some geologists have maintained that the rocks in question represent the original cooled crust of the globe, while the majority consider them to be all metamorphic. It is enough for our present purpose to know that a pavement of such rocks appears everywhere to underlie the sedimentary fossiliferous formations.
Fig. 1. Section Of Horizontal Strata.
The upper continuous line, A-B, = surface of ground ; the lower continuous line, C-D, = sealevel; /, limestone ; x, sandstones and shales.
The great bulk of the derivative rocks being of sedimentary origin, it is obvious that they must have been at the time of their formation spread out in approximately horizontal layers upon the beds of ancient lakes and seas. This we are justified in believing by what we know of the accumulation of similar sediments in our own day. The wide flats of our rivervalleys, the broad plains that occupy the sites of siltedup lakes, the extensive deltas of such rivers as the Nile, the Po, the Amazon, the Mississippi, the narrow