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in valleys, but upon hill-slopes and hill-tops. Obviously hundreds of feet of rock have been removed from the whole land-surface since those dikes were injected.
In fine, then, we conclude that many most conspicuous and characteristic features of the land owe their origin to igneous action. In some places the intrusion of masses of molten rock has produced more or less prominent swelling and bulging at the surface, while the outpouring of volcanic materials has resulted in the formation of hills and mountains, and of plains and plateaux of accumulation. Ere long, however, all such land-forms become modified by epigene action, and more or less completely changed. Intrusive masses formerly deeply buried are eventually exposed, and, owing to the more rapid removal of the rocks through which they rise, may come to form mountains of circumdenudation, while these in their turn tend to be reduced to a base-level. Volcanoes, in like manner, are broken down and crumble away, until it may be the only relics that remain are plugged-up vents, and the dikes proceeding from them, every fragment of the cones having vanished. Or the lavas of former times, having been interbedded with and deeply buried under strata of aqueous formation, may, owing to their superior durability, come to form escarpment-hills and mountains, when the strata originally deposited above them have been removed by denudation. So again volcanic plateaux are dug into by erosion, and pass
through a well-marked cycle of changes. The plateaux are broken up into groups of pyramidal mountains, and these in time are reduced, and may even be entirely replaced by plains of erosion. Thus in lands which have been for long periods of time exposed to erosion, although evidence of former igneous action may abound, and irruptive and eruptive rocks may enter prominently into the formation of the more striking surface-features, the shape of the latter we see is entirely the result of denudation and erosion. If the igneous rocks now form hills and mountains, it is because of their superior durability. Intrusive and effusive rocks alike appear at the surface, and the forms they assume depend chiefly upon the geological structure and mineralogical character of the masses.
INFLUENCE OF ROCK CHARACTER IN THE
JOINTS IN ROCKS AND THE PART THEY PLAY IN DETERMINING
THE origin of surface-features, as we have now learned, is frequently complex. Only in very few cases, can we assert that any prominent feature is the direct result of crustal movement alone. In time all features due to plutonic or subterranean action become more or less modified. We are justified in maintaining that the great mountain-chains of the globe owe their origin indeed to folding and fracturing of the crust; but even the youngest of these has yet been so profoundly modified by epigene action, that the external configuration no longer coincides, save in a general way, with the internal geological structure. Each chain as a whole owes its existence to crustal deformation, but the individual mountains of which it consists are largely monuments of erosion. And so of land-surfaces generally we may say that their more prominent features are the result of denudation, guided and controlled by geological structure. We cannot study the configuration of the land, however, without perceiving that the relative durability of rocks has also had some share in determining the form of the surface. In regions composed largely of "soft" rocks we may note a general absence of abrupt and broken outlines; the surface even when hilly is usually rounded and gently undulating. It is otherwise when "hard" rocks predominate, the features assumed by these tending to be less smooth and (lowing. The surface becomes more diversified still, however, when both soft and hard rocks occur together. In a word, hard rocks at all elevations offer most resistance, while soft rocks more readily succumb to epigene action. We thus arrive at the general conclusion that the form assumed by the land under long-continued erosion and denudation is determined directly by the character of the rocks and the mode of their arrangement, and init■ito.</Y, of course, by igneous action and crustal movements, to which the most striking and conspicuous geological structures are due.
These general conclusions have now been sufficiently illustrated, and we may next consider certain surface-features a little more closely. Rocks, as we have seen, consist roughly of two great classes—those which occur in more or less distinct beds or strata, and those which show no such arrangement, but appear as amorphous masses. The former class is typically represented by sandstones, shales, and limestones, the latter by granite, syenite, and other eruptive rocks. Most of the bedded rocks are fragmental or clastic; but crystalline rocks, such as the various lavas, not infrequently assume bedded forms. With few exceptions all great amorphous rock-masses are crystalline. There is yet another important group of crystalline rocks—the schists—which to some extent simulate the characteristic structures of clastic rocks. Thus they often show a kind of bedding, and their foliation mimics, as it were, the lamination of shaly strata. The foliation and bedding, however, are commonly more or less puckered and contorted.
Now all rocks are traversed by natural divisionplanes or joints, and these, in the case of well-bedded strata, are usually disposed at approximately right angles to the planes of bedding. Thus, as we have seen, beds of sandstone, etc., are divided up into somewhat quadrangular or cuboidal blocks. Old lava-flows, in like manner, often show at least two similar sets of vertical joints, and not infrequently these are cut by a third set, disposed at approximately right angles to the others. Not a few bedded igneous rocks and intrusive "sheets," again, assume a more or less columnar aspect, owing to the symmetrical arrangement of the joints. In amorphous masses of crystalline rocks, on the other hand, uniform jointing as a rule is absent. Their divisionplanes run in various directions, and are often extremely irregular. In some places they may be very closely set, in other places they are far apart.