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CHAPTER IX

INFLUENCE OF ROCK CHARACTER IN THE
DETERMINATION OF LAND-FORMS.

JOINTS IN ROCKS AND THE PART THEY PLAY IN DETERMINING
SURFACE-FEATURES — TEXTURE AND MINERALOGICAL COM-
POSITION OF ROCKS IN RELATION TO WEATHERING—FORMS
ASSUMED BY VARIOUS ROCKS.

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. Thus while bedded strata of all kinds, breaking up along the joints, tend to give rise to rectangular features at the surface, amorphous crystalline rocks, quarried by epigene action, generally yield irregular contours. And the same is the case with the crystalline schists, the jointing of which is as a rule capricious and uncertain.

It is obvious, therefore, that surface-features must be greatly influenced by the character of rock-joints. Apart altogether from other geological structures, joints must largely determine the physiography of the surface. To such an extent is this the case, that it is generally easy to tell at a glance whether any particular mountain is composed of amorphous crystalline rocks, of schists, or of regularly bedded strata. Mountains carved out of horizontal strata tend, as we have seen, to assume pyramidal forms, while in the case of inclined beds erosion and denudation result in the formation of escarpments and dip-slopes. This, however, only holds true when relatively hard beds are intercalated among a series of softer strata. Should the rocks throughout be of much the same consistency no escarpments will be developed, but the whole will wear away equally, and so give rise to a gently undulating surface. Usually, however, a thick series of strata will be found to comprise rocks of various degrees of durability; and in general, therefore, bedded rocks, whether horizontal or inclined, tend to yield rectangular outlines. But when the dip greatly increases, and the strata are more or less violently contorted, the beds are often crushed and confusedly shattered or jointed, while at the same time the rocks themselves may become metamorphosed, and eventually pass into the condition of schists. Rectangular outlines are thus gradually replaced by the jagged, rough, and abrupt configuration which is so characteristic of slaty and schistose or foliated rocks.

Amongst the crystalline schists rectangular outlines are not common. Now and again, however, when different kinds of schists rapidly alternate in successive sheets or beds, some will almost certainly weather more rapidly than others. The outcrops of the less yielding rocks will thus tend to project; but as jointing is usually irregular and confused, such outcrops seldom show rectangular outlines. Exceptionally, well-marked escarpments may be met with, but the general high dip and contorted character of the rocks forbid such formations. When steep wall-like outcrops of schists occur, they have very often been determined by the presence of normal faults or of thrust-planes. In short, while the foliation and pseudo-bedding of schistose rocks now and again give rise to surface-features which are more characteristic of truly bedded strata, yet such features are apt to be strongly modified by the vagaries of the jointing.

In amorphous crystalline masses, which show neither bedding nor foliation, the character of the joints usually varies with the nature of the rock. In

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