« PreviousContinue »
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
granite, for example, there are usually three sets of joints, one of which traverses the rock in an approximately horizontal direction, or may have a dip now in one direction, now in another. The vertical joints often cut each other at right angles, but not infrequently they meet at more or less acute angles. In addition to these main joints, however, there are often others. Sometimes the joints are wide apart, and they then enclose large rectangular or rhomboidal blocks. At other times they are set so closely together that the rock when exposed breaks up into a mass of angular dSrts. As the character of the jointing varies in this way within narrow limits, the rock tends to assume broken interrupted contours. On the other hand, when the disposition of the jointplanes is more regular and better denned, the horizontal joints maintaining their direction for some distance, granite not infrequently breaks up as if it were a bedded igneous rock. A mountain-wall so constructed rises in a series of gigantic steps, like tiers of cyclopean masonry, interrupted by entering and re-entering angles. Where the "horizontal" joints are much inclined a corresponding change in the direction of the main rock-ridges and reefs may be observed. Not infrequently, however, the horizontal jointing is obscure and ill-defined or even wanting, and the chief contours of the surface are then determined by the vertical joints alone. Under such conditions the mountain-slope shows irregular vertical or steeply inclined walls, ridges, and but