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tresses, which often run into each other as they are followed upwards, and may eventually taper off to a point. (See Plate I.)

The influence of joints, however, is apt to be greatly obscured by the manner in which rocks themselves disintegrate and crumble down. The sharply angular rock-faces defined by joints are slowly or more rapidly eaten into by epigene action, and the rock exfoliates or crumbles down irregularly according to its character. Indeed, this rotting action has often proceeded very far before the joint-faces are laid bare. When a mass of rock, losing its support, falls away, the new surface exposed has already become to a larger or smaller extent disintegrated and decomposed, so that frost and rain are enabled rapidly to reduce and modify it. Hence the sharp irregular outlines which joints naturally tend to produce are, in the case of such rocks as granite, generally rounded off. Basalt-rocks in like manner often weather readily and become decomposed and disintegrated along planes of jointing, and thus give rise to a somewhat rounded and lumpy configuration. But there is often much diversity of surface displayed by one and the same rock-mass, the basalt in some places weathering rapidly into rounded forms, while in other places, especially where the rock is fine-grained and compact, the sharp angles of the jointing are better preserved. (See Plate II.)

The usually finer-grained rhyolites, trachytes, andesites, and phonolites are not as a rule so readily disintegrated as normal granites and basalts. Their joints, moreover, are not only less uniform, but frequently very abundant and closely set. Such rocks, therefore, are readily broken up. Mountains carved out of them usually show sharp crests and peaks, while their slopes are hidden under curtains of angular ddbris, through which ever and anon are protruded reefs, ridges, buttresses, and bastions of such portions of the rock-mass as are less profusely jointed. (See Fig. 78, p. 203.)

In short, we may say that every well-marked rocktype breaks up and weathers in its own way, so that under the influence of denudation each assumes a particular character. We see this even in the case of well-bedded aqueous rocks. Planes of bedding and jointing no doubt are the lines of weakness along which rocks most readily yield, but each individual rock-species weathers after its own fashion—the different kinds of shale, sandstone, conglomerate, and limestone are decomposed, disintegrated, and crumbled down at different rates, and each in a special way, according to its mineralogical composition and state of aggregation. Thus, although a region built up of bedded aqueous rocks may show the same general configuration throughout—horizontal strata giving rise to pyramidal-shaped hills and mountains, while inclined strata of variable consistency present us usually with a series of escarpments and dip-slopes —yet with all this sameness the details of rock-sculpturing may be singularly varied. And the same is

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