All the various kinds of rocks are differently affected by the atmospheric influences of decay, and so present different external appearances and shapes, so that after a little experience the geologist can recognize the presence of certain rocks by the kind of scenery they produce; and this knowledge is often of great use in helping him to unravel the geological structure of a difficult region. Thus granite, crystalline schists, slates, sandstones, and limestones, all "weather" in their own ways, and moreover split up differently, because their joints and other natural lines of division run in different ways.
Thus granite is jointed very regularly, some of the joints running straight down and others running horizontally, so that the rain and atmosphere seize on these lines and widen them very considerably; and thus the granite is weathered out either in tall upright columns, like those seen at Land's End, or else into great square-shaped blocks with their corners rounded off, presenting the appearance of a number of knapsacks lying one over the other. In this way we can account for the well-known "Tors" of Devonshire, and the "Rocking Stones." Granite weathers rapidly along its joints, and its surfaces crumble away more rapidly than might be expected, considering how hard a rock it is; but the felspar which is its chief mineral constituent is readily decomposed by rain water, which acts chemically upon it. The deposits of China clay in Devonshire are the result of the decomposition and washing away of the granite of Dartmoor.
Granite mountains are generally rounded and "bossy," breaking now and then into cliffs, the faces of which are riven by huge joints, and present a very different appearance from those composed of crystalline schists with their sharp crests and peaks. Ben Nevis and the Cairngorms are partly composed of granite.
Gneiss is a rock composed of the same minerals as granite; namely, mica, quartz, and felspar. And yet mountains composed of this rock have quite a different aspect, and sometimes, as in the Alps, produce very sharp and jagged pinnacles. The reason of this is that gneiss splits in a different way from granite, because its minerals are arranged in layers, and so it is more like a crystalline schist.
Mica-schist is another rock very abundant in mountain regions. This rock is composed of quartz and mica arranged in wavy layers. The mica, which is very conspicuous, lies in thin plates, sometimes so dovetailed into each other as to form long continuous layers separating it from those of the quartz; and it readily splits along the layers of mica. This mineral is easily recognised by its bright, shiny surface. There are, however, two varieties,—one of a light colour and the other black.
Mica-schist and gneiss are often found in the same region, and are the materials of which most of the highest peaks in Europe are composed. We find them abounding in the district of Mont Blanc; and all the monarch's attendant aiguilles, with the splintered ridges enclosing the great snowfields in the heart of the chain, consist mostly of these two rocks. The Matterhorn, Weisshorn, Monte Viso, the Grand Paradis, the Aiguille Verte and Aiguille du Dru are examples of the wonderful forms produced by the breaking up and decay of these two rocks.
The different varieties of slate split in a very marked way. Slates are often associated with the schists, and exert their influence in modifying the scenery.
Limestone ranges, though less striking in the outlines of their crests than those composed of slates and crystalline schists, and not reaching to such heights, are nevertheless not at all inferior in the grandeur of their cliffs, which frequently extend for miles along the side of a valley in vast terraces, whose precipitous walls are often absolutely inaccessible. The beauty of limestone mountains is often enhanced by the rich pastures and forests which clothe their lower slopes. The dolomitic limestone of the Italian Tyrol, being gashed by enormous vertical joints and at the same time having been formed in rather thin layers which break up into small blocks, produces some very striking scenery. But wild as these mountainous ridges may be, their forms can never be confounded with those of the crystalline schists; for however sharp their pinnacles may appear at first sight, careful examination will always show that their outline is that of ruined masonry, suggesting crumbling battlements and tottering turrets, and not the curving, flame-like crests and splintered peaks of the crystalline schists.[30]
It has already been explained that all sedimentary rocks have been formed under water in layers or strata, and it must be obvious that the stratification of such rocks has an important influence on scenery; and very much depends on whether the strata have been left undisturbed, with perhaps just a slight slope, or whether they have been folded and crumpled; for the position of the strata, or "bedding," as it is called,—whether flat, inclined, vertical, or contorted,—largely determines the nature of the surface. Undoubtedly the most characteristic scenery formed by stratified rocks is to be seen in those places where the "bedding" is horizontal, or nearly so, and the strata are massive. A mountain constructed of such materials appears as a colossal pyramid, the level lines of stratification looking like great courses of masonry. The joints that cut across the strata allow it to be cleft into great blocks and deep chasms; so that, as in the case of the dolomitic limestone above mentioned, we find a resemblance to ruined buildings.
We cannot find a better example of this in our own country than the mountains of sandstone and conglomerate (of the Cambrian age) that here and there lie on the great platform of old gneiss in the west of Sutherland and Ross. Sir A. Geikie says,—