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.