It can hardly be too often repeated that the fundamental law in the universal decay and sculpture of the land is that the waste is proportioned to the resistance offered to it: the softer rocks are worn down with comparative rapidity, while the harder varieties are left projecting above them. As a general rule, volcanic rocks are more durable than those among which they are interstratified, and hence project above them, but this is not always the case. No universal rule can, indeed, be laid down with regard to the relative durability of any rocks. While, therefore, topographic contours afford a valuable indication of the nature and disposition of the rocks below the surface, they cannot be relied upon as in all circumstances an infallible guide in this respect. No better proof can be offered of the caution that is needed in tracing such contours back to their origin than is furnished by the old volcanic rocks of Britain. These eruptive masses, consisting usually of durable materials and ranging through a vast cycle of geological time, usually rise into prominent features and thus support the general law. But they include also many easily eroded members, which, instead of forming eminences, are worn into hollows. They include, in short, every type of scenery, from featureless plains and rolling lowlands to craggy and spiry mountains. The first point, then, which is established in an investigation of the topographical influence of old volcanic rocks is that their prevailing prominence arises from relative durability amidst universal degradation. When we proceed further to inquire why they vary so much from each other in different places, and how their complicated details of feature have been elaborated, we soon learn that such local peculiarities have arisen mainly from variations in the internal structure and grouping of the rocks themselves. Here again the general law of sculpture comes into play. The local features have depended upon the comparative resistance offered to the sculpturing agents by the different portions of a volcanic series. Each distinct variety of rock possesses its own characteristic internal structure. The lines along which atmospheric disintegration will most effectually carry on its carving work are thus already traced in the very substance and architecture of the rock itself. Each rock consequently yields in its own way to the processes of disintegration, and thus contributes its own distinctive share to topographical feature.