The basic rocks present far more rugged outlines. When a cauldron occupied by basic diorite or by gabbro comes under denuding action, the numerous crossing joints oppose the formation of domes or tables. The weather widens one groove here, another there; the rock breaks away in angular fragments rather than as a powder over a broad surface, and serrated edges and jagged pinnacles arise along the crests. The diorites among our old metamorphic rocks in Scotland or in Ireland can be recognised on the sky-line at considerable distances. Sir A. Geikie, in his "Scenery of Scotland," has made the contrast between granite and gabbro in the centre of the Isle of Skye familiar to all geologists. Here the two types of rock were erupted at no long interval, and they have been exposed to denudation under the same conditions. J. Macculloch dwelt in 1819[91] on the relative resistance of the gabbro and the rapid disintegration of the granite hills, quaintly remarking of the latter that "the loose stones, by their constant descent from the summits, obscure the rocky surface, covering the sides with long torrents of red rubbish even more unpleasing to the sight than their conoidal forms." Macculloch noted that the loose blocks in the gabbro region lay much as they had fallen, without the production of a sand. In most mountain-chains produced by folding, igneous matter has been forced up as an accompaniment of the earth-movements. The local knots and laccolites, or the great cores admitted along certain anticlines, stand out on weathering among schistose or stratified hills. Their surfaces are marked by accidents, and each peak as it comes into view offers something of a new surprise. The wall of Mont Blanc from the angle near Entrèves, and the huge crag of the Matterhorn above the valley of the Visp, have illustrated to every traveller the dominance of igneous masses in the landscape. In our own islands, the granites of Ben Cruachan and Cairn Gorm have resisted long ages of denudation; an intrusive sheet of finer grain forms the long sheer wall of Cader Idris; while obsidian lava-flows, now grey and dull and crystalline, have furnished on Snowdon the finest scenery of Wales. The fortress-town of Edinburgh has arisen on the relics of a dead volcano; and the high moor of Leinster, so long the peril of the English, records an igneous cauldron that has been exposed to denudation from the opening of Devonian times. CHAPTER VI METAMORPHIC ROCKS INTRODUCTION[92]