Granite, which seems to be the base of the whole continent, is widely spread to the east and south: it appears in Tierra del Fuego and in the Patagonian Andes abundantly, and at great elevations, and in Chile and southern Peru forms the line of hills parallel to the Pacific, and where are situated the mineral riches of the former republic; but it comes into view so rarely in the northern parts of the chain, that Baron Humboldt says a person might travel years in the Andes of Peru and Quito without falling in with it. He never saw it at a greater height above the sea than 11,500 feet. Gneiss is here and there associated with the granite, but micaschist is by much the most common of the crystalline rocks. Quartz rock, probably of the Devonian period, is much developed, generally mixed with mica, and rich in gold and specular iron. It sometimes extends several leagues in the western declivities of Peru 6000 feet thick. Red sandstone, with its gypseous and saliferous marls, of the age of our English red marl, of vast dimensions, occurs in the Andes, and on the table-land east of them, where, in some places, as in Colombia, it spreads over thousands of square miles to the shores of the Atlantic. It is widely extended at altitudes of 10,000 and 12,000 feet— for example, on the plains of Tarqui and in the valley of Cuença. Coal is sometimes associated with it, and is found in the Andes of Pasco, in Peru, 14,750 feet above the sea. Porphyry abounds all over the Andes, from Patagonia to Colombia, at every elevation, on the slopes and summits of the mountains rising to the greatest elevation, but of very different ages and mineralogical characters. One variety which frequently occurs is rich in metals, and hence has been designated as _metalliferous_: in it are situated some of the most celebrated silver mines of Peru, those of Potosi, Oruro, Puno. The bare and precipitous porphyry-rocks give great variety to the colouring of the Andes, especially in Chile, where purple, tile-red, and brown are contrasted with the snow on the summit of the chain.[55]