XXIX. On Rocks and Minerals in Cornwall

Abstract

Mr. White said that the strata in Cornwall are chiefly Devonian, but are much broken up and metamorphosed by the action of igneous rocks, traps, porphyries (locally known as “Elvans”), and granites. These latter are of many varieties of hardness and texture, and are used—the harder, for ornamental and heavy building purposes, like our own Aberdeen, Peterhead, and Mull granites; the softer and more earthy varieties, for the manufacture of china clay, of which about 80,000 tons are produced annually. London Bridge, and Waterloo Bridge, in London, and the docks at Chatham, are built of Cornish granite. The chief feature of the county, however, and that which has brought Cornwall its greatest riches, is the abundance of its metallic veins. The Cornish mines have been known and worked from time immemorial, in the ordinary, though not the geological, sense of that well-worn phrase; it is known that long before the Christian era Phœnician sailors visited the coast and other parts of the county for the purpose of trading in tin and various other metals. Tin occurs nowhere else in the British islands, and is here found in two forms—stream tin and vein tin. The former is found from the size of small grains to lumps of 10lbs. to 12lbs. weight, while occasionally large masses of rock occur richly impregnated with the mineral. These are found scattered over the surface of the hard rocks and overlaid by soft deposits, and are also got in gullies and water-courses. As stream tin is