Page:Cornwall (Salmon).djvu/22

 CORNWALL parts of Cornwall, Lower Silurian beds near Dodman Point, Pliocene beds at St. Agnes and St. Erth, and various raised beaches and tracts of blown sand round the coast. There are certain dykes or veins of cjuartz- felsite or quartz-porphyry locally known as Elvans. They traverse both granite and slates, and often coincide in direction with lodes. Elvan is a durable stone and useful for road- making and building purposes. Of the mineral products of Cornwall by far the most important from some points of view is tin, but this will be dealt with under the head- ing of mining. From the killas or clay slates in the Devonian rocks of Cornwall, in which tin mainly occurs, copper, silver, lead and iron are also extracted. Slates are largely procured from the same rocks, and at the Delabole Quarries slates of excellent quality for roofing purposes and for tombstones are dug. Rock crystals, called " Cornish dia- monds," are found in many parts of Cornwall. Serpentine, for which the Lizard district is famous, is a hydrous silicate of magnesia, etc., and is so called from the fancied resemblance of the rock to a serpent's skin. This stone, which may claim to be one of the most beautiful of English rocks, occurs principally in Kynance Cove, where it presents some re- markably beautiful colour effects, heightened by contrast with the white sands on the beach. Various fancy articles are made of Serpentine and sold to visitors.