A HISTORY OF CORNWALL While subsequent upheaval followed by denudation has afforded us the opportunity of studying the sea bottoms of those ancient periods, it has also laid bare for our inspection the subterranean regions of volcanic energy. The deeper seated magmas, the cooling of which occupied long periods of time, are represented in Cornwall by the extensive masses of granite which enter so largely into the geology of the county. But be- sides the granite which forms a series of bosses along the watershed and constitutes the most elevated portions of the area, smaller intrusions of igneous material have been forced into the killas as sills and dykes, that are represented by the greenstones, mica traps and elvans. In discussing these various products of igneous intrusion sealed up within the killas, priority naturally attaches to the granite from its greater superficial extent, and the important position it occupies as the seat of the staple industries of the county ; in the raising of metal, stone and clay, both as regards the granite itself and the killas which flanks its margins. The granite which comes to the surface in Cornwall resolves itself into five well marked bosses, along a line which approximates to the central axis of the county. To the north-east lies the Brown Willy mass which extends over Bodmin Moors ; the Hensbarrow granite forms a somewhat smaller boss in the neighbourhood of St. Austell ; and west of the Falmouth estuary the Carn Menelez granite, with its two out- liers of Carnbrea and Carnmarth, occupies an extensive ring within the limits of Falmouth, Helston and Camborne. Another large mass forms the western peninsula of Cornwall extending from St. Ives to the Land's End ; while the fifth mass constitutes the Isles of Scilly. Of the smaller granite patches are those of St. Michael's Mount, and Tregonning and Godolphin Hills, those of Carnbrea and Carnmarth already noticed, Cligga Point (near St. Agnes), Castle-an-Dinas and Belovely Beacon (near St. Colomb Major and Roche), Kit Hill and Kingston Down (near Callington), and other minor intrusions in close proximity to the larger masses. The granites vary considerably in texture, but their mineral composition is so closely allied that we are driven to the conclusion that the various individual bosses derived their material from a common source of supply. Mining operations have demonstrated the underground connection of the granites of Carnbrea and Carn Menelez. Similarly the smaller patches which occasionally flank the larger bosses are in all probability united below ground with the main mass. From this reasoning it might be in- ferred that if we could strip off the overlying killas the larger granite masses would be similarly connected ; but in spite of their close petro- logical relationship these larger bosses have acquired a marked individu- ality both as regards their texture and the distribution of their chemical contents. It would appear therefore that each of these bosses marks the site of individual eruptive centres, the materials of which have been derived from a common magma at lower depths. Moreover, although 24
