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 



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