232 CORNISH EL VANS. 



gated. The quartz, instead of forming a kind of crystalline residue as 

 in granite, is usually enclosed with the crystallised felspar in a f el- 

 spathic or quartzose-felspathic base. Mica, schorl, chlorite, and pinite 

 are sometimes accessory minerals ; the quartz is often in double hexa- 

 gonal pyramids with the angles removed and rounded. The felspar 

 is usually in large well-defined crystals, though sometimes so minute 

 as only to be seen with the lens. When the elvan occurs in granite, 

 its texture is generally finer than when it cuts through slate. It 

 rarely penetrates between the cleavage planes of slate. In the prin- 

 cipal mining districts the elvans are parallel to most of the productive 

 tin and copper lodes, running a few degrees north of east, though in 

 other parts of the country the elvans run nearly north and south, and 

 almost coincide in direction with the cross courses which often yield 

 ores of lead and iron. Under the microscope the quartz is seen to 

 contain hair-like crystals of schorl. 1 



Distribution of Elvans. The elvan dykes of Cornwall vary from 

 a breadth of a few feet up to 300 or 400 feet ; they are parallel to 

 each other, and can be traced for several miles. One of the longest 

 runs from Wheal Darlington, near Marazion, for twelve miles by 

 Wheal Fortune, Corbus, Treganhorn, Cayle, Roseworthy, and Cam- 

 borne to Pool, sending off a branch near Cayle about five miles long, 

 which passes by Carnbrae Green, Cassawson, and Tregear into the 

 Carnbrea granite on the west of Camborne Beacon. Another runs 

 along Penzarrce Pier. Only one has been observed in the Scilly Isles, 

 on the northern part of St. Mary's, running 25 N.W. It has a grey 

 felspatho-quartzose base, containing crystals of light-coloured felspar 

 and quartz. At the Land's End elvans sometimes contain schorl ; in 

 the central part of the dyke they are often granitic in texture. The 

 elvan which runs through St. Hilary abounds in crystals of pinite. 

 Elvans are numerous between Redruth and Gwennap, and seem to cut 

 into the granite. The elvan on the west of Killaganoon is so decom- 

 posed as to be worked for crucible clays, but they are usually hard. The 

 colour of the rock sometimes changes in a short distance from a rose 

 tint to greenish, and the texture from porphyritic to arenaceous. The 

 Cubert elvan in the centre is a compound of quartz-felspar and mica, 

 though the mica is rare. The upper part of the elvan of "Watergate 

 Bay appears to have a concretionary structure. North of the Hens- 

 borough granite the elvans are very variable in colour. A group of 

 elvans near St. Austell runs 20 N.E. They sometimes contain nests 

 of sulphide of copper, and occasionally small nests of plumbago. The 

 Pentnau elvan contains fragments of slate rocks, especially in the 

 branch from it which runs along the shore to the Black's Head. 

 Elvans also occur round Brown Willy. The elvan called Roborough 

 stone, on Morwell Downs, abounds in crystals of quartz formed of 

 hexagonal pyramids base to base. Elvans are much divided by joints 

 like the granite. Elvans everywhere have the appearance of having 

 been produced towards the close of the eruption or upheaval of the 

 granite masses near which they occur. 



1 Q. J. G. S., J. A. Phillips, vol. xxxi. p 334. 



