S50 Mineralosical Observations on Cornwall. [NoV» 



sence of metal. It seems to be composed of nothing else than 

 silica and alumina. I do not know, indeed, that it has ever 

 been subjected to a chemical analysis ; but this ought to be its 

 composition, on the supposition that it consists of pure felspar 

 and quartz, and that no metallic matter is present. 



Almost all the mines in Cornwall are confined to a tract about 

 six miles broad, running from the Land's End to the extremity 

 of Dartmoor, seemingly in the same direction with the granite 

 range, and just beside it. The ores are all in veins, the most 

 important of which run in an east and west direction. It has 

 been observed that the principal tin veins run from south-west 

 to north-east, while thejirincipal copper veins run from west to 

 east. There are a considerable number of these veins not 

 exactly parallel to each other, but approaching to that direction. 

 In Dolcoath mine, for example, I counted eight veins. These 

 veins in Cornwall are called lodes. During their course they 

 vary considerably in thickness, and even in their direction. 

 They go to a very considerable depth ; and not one of them, as 

 far as I could learn, has ever been mined to its extreme depth. 

 The ores have generally been observed to increase in value as 

 the vein deepens. One of the deepest mines in Cornwall is 

 Dolcoath, about a mile south-west of Redruth. Its present 

 depth is 220 fathoms. I believe Huel Unity was still deeper ; 

 but of late the deepest part of it has been abandoned. This was 

 owing to a mine on the south side of it having been forsaken. 

 The water, in consequence, accumulated to such a degree that 

 the expense of drawing it up was greater than the mine could 

 afford. This obliged them to forsake the lowest part altogether. 

 It is common to find tin at the top of copper veins ; but never 

 copper at the top of tin veins. . The upper part of the vein is 

 called gossan by the miners. It contains less ore than the lower 

 part, and the veinstones are always stained yellowish red, as if 

 they had been steeped in ochre. The rock which borders on the 

 vein on both sides is called the country. This, about Redruth, 

 is always transition slate ; except at a considerable depth. Thus 

 at Huel Unity, at the depth of 50 fathoms, the country becomes 

 granite. To this granite they give the name of grauen. 



The veinstones are frequently quartz. Sometimes they consist 

 of hornstone porphyry, a stone to. which the miners give the 

 name of elvan. 1 got specimens of elvan from the overseers ot 

 five or six different mines, and all of them consisted of hornstone 

 porphyry, differing indeed somewhat in its colour, and in the 

 size of the crystals which it contained ; but in other respects the 

 same. Hence I conceive there can be no doubt that elvan in 

 Cornwall signifies hwnstone porphyry. Sometimes clay occurs 

 in the veins. To tills the miners give the name of jleuckan. 



The only copper ore of any consequence that occurs in these 



