Metalliferous Veins. 157 



more quartzose, and in the slate it is also accomplished by the 

 latter undergoing a like change ? Sometimes, indeed, the vein 

 itself, at these points of union, appears to partake of the nature 

 of the containing rock ; but much more commonly it entirely 

 includes portions of the rock of various dimensions, according 

 to the size of the vein. 



These horses are of the same nature as the contiguous rock, 

 being slaty when the walls of the vein are slate, and granitic 

 when they are of granite. 



As a general fact, though, with many exceptions, it may 

 be said that tin ore prevails in the granite, and copper ores 

 in the slate ; notwithstanding it may, perhaps, be true, that 

 the largest single masses may have been found in the oppo- 

 site rock, of tin ore in slate, at Wheal Vor, and of copper in 

 the granite, at Tresavean ; for example, a pretty fact of the 

 prevalence of different ores in various rocks, I noticed in Bo- 

 tallack. There were two or three alternations of granite and 

 slate of no great extent ; the lode, when in the granite, contain- 

 ed tin-ore, and when in the slate, copper. Indeed, it is a very 

 well known fact, that the same vein is seldom productive in two 

 different rocks ; thus the immense mass of tin-ore, I believe 

 more than a million sterling worth, in Wheal Vor, was in slate, 

 whilst the same vein is entirely unproductive in the granite. 

 The adjoining mine of Great Work gives all its tin-ore in gra- 

 nite, and is poor in the slate. Again, the lode of Tresavean 

 yields its copper-ores in the granite, being worthless in the 

 slate ; whilst the neighbouring mines have given almost the 

 whole of their copper in the latter. These respectively are on 

 parallel veins. There is a prevailing proverb of " ore against 

 ore, 1 ' meaning that in the same neighbourhood there is a greater 

 probability of it in parallel veins, near the same north and south 

 line, than eastward or westward, even on the same vein. 



It has been already quoted, that even the richest metallife- 

 rous veins contain, compared with their total mass, but a small 

 proportion of ore, and that this is irregularly distributed. 

 These masses are called shuts or shoots, and appear by their 

 dip in the vein as if obedient to some influence of the granitic 

 masses in their vicinity, always dipping from and seldom to- 



