412 THE WALLS OF VEINS. 



local production of metallic substances is in any special manner de- 

 pendent upon the chemical or mineralogical composition, or the 

 circumstances of the formation of the adjacent rocks, though in many 

 instances we observe the aggregation of the substances in the vein to 

 have been influenced by peculiar conditions of the including rocks. 



Alteration of Substance of Walls of a Vein. The walls or 

 cheeks which form the more or less definite boundaries of the vein 

 are in some instances highly indurated, and very often fissured, so 

 as to break parallel to the vein ; in other cases certain sorts of rock 

 (as clay slate, both in Cornwall and Germany) are greatly softened, 

 and even converted to clay, along one or both sides of a vein. 

 Werner mentions the decomposition of f elspathic and hornblendic rocks 

 for a fathom from the vein. We have also witnessed the fact of lime- 

 stone, usually a blue or grey crinoidal rock, burnt, as the miners term 

 it; that is, converted to a brown granular crystalline rock in Tees- 

 dale. Another remarkable effect in the walls is the production of 

 slickenside, so long known in the mines of Derbyshire, which are 

 situated in limestone, and tilled with fluoric and barytic spars, and 

 yield lead ; in those of Cornwall, which are in killas, and with a 

 matrix of quartz, and yield copper ; in the magnesian limestone of 

 Yorkshire, where copper or lead lines the limestone cheeks ; and in 

 the faults of the coal system of Yorkshire, where neither spar nor 

 metallic matters are common. These and many other occurrences of 

 rubbed surfaces along planes of fissures speak a clear language, and 

 prove to the fullest conviction the mechanical movement of the sides 

 of the fissure upon one another or upon the contained substances. 

 The groovings of the surfaces, thus produced by rubbing, indicate, of 

 course, the line of the movement ; the circumstance that the polished 

 faces are partially covered by lead ore, copper ore, &c., as the nature 

 of the vein is, proves, moreover, that the movement was, in such 

 cases, posterior to the introduction of the whole or a part of the 

 mineral impregnation, so that the same fissure has been, in such 

 cases, the plane of more than one convulsive movement. 



Eelation of Veins to the Different Ages of Rocks. It is in 

 the palaeozoic rocks, and in the metamorphic and igneous rocks 

 associated with them, that all the veins in Great Britain are worked. 

 In a few instances veins of small value, producing lead and copper, 

 pass through the magnesian limestone ; but not a single example is 

 known of a true metallic vein in the Oolitic, Cretaceous, or Tertiary 

 strata. The connection of metallic veins with the older rocks is 

 not an accidental coincidence, but a constantly recurring pheno- 

 menon; and the absence of such veins from the newer strata in 

 England cannot be accounted for by any circumstances of the geo- 

 graphical position of these strata ; for both around the metalliferous 

 slates of Cumberland and limestones of Derbyshire the trias is exten- 

 sively spread, and yet not one lead or copper vein occurs in it. Any 

 one who should confine his attention to the British Isles might infer 

 that the causes of the production of mineral veins had been almost 

 wholly inactive ever since the Carboniferous epoch ; and as a general 



