METALLIC VEINS. 



287 



Somelimes, the ore extends in a compact mass, from one side of 

 the vein to the other; but, not unfrequently, there are hollow spaces 

 in veins, called druses^ which are lined with crystals; in these cavi- 

 ties, the most beautiful and regular crystalline forms are obtained. 

 Metallic veins, often, divide and unite again, and, sometimes, they 

 separate into a number of smaller branches, called strings. A gen- 

 eral idea of the different modes in which metallic veins intersect 

 rocks, and are sometimes intersected by each other, is represented in 

 I Plate IV. fig. 4. 



To what depth metallic veins descend, is not known, nor is it as- 

 certained, whether they generally grow wider or narrower in their 

 descent. The opinions of miners on this subject are so various,, 

 that it may fairly be inferred, that veins differ, in this respect, in dif- 

 ! ferent situations. No instances, I believe, have occurred of a vein 

 j being absolutely worked-out in depth, though it often grows too poor 

 ' to repay the labour of working deeper : more frequently, the further 

 descent of the miner is stopped, by the difficulty or expense of re- 

 moving the water. Veins are seldom rich in ore near the surface, 

 ibut increase in richness as they descend, and at greater depths be- 

 come poorer again. When Pryce wrote the "Mineralogy of Corn- 

 iwall," it was believed that the richest state of a mine for copper in 

 that county, was from eighty to one hundred yards deep; and for 

 tin, from forty to one hundred and twenty yards. This account by 

 no means agrees with the present state of the Cornish mines. Cop- 

 per and tin are procured in considerable quantities at the depth of 

 four hundred and fifty-six yards, in the Dolcoath mine. The Ecton 

 copper mine, in Staffordshire, is now worked at the depth of four 

 hundred and seventy-two yards : it is the deepest mine in England. 

 The deepest mine that has been worked in Europe, or in any part of 

 the world, is one at Truttenberg, in Bohemia, which is one thousand 

 yards below the surface. 



'I Metallic veins frequently contain different ores at various depths. 

 Ilron ore, copper ore, cobalt ore, and silver ore, succeed each other 

 in some of the mines in Saxony, 



In France, there are mines which contain copper ore in the lowest 

 part, silver ore above, and over that iron ore. 



I In Cornwall, blende, a sulphuret of zinc, frequently abounds in 

 the upper part of veins that become rich in copper as they descend ; 

 the blende rarely continuing to any considerable depth. In the same 

 district, tin is also commonly found at a small depth, in veins which 

 afterwards prove rich in copper. " Among other instances that might 

 be quoted, are the two deep extensive copper mines called Huel 

 Unity, and Cook's Kitchen, both of which were at first worked for 

 tin. In both, the tin was soon extracted ; but it should be noted as 

 an uncommon circumstance, that in the latter mine, after working to 

 the depth of one hundred and eighty fathoms, first through tin, and 

 afterwards through copper, tin was found again, and has continued 



