MINERAL VEINS. 



239 



according to the metallic contents, but also according to the nature of 

 the vein-stuffs, and therefore must be learned by observation in each 

 country. We will give three of the most constant as illustrations. 



Cupriferous Veins. — The original form in which copper seems to 

 exist in veins is copper pyrites, a double sulphide of copper and iron 

 (CuFeS 2 ). Now, along the bach or outcrop of copper-veins, to a depth 

 of thirty to sixty feet, the vein usually contains no copper at all, but 

 consists of vein-stuff (more or less changed, according to its nature), 

 among which are scattered masses of a dark reddish or brownish 

 hydrated peroxide of iron, in a light, spongy condition. This peculiar 

 form of peroxide of iron, so characteristic of the outcrop of copper- 

 veins, is called by the Cornish miners gossan, and by the German and 

 French miners iron hat (eiserner hut ; chapeau defer'). Below the in- 

 fluence of atmospheric agencies the vein is in its original condition, i. e., 

 consists of vein-stone containing disseminated masses of copper pyrites. 

 Just at the junction of the changed with the unchanged vein — i. e., 

 running along the back of the vein at a depth varying from thirty to 

 sixty feet — occur rich accumulations of copper, as native copper, red 

 and black oxides of copper, green and blue carbouates of copper, etc. 

 These facts are illustrated by Fig. 216, which is a section of the Duck, 



town mines of Tennessee. The 

 irregular line, s s, is the out- 

 line of a hill, along the crest of 

 which the vein outcrops ; the 

 part b consists almost wholly 

 of gossan, with only small 

 masses of quartz- vein stuff ; a 

 i» the rich accumulation of 

 copper ore, here about two or 

 three feet thick ; and c is the 

 unchanged vein, consisting of 

 vein-stuff, inclosing arsenical 

 pyrites, and copper pyrites in 

 very large quantities. 

 These phenomena may be explained as follows : There can be no 

 doubt that the gossan represents copper pyrites, from which the copper 

 has been entirely washed out, leaving the iron in an oxidized condition. 

 Thus the whole of the copper from b (and probably from much more 

 than b, for the process of denudation has gone on pari passu with the 

 process of leaching) has been leached out and accumulated at a. Fur- 

 ther, it is probable that the process was as follows : When copper 

 pyrites is exposed to moist air it slowly oxidizes into sulphates of iron 

 and copper (CuFeS 2 +80=FeS0 4 +CuS0 4 ). The iron sulphate (prob- 

 ably assisted by reaction with alkaline or earthy carbonates) quickly 



Fig. 216.— Ducktown (Tennessee) Copper Ve^n, show- 

 ing Surface-Changes (after Safford). 



