240 STRUCTURE COMMON TO ALL ROCKS. 



passes into ferric oxide and is left in a spongy condition, while the 

 copper sulphate is carried downward. This much seems certain, but, 

 by what subsequent process the copper takes all the forms actually 

 found at «, is little understood, although it is probable that the car- 

 bonate is produced by the reaction, on the sulphate, of waters contain- 

 ing alkaline carbonate or bicarbonate of lime.* 



Plumbiferous Veins. — The natural or original form in which lead 

 occurs in veins is sulphide of lead, or galena. But along the backs or 

 outcrops of lead- veins it is found more commonly as carbonate.. The 

 explanation seems to be as follows : Lead occurs mostly in veins inter- 

 secting, or in sheets between, strata of limestones. It is probable that 

 the galena (PbS) is oxidized by meteoric agencies and becomes sulphate 

 (PbS0 4 ), and then the sulphate, by reaction with the carbonate of lime 

 derived from the wall-rock or from the calc-spar of the vein-stuff, be- 

 comes carbonate, thus: PbS0 4 +CaC0 3 =PbC0 3 +CaS0 4 . In proof 

 of this process it is stated f that galena, thrown out of the old mines of 

 Derbyshire among rubbish of limestone, has all, in the course of ages, 

 been changed into carbonate. Moreover, it is not uncommon to find 

 in lead veins masses of sulphide changed on the outside into carbonate. 



Auriferous Quartz- Veins. — Gold is found either in quartz- veins, in- 

 tersecting metamorphic slates (quartz-mines) or in gravel-drifts in the 

 vicinity of these (placer-mines). Originally it existed in the quartz- 

 veins usually associated with metallic sulphides, particularly the sul- 

 phide of iron (pyrites). If the pyrites be dissolved in nitric acid, the 

 gold is left as minute threads and crystals. Evidently, therefore, it exists 

 in minute threads and crystals scattered through the pyrites. Now, 

 when such a vein is exposed to meteoric agencies, the pyrites is oxi- 

 dized, partly as soluble sulphate, and carried away, and partly as insol- 

 uble reddish peroxide, which remains. J The quartz-vein stone is, there- 

 fore, left in a honey-comb condition by the removal of the pyrites, and 

 more commonly stained of a rusty color by the peroxide. Among the 

 cells of this rusty cellular quartz the gold is found in minute, sharp 

 grains, evidently left by the removal of the pyrites. Hence, in an 

 auriferous quartz-vein, along the outcrop to a depth of thirty to sixty 

 feet (i. e., as far as meteoric agencies extend) gold is found free in 

 small grains among the cellular quartz ; but below the reach of these 

 agencies it is inclosed in the undecomposed pyrites. 



Placer-Mines. — If a mountain-slope, along which outcrop auriferous 

 quartz-veins, be subjected to powerful erosion by water-currents, then 



* Biscbof, Chemical and Physical Geology, vol. iii, p. 509. 



f De la Beche, Geological Observer, p. 794. 



% Probably the iron sulphide is oxidized to the condition of sulphate, then reduced to 

 carbonate by water containing alkaline carbonate or bicarbonate of lime, and lastly per- 

 oxidized by exchanging carbonic acid for oxygen (Bischof). 



