ORIGIN OF SOME ORES OF COPPER. 173 



evolved and a mixture of ferric hydrate and hydrated cu- 

 prous oxide is precipitated, the latter becoming crystalline 

 after the expiration of some time. 



Of the three methods given above, that of Knop undoubt- 

 edly furnishes one of the most probable processes by which 

 many large deposits of the so-called tile-ore and copper- 

 pitchblende have arisen ; but there are again many other 

 deposits of cuprite which may easily have been formed in 

 other ways. I will not discuss at the present time the 

 various opinions upon the formation of copper-ores ex- 

 pressed by some very eminent and competent authorities ; 

 but I may mention that Dr. Ferdinand Wiebel, in an ex- 

 haustive treatise {' Das Gediegen-Kupfer und Roth-Kup- 

 fererz/ Hamburg, 1864), concludes that the native metal 

 and cuprite have in all cases been formed from a cupric 

 sulphate solution (obtained by the oxidation of copper- 

 pyrites) through its reduction by a solution of ferrous sul- 

 phate (also derit^ed from the oxidation of copper-pyrites). 

 This statement is doubtless correct in the case of some 

 formations ; but there are many others where it is ques- 

 tionable. Gustav Bischoff points out, in his ' Chemical and 

 Physical Geology,^ that " It is certain that the contents of 

 a lode are of later date than the adjoining rock ; so that it 

 can be shown that these contents originate from the adjoin- 

 ing rock. If, then, it can be ascertained what compounds 

 of the metal exist in that rock, the previously existing 

 minerals may be distinguished from those of later date ; 

 but the determination of this point is attended with great 

 difficulties, and is generally impossible.'^ It is a well-as- 

 certained fact that all the ores of copper are found mostly 

 in crystalline rocks or metamorphic rocks derived from 

 them (some ores, however, being found in newer formations) ; 

 and Struve (' Ueber die Nachbildung der natiirlichcn Hc;il- 

 qucUen,' Heft ii. 17) has proved in the most conclusive 



