340 ORIGIN AND DEVELOPMENT OP OBE-DEPOSITS. 



talline aggregates of chalybite, which had been subsequently 

 permeated by siliceous solutions, and so coated with quartz. A 

 similar oxidation of sulphides and removal or alteration of 

 carbonates has been observed in all mining countries, and to 

 this is due the greater part of the difference between "free- 

 milling " and " refractory " gold and silver ores in the Eocky 

 Mountains. 



The production of gozzan seems in most eases to be due to 

 the action of surface-waters percolating through the fissures or 

 their mineral contents. Hence it is, that when once the water- 

 level is reached, or where the lode is so solid as not to admit of 

 any considerable circulation, there is little or no oxidation- 

 "When, however, the gozzan extends below the water-level, it is 

 more likely to have been produced by thermal springs since 

 exhausted, and the large quantities of gozzan material spread 

 over the surface in some situations supports this view, and 

 evidences the recent character of the extinction. 



The experiments of Fox, Becquerel, and Hunt, show that 

 electrically-excited salt water acts powerfully on chalcopyrite, 

 converting it into erubescite and setting free peroxide of 

 iron. Supposing this process to take place in the depths of the 

 earth in connexion with an upward current, the iron oxide would 

 be deposited at or near the surface as gozzan. In this case, the 

 gozzan would be an indication not of bodies of iron-ore but of 

 chalcopyrite and erubescite, and traces of copper would be more 

 or less abundant in it.* Some fiirther characters of gozzans 

 and of their relations in their underlying mineral deposits, as 

 well as to the surface contours of the regions in which they 

 occur, will be dealt with in the fourth chapter.f 



* In this connexion some recent experiments of Mr. W. N. Steernwitz are of 

 interest. He placed crystals of ferrous sulphate in a cold solution of an alkaline 

 silicate. After a time, thin nearly colourless threads began to rise from the 

 crystal through the solution to the surface, where they became oxidised, spread 

 about, and finally were deposited as a brown ferruginous silicate " resembling 

 that of the ii'on outcrops which indicate ore-veins." When salts of other metals 

 were present, traces of them were found in the ferruginous deposit. (See School 

 Mines Quarterly, U.S.A., 12, 181-186.) 



fFor interesting particulars relating to gozzans, see Henwood, Trans. 

 B.O.S.C., v. ; Argall, Rep. M.A., 1864and 1872 ; Phipson, Mining Journ., 1864 ; 

 ColHns, Joiim. E.7.C., 1888. 



