1104 A TREATISE ON METAMORPHISM. 



which small amounts of copper are not found. Almost every porous amyg- 

 daloid shows flakes of it. In many localities it is so abundant that exten- 

 sive exploration has been undertaken, with the hope of finding- large ore 

 bodies, as, for instance, in Douglas County, Wis., Isle Royale, and Mamainse. 

 But all of these explorations have resulted in failure. To me the almost 

 universal association of small quantities of copper with the Keweenawan 

 lavas is the most conclusive evidence that these lavas are the source of 

 the metal. 



While copper commonly occurs in many of the igneous rocks in its 

 native state, especially in those which are porous, it is found in the less 

 porous and therefore less altered rocks in minute quantities in the form of 

 a sulphide, and this is thought to be the original form of the metal. That 

 is to say, at the time the lavas crystallized the copper separated as copper 

 sulphide or copper-iron sulphide. When the lavas were upturned by the 

 formation of the Lake Superior syncline, and denudation began truncat- 

 ing them, the segregation of the copper deposits was inaugurated. The 

 descending oxidizing waters transformed the copper sulphides into copper 

 sulphates and took them into solution. The underground water was finally 

 converged into trunk channels, and there met solutions bearing ferrous salts 

 or came into contact with ferrous compounds. At such places reduction 

 and precipitation took place. Where there were good trunk channels, as 

 in the upper surfaces of many of the amygdaloids, there was sufficient segre- 

 gration to encourage widespread exploration, as has already been noted. 

 But only a few of the amygdaloids were open and scoriaceous enough to 

 become the centering points of a sufficiently extended circulation to pro- 

 duce workable ore deposits. The greatest trunk channels were in the con- 

 glomerates. Where these conglomerates were interstratified with abundant 

 lavas bearing a sufficient amount of copper, and there existed other neces- 

 sary favorable conditions, the rising circulation hao fortunately segregated 

 the metal in great quantity. The most notable of these deposits is in the 

 conglomerate upon which the Calumet and Hecla and the Tamarack mines 

 are located 



SILPHIDES. 



It has been stated that metals which occur as sulphides comprise iron, 

 copper, lead, zinc, nickel, arsenic, antimony, mercury, and silver. Of 

 these sulphides iron is the most abundant. Indeed, the sulphides of iron — 



