PRECIPITATION OF COPPER. 1103 



to say that where the ferrous salts are very abundant well-known chemical 

 principles lead to the conclusion that a partial oxidation of the ferrous 

 compound is far more probable, since by the reduction of the copper an 

 amount of oxygen is liberated sufficient to transform only a small part of 

 the iron to the ferric state. In this connection it is to be remarked that in 

 the Lake Superior native copper deposits ferrous compounds are very 

 abundant, and magnetite is a very common mineral, both in the amygdaloid 

 and conglomerate deposits. For example, Irving illustrates the intimate 

 association of the native copper and the magnetite in the cupriferous sand- 

 stones of the Nonesuch mine, the copper frequently surrounding grains of 

 magnetite." My first interpretation of this relation was that the magnetite 

 was probably the reducing agent which threw down the copper, but if this 

 were so it should be transformed to hematite. My present view is that 

 the native copper and the magnetite were both precipitated as a result 

 of the reaction of ferrous salts upon copper salts. According to Stokes's 

 work these would not be simultaneously precipitated at the same place. 

 The magnetite would form where the temperature is higher and the copper 

 where the temperature is lower. To explain the existence of both at the 

 same place one is obliged to suppose that the magnetite formed first, 

 and that later when the temperature was lower the copper was thrown 

 down. It is notable that in the illustrative case mentioned, the copper 

 does surround the grains of magnetite, and thus corresponds with Stokes's 

 experimental work. 



It is hardly necessary to say that in one district the chief reaction 

 precipitating the copper may produce ferric salt and in another district may 

 produce magnetite, while in still other districts the precipitation of the 

 copper may be due to the combination of both reactions. 



The Lake Superior copper deposits are believed to be an ideal case of 

 ores deposited by ascending waters, the sources of which are the igneous 

 rocks of the Keweenawan. In this region the only locality at which the 

 ore has been found in paying quantities is at Keweenaw Point, and the 

 productive district is at present confined to a very small area about Calu- 

 met and Houghton. Notwithstanding' this fact, there is scarcely a locality 

 in the Lake Superior region where the Keweenawan basic lavas occur in 



a Irving, R. D., The copper-bearing rocks of Lake Superior: Mon. IT. S. Geo]. Survey, vol. 5, 18S3, 

 fig. 1, PI. XVI, p. 127. (Description of same pp. 131-132.) 



