308 THE JOURNAL OF GEOLOGY. 



the ore deposit have been concentrated partly by segregation 

 during alteration, and partly by the leaching of the asso- 

 ciated materials. As a result of this, these minerals occur as 

 seams, pockets or irregular bodies, often a hundred feet or more 

 in diameter, generally enclosed by, and often intimately asso- 

 ciated with, the oxidized iron materials which represent the 

 gangue. 



In the case of the Arizona deposits, alteration has progressed 

 just far enough to increase greatly the value of the deposits with- 

 out to any extent injuring it. Such products of alteration, how- 

 ever, are more or less soluble in surface waters containing various 

 organic and inorganic compounds, so that in a moist climate there 

 is a constant tendency to leach them out and leave only the less 

 soluble parts of the gangue. In Arizona, this stage has not yet 

 progressed to a noticeable degree, and one reason for this may 

 be the extreme dryness of the climate, which affords opportunity 

 for only comparatively slight percolation of surface waters. 



In the copper deposits of Montana and the Appalachian 

 region, however, a further stage of alteration is often observable. 

 The copper deposits at Butte City, Montana, are composed 

 largely of chalcocite, with copper pyrites, bornite, enargite, iron 

 pyrites and other minerals in a siliceous gangue. On the surface 

 the copper in these deposits has been almost entirely oxidized 

 and leached out, and the ore consists of a porous, rusty, siliceous 

 mass which was once mined for the small percentage of silver it 

 contained. As depths were reached, the oxidized copper min- 

 erals began to appear, and eventually the sulphides formed the 

 mass of the veins. In this case, a further stage of alteration is 

 seen than that in Arizona. 



At Ducktown in eastern Tennessee,' deposits of mixed iron 

 and copper pyrites occur and have been altered in a somewhat 

 similar manner on the surface. The copper minerals have been 

 leached out of the ferruginous gangue in the upper parts of the 

 deposits, and for a depth of from 20 to 80 feet or more, the 

 deposits are composed simply of a porous mass of more or less 



' J. D. W^HITNEY, The Metallic Wealth of the United States, pp. 322-324. 



