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the ore deposit have been concentrated partly by segregation during alteration, and partly by the leaching of the associated 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 associated 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 without to any extent injuring it. Such products of alteration, however, are mor 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 minerals 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,^[1] 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