300 THE JOURNAL OF GEOLOGY. 



residual products of alteration have accumulated to great thick- 

 nesses. This region, however, had once a much more moist 

 climate than now, and some of the alteration may have occurred 

 then. Many of the Arizona copper deposits in this region 

 originally contained their copper in the form of copper pyrites, 

 which, under similar conditions, is probably more resistant to sur- 

 face alteration than the native copper of Michigan, and yet it has 

 been changed to various other copper minerals for depths often 

 reaching from lOO to over 700 feet. In Chile some of the 

 copper sulphide deposits are said to have been altered to a 

 depth of 1,500 feet, but it is very rare that much alteration 

 extends in any ore deposits to greater depths than this. In the 

 more moist climate of Tasmania, the results of alteration are also 

 Very marked. 



The depth of alteration of ore deposits in unglaciated regions in 

 the United States varies from a few feet to over 1,000 feet. In the 

 Appalachian region, many of the deposits of auriferous quartz, iron 

 pyrites, copper pyrites, etc., are altered to depths varying from less 

 than one to a hundred feet or more. Many of the Clinton iron ore 

 deposits are altered to still greater depths. The depth of altera- 

 tion in these Appalachian deposits is usually much greater, other 

 things being equal, south of the limit of glaciation than north of 

 it. In the silver, lead, gold, and copper deposits of the Rocky 

 Mountains and the western arid region, such as at Butte City, 

 Leadville, Central City, Cripple Creek, Silver City, Lake Valley, 

 Eureka, Virginia City, Park City, the Coeur d'Alene district, and 

 elsewhere, the alteration has reached depths ranging from 50 to 

 600 or 700 feet, and in some rare cases still more. At Granite 

 Mountain in Montana, signs of alteration are seen in the argen- 

 tiferous quartz deposits of that region, even at depths of 900 feet, 

 though of course at such depths the alteration is slight as com- 

 pared with that nearer the surface. 



Complete alteration rarely extends to these greater depths, 

 and usually parts of a deposit which have as yet escaped altera- 

 tion appear comparatively near the surface. These are at first very 

 few and may be entirely enclosed by altered products, but with 



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