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residual products of alteration have accumulated to great thicknesses. 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 surface alteration than the native copper of Michigan, and yet it has been changed to various other copper minerals for depths often reaching from 100 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 alteration 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 argentiferous quartz deposits of that region, even at depths of 900 feet, though of course at such depths the alteration is slight as compared with that nearer the surface.

Complete alteration rarely expends to these greater depths, and usually parts of a deposit which have as yet escaped alteration appear comparatively near the surface. These are at first very few and may be entirely enclosed by altered products, but with