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and the amount that would immediately run off the surface or be evaporated and thus have but little altering effect. The rate and degree of variation in temperature affect the amount of breaking in the rock by expansion and contraction, and, therefore, the accessibility of the rock to surface influences. The character of the climate also influences, to a certain extent, the nature and amount of vegetation, and from the vegetation are obtained many organic acids which assist the action of surface waters. In other ways, also, such as in the generation of nitric acid in the atmosphere, the character of the climate influences the agents of alteration.

As a result of all these influences, surface alteration is found to extend in different ore deposits to depths varying from only a few inches, or in fact only a fraction of an inch, to several hundred and even a thousand or more feet. In glaciated regions the products of decay have often been swept away by glacial action, and the time which has elapsed since then has not been sufficient for alteration to have extended to any great depths; while in regions of moist climates, the erosion sometimes, though not always, keeps pace with the alteration, so that the depth of the change is shallow. In those regions, however, which have not been recently glaciated and which have dry or only moderately moist climates, so that erosion is slight, or in places which have moist climates, but which, on account of their topography, are not subjected to very active erosion, the products of alteration collect, and the changes are traceable downwards often to great depths.

In the copper regions of Michigan, the deposits have been exposed to glaciation, and are still exposed to the active effects of erosion in a moist climate, so that here, though the native copper of the region is a material very easily affected by surface alteration, yet the only change observable is a slight stain of copper carbonate or oxide on the surface of some of the native copper, and even this is not always present. On the other hand, in the arid region of the west, most of which has not been recently glaciated and which has an exceedingly dry climate, the