SUPERFICIAL ALTERATION OF ORE DEPOSITS. 299 



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 atmos- 

 phere, the character of the climate influences the agents of alter- 

 ation. 



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 hun- 

 dred 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 moder- 

 ately 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 altera- 

 tion 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 cop- 

 per 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 



