302 THE JOURNAL OF GEOLOGY. 



The materials in their original environment may or may not 

 have been sufficiently concentrated to serve as commercial 

 sources of supply, but the fragmental deposits mentioned almost 

 always represent a further concentration. This class of deposits 

 is of great importance, but the present discussion relates more 

 especially to the superficial alteration of deposits that remain 

 in situ, and therefore these will be treated more in detail than 

 the other class, (No. 2), though the latter will be mentioned as 

 occasion requires. 



SUPERFICIAL ALTERATION IN DIFFERENT DEPOSITS. 



Alteration in iron deposits. — It was once generally believed that 

 most iron deposits were the result of direct precipitation from 

 aqueous solution, or in rarer cases, were igneous masses. It has 

 long since been shown, however, that most workable iron depos- 

 its are the result of a concentration subsequent to their deposi- 

 tion, while very few are due to a direct precipitation during the 

 formation of sedimentary rocks, though some may be due to a 

 process of differentiation in the cooling of eruptive magmas. ^ 

 The original presence of the iron in sedimentary rocks was doubt- 

 less due to a direct precipitation during the formation of the 

 enclosing rock, but it was then in a finely disseminated condi- 

 tion, and it was only by being subsequently taken into solution 

 again by percolating waters and concentrated, that it was con- 

 verted into bodies of greater or less purity. Generally, though 

 possibly not always, this process is superficial, and though it may 

 extend to a depth of several hundred or even a thousand feet or 

 m.ore, it can be traced directly to surface influences, and its 

 effects are seen to decrease gradually with depth. Shaler, ^ in 

 1877, showed that some of the limonites of Kentucky, Ohio and 

 elsewhere were concentrations of iron derived in solution from 

 shales and other rocks and reprecipitated in underlying lime- 

 stone. 



' See foot note on second page of present article. 



^N. S. Shaler, Kentucky Geol. Survey, Report of Progress, Vol. III., New 

 Series, 1877, p. 164. 



