Page:The Journal of geology (IA journalofgeology21894univers).pdf/318

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.

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 deposits are the result of a concentration subsequent to their deposition, 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.^[1] The original presence of the iron in sedimentary rocks was doubtless due to a direct precipitation during the formation of the enclosing rock, but it was then in a finely disseminated condition, and it was only by being subsequently taken into solution again by percolating waters and concentrated, that it was converted 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 more, it can be traced directly to surface influences, and its effects are seen to decrease gradually with depth. Shaler,^[2] 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 limestone.