Huronian ores of Lake Superior have the same composition as the Silurian deposits. Lastly, the Laurentian magnetites constitute the other extreme of the ferruginous series. Both the water and a part of the oxygen have disappeared, leaving a compound richer in metal, and therefore more highly prized by the smelter. The application of a gentle, continuous heat is adequate to explain the change of the limonites into hematites and magnetites.
The process of change may be seen in the manufacture of common bricks, or the purification of quartz for the production of glass. The blue clay becomes red when burned, because it parts with its water of composition; and likewise the small percentage of hematite in the quartz becomes magnetic on the application of heat, and, after pulverization, has the iron removed by magnets, so that the silica-flour may be perfectly pure, and not impart a green tint to the glass. It is not maintained that the native limonites have been converted into magnetites in precisely the way in which the same results have been accomplished artificially; but the manipulation of the manufactured products shows that the metamorphosis is a feasible process, and by no means of difficult accomplishment in Nature.
In a review of a report by the author, in which this theory of iron ore origin is elucidated, Prof. Dana objects[1] to its value, because "carbonic acid, which does now some of the work of iron-transportation, may have done far more then," on account of its presence in the atmosphere in great abundance. No doubt exists as to the assistance afforded by carbonic acid in this work, but this fact only confirms the truth of our argument, since no chemist will allow that carbonic acid can remove the iron-rust from the soil without the help of some deoxidating agent, such as vegetation. The chemical change for which we require the presence of vegetation is the same, whether carbonic acid be involved or not. Indeed, an excellent authority for the form in which this change is effected is the professor's own treatise on mineralogy,[2] where he says, "The iron is transported in solution as a, protoxide carbonate in carbonated waters, a sulphate, or as a salt of an organic acid." Each of these methods requires the presence of a deoxidating agent like vegetation; and nothing better has yet been suggested. The iron-ores produced by volcanic ejections are of very limited amount, and mingled with too much dead rock to be capable of utilization. Nor does the suggestion of the decomposition of pyrites by atmospheric agents to form limonite necessitate the origin of all iron ores in that way.
Accepting the validity of the argument, it follows that vegetation must have been extremely abundant in the Laurentian and Huronian ages on account of the presence in them of enormous deposits of iron ores, as on Lake Superior, in the Adirondacks, Missouri, etc. Some of the beds are hundreds of feet in thickness.
