Page:Popular Science Monthly Volume 10.djvu/272

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selves into two groups, or those relating to plants and animals, which we will consider in their natural order:

Evidences of Plant-Life.—An interesting evidence of the existence of vegetation in Eozoic times is derived from the presence of iron-ores, an argument first set forth by Sterry Hunt. The ores are first formed in the hydrated condition, and then lose their water by metamorphic agencies, becoming specular and magnetic, or the state in which the Laurentian irons are now known. Ores of iron are conceived to have been formed under similar conditions in all ages. At the present day they accumulate in swamps and low grounds in the condition of the hydrated peroxide (ferric), or bog-ore, oftentimes in company with manganese. The presence of organic vegetable matter is requisite in order to extract the iron from the rock or soil and effect its deposition. The metal present in slight amount in the soil is the insoluble ferric oxide, or the familiar condition of iron-rust. Water charged with soluble vegetable infusions, like that in swamps too full of the disagreeable extract of leaves, etc., to be palatable, has the power of dissolving ferric oxide. The process consists in the removal of a part of the oxygen by the vegetable compound, or deoxidation, when the compound becomes changed into the readily-soluble ferreous oxide. But this is not a stable compound in the presence of our atmosphere. The rejected oxygen is brought back again, and in its recombination takes water with it, producing the hydrated ferric oxide, which, being insoluble, is precipitated, and covers the ground on the bottom of the pool. On visiting almost any swamp at the present day, this reddish-brown coating of hydrated iron-rust may be seen abundantly. Where streams of water cause the swampy water to flow to lower regions, the iron compound is also conveyed in suspension, and in the course of a few years a thick deposit of ore is accumulated. Our New England ancestors used these beds for the manufacture of their pig-iron in localities where only the name now exists for the village, such as the Tamworth or Gilmanton Iron-Works. All tradition of the manufacture there has disappeared. The Katahdin Company, in Maine, however, and some others, still derive their ore-supplies from this bog-compound.

Our theory supposes that the principal iron-ores in every age of the world had their origin in this way. There is no other agent save this organic extract which produces iron-ore on a large scale at the present day; hence it is rational to explain the origin of ancient ferruginous beds in the same way. If we examine the formations in order, we find the very ores themselves obviously thus accumulated: 1. There are the early Tertiary limonite beds of Western New England, New Jersey, and Pennsylvania, still scarcely removed from the bog form, with the accompanying clays. 2. There are the older Carboniferous nodules and the celebrated Clinton hematites, differing from the limonites only by the absence of water. 3. The specular