SEDIMENTARY DEPOSITS.
We distinguish the sedimentary deposits of the Palaaozoic formations from those of the Azoic, because they have been formed under different circumstances. Though per haps the greatest amount of material forming the immense Palmozoic deposits of the Appalachian basin was derived almost direct from volcanic sources, it is evident that those sources were remote, and that the dust, sand, and ashes of volcanoes were carried great distances into the interior basin. It is also evident that much of the material was derived from the debris of older formations brought into the central basins by rivers, waves, and tides. Consequently, the ores deposited in these strata must have been the result of precipitation from a solution held by the waters.
Some of these stratified beds are of great extent, particularly those of the Carbo niferous era, found in our coal measures. We can hardly conceive how those immense ore beds, which are almost coextensive with the fields in which they exist, could have been derived from the debris of older formations, since the material derived from this source would be extremely diversified, and not at all likely to produce a uniform deposit either of ore, sandstone, or slate. It would be a promiscuous breccia of every older rock, and an olla-podrida of every lithological formation. We cannot comprehend any natural process by which the changes of beds and strata could be formed, except through volcanic means. A season of rest and quiet forms limestones, slates, shales, or coal-beds; while seasons of volcanic violence form the coarser beds of sandstones, &c.
Since all the material of the earth contains more or less iron, it would be natural to suppose that the lava vented from even distant volcanoes, and carried by whatever process into the interior basins, would precipitate this metal in beds. It would find its peculiar horizon of stratification by specific gravity, but the character of the ore would be influenced by the chemical action to which it was subjected.
The fossiliferous ore-beds in the vicinity of the limestones partake of its calcareous nature, while the Carboniferous beds of the coal measures are impregnated with the carbon of the coal.
Chemical action, electricity, gravity, and common segregation may, and in all proba bility do, exert a great influence in forming either veins or beds of ore; but the chief causes of their production are:— FIRST.-By the sediment of volcanic matter thrown directly in water, forming beds of magnetic and specular iron ores, gold,* &c.
SECOND.-By sublimation in true veins and fissures, producing sulphurets of copper, silver, lead, &c. &c.
THIRD.-By gravitation or segregation from beds of lava and trappean formations generally, forming veins or seams of limited extent and irregular spread.
FOURTH.-By precipitation from water holding the ores in solution, and derived chiefly from volcanic sources, but influenced and changed by chemical action and affinity with the proximate formations.