FACTS AND THEORIES.
The many theories of the formation of the metallic ores now existing tend to confuse the mind of the student, and it is a difficult matter to select the most probable, since those theories are generally supported by arguments that are convincing to those who are familiar only with the localities to which they apply. But, as most of the existing theories in relation to the formation of metallic' veins, seams, and beds are the result of local experience and study, they do not apply generally. For instance, one theory makes all metallic and mineral veins, seams, and beds the result of sedimentary deposits in water ; another considers them to be the direct result of volcanic eruptions ; a third advocates the theory of sublimation; a fourth refers the result to electricity, and others to segregation, attraction, gravitation, &c. &c. Now, to any one familiar with the numerous forms and the varied manner in which the metallic ores exist, it will appear evident that none of the theories advanced will cover the ground or account for all the coincidents, but that all are required as a general application.
The Cornish copper lodes of Great Britain are true or fissure veins, and evidently connect with the interior of the earth, and have their origin direct from a Plutonic source. They were filled by sublimation, and condensed by the effects of water and steam. Those of Mansfeld, in Prussia, are stratified in the Permian, and, though the indirect results of volcanic agencies, are deposits or sediments from water. The veins or beds of the Southern States are the results of sublimation, segregation, and sediment, since they exist as sulphurets in connection with copper, lead, gold, and sulphur in true veins, in masses or seams mixed with native copper, and in beds lying in the Azoic slates and the lower Palazozoic slates and sandstones. The copper masses of Lake
Superior may belong to later formations than those named, but they owe their origin more directly to volcanic agencies.
The ores of iron result in much the same manner, but perhaps less from sublimation than from other causes, though the oxides of iron generally form the outcrops of all metalliferous veins,--the "gassan" of the Cornish miners, and the "iron hat" of the Germans. This oxidization is always superficial, and results from atmospheric causes. Nearly all metalliferous veins are accompanied by iron pyrites, or oxides of iron. The oxidization of the outcrops sets free the more volatile ores, or, being oxidized, they are washed away by water ; while the iron, being harder, and naturally cemented, remains behind. Many such apparent beds of iron exist in the older rocks, and in nearly all formations; but they are neither true beds nor true veins, and, though they occasionally yield a large amount of good iron ore, they are superficial, and depreciate rapidly in quantity and quality as they descend from the surface.
Iron is universally distributed through all animate and inanimate creation, as far as our earth is concerned, and exists in greater or less extent in all rocks,—generally so limited, however, as to be scarcely perceptible, but frequently in large and evident quantities, though not enough for practical purposes, and often in masses pure enough for use, even in those ferriferous rocks which cannot be termed ore-beds.