Mineral veins are not, however, generally filled by injection from below. It is found that those veins only are productive which have an east and west direction. But injected dikes run in all directions. The ore often varies in richness at different depths in the vein, or passes into ore of some other metal. The ore also varies in kind and quality, according to the character of the rock through which the vein passes. These phenomena are best explained by supposing that the sediment of which the strata were formed contained the mineral substances of these veins in small proportion. After they were solidified, and fractures had been formed, the mineral substance was transferred by molecular action to the fissures, and deposited. It was shown by the early experiments of Davy, that voltaic currents are capable of taking up mineral substances from their solutions, and removing them from one cup to another. It has been ascertained that in most mineral veins a proper apparatus will detect the existence of electric currents. It may be regarded as certain, that the unequal heating of different parts of the surface at the same time, by the sun, causes a vast current of feeble intensity to circulate around the earth once in twenty-four hours. The unequal distribution of heat below the surface may also produce currents subject to other laws. We should expect that these currents would take up the mineral substances diffused through rocks, and deposit them by themselves. It seems probable, therefore, that the molecular action, from which the segregation of metallic veins has resulted, was that of voltaic currents. SECTION III.--ORGANIC CAUSES. The effects of all organic causes in producing geological changes are inconsiderable, compared with those of inorganic causes. With the exception of the coral formation, the most important of these effects are those produced by human agency. We find examples of this agency in the distribution of animals and plants beyond the regions where they are indigenous; in the increased numbers of certain species, and in the diminution, if not extinction, of others; in the modifications of climate, dependent on the destruction of the forests and the cultivation of the soil; in controlling the course of rivers; in arresting by embankments the encroachments of the sea; in breaking up and changing the place of great quantities of rock by mining and engineering operations; and in the increased quantity of sediment furnished to streams by cultivating the surface, and thus preventing the protecting influence which the matted roots of trees and the smaller vegetables would otherwise have. Such effects, though attributable mainly to man, are produced in some degree by all other animals.