The rocks, taken as masses, will not conduct an electric current when dry: granite, porphyry, slate, and limestone, obstructing its passage even through the smallest spaces. But all the metallic formations admit of its circulating with great freedom. This fact it must, however, be remembered does not in any way interfere with the hypothesis of the existence of electricity in all bodies, in what we must regard as its latent state, from which, under prescribed conditions, it may be readily liberated. Neither does it affect the question of circulation, in relation to the great diffusion of electricity which we suppose to exist through all nature, and to move in obedience to some fixed law. We know that through the superficial strata electric currents circulate freely, whether they are composed of clay, sand, or any mixture of these with decomposed organic matter; indeed, that with any substance in a moist state they suffer no interruption.
The electricity of mineral veins has attracted much attention, and numerous investigations into the phenomena which these metalliferous formations present, have been made from time to time.[157]
By inserting into the mass of a copper _lode_, or vein, in situ, a metallic wire, which shall be connected with a measurer of galvanic action, a wire also from the instrument being brought into contact with another _lode_, an immediate effect is generally produced, showing that a current is traversing through the wires from one _lode_ to the other, and completing the circulation probably over the dark face of the rock in which the fissures forming the mineral veins exist.[158] The currents thus detected are often sufficiently active to deflect a magnetic needle powerfully, to produce, slowly, electro-chemical decomposition, and to render a bar of iron magnetic. These currents must not be confounded with the great electrical movements around the earth. They are only to be detected in those mineral formations in which there is evidence of chemical action going on, and, the greater the amount of this chemical operation, the more energetic are the electrical currents.[159] We have, however, very good evidence that these local currents have, of themselves, many peculiar influences. It not unfrequently happens that owing to some great disturbance of the crust of the earth, a mineral vein is dislocated, and one part either sinks below, or is lifted above its original position; the fissures formed between the two being usually filled in with clay or with crystalline masses of more recent formation than the fissure itself. It is frequently found that these “_cross courses_,” as they are called in mining language, contain ores of a different character from those which constitute the mineral vein; for instance, in them nickel, cobalt, and silver are not unfrequently discovered. When these metals are so found, they almost invariably occur between the ends of the dislocated lode, and often take a curvilinear direction, as if they were deposited along a line of electrical force.[160]