All those changes in which the action is molecular,—that is, between the molecules as such, and not between the masses,—including the effects of the imponderable substances, we regard as resulting from chemical agency.
Under the control of these molecular forces the crystalline rocks have taken their form; and if the crust of the earth could have remained in a fixed condition, in which these forces would have been in equilibrium, no further chemical action could have taken place. But, instead of being in a fixed condition, the present system is one of perpetual change. Various disturbances of this equilibrium of forces,—such, for instance, as the diurnal and annual changes of temperature at the surface, and the still greater secular changes of temperature at great depths,—will bring the chemical forces into operation. The mechanical disintegration of the crystalline rocks, and the deposition of them in strata independently of the chemical affinity of their particles, will give occasion for chemical changes,—that is, for a rearrangement of the particles in accordance with their affinities,—whenever any movement of the particles among themselves can take place. These movements take place, to a very great extent, under the influence of electrical currents, and of change of temperature, even while the masses retain their solid form.
Chemical affinity has exhibited itself on the largest scale in the formation of the various mineral species of which the crust of the earth is composed; but we may also refer to the same cause the formation of divisional planes in rocks, the concretionary arrangement, and mineral veins.
1. Divisional Planes.—It has before been stated, that the older rocks, in many cases, cleave freely in planes not parallel with the stratification. (See [Fig. 48].) In some instances, in beds of lava, a similar cleavage exists, sufficiently perfect to allow of its use as a roofing material. In these cases, there must have been a rearrangement of the particles, so that their axes of greatest attraction would lie in parallel planes; the same arrangement which exists in mica and other crystalline substances, which have one and but one free cleavage.
A similar arrangement has sometimes taken place under such circumstances as to submit the process to more careful scrutiny. In the gold mines of Chili, the powder from which the gold has been washed is “thrown into a common heap. A great deal of chemical action then commences; salts of various kinds effloresce on the surface, and the mass becomes hard, and divides into fragments which possess an even and well-defined slaty structure.” When a portion of clay, worked into a paste with a very weak acid, is submitted to a weak voltaic action for several months, and then dried, it is found to have acquired a distinct though imperfect cleavage structure.
It appears, then, that both electrical currents and ordinary chemical action are capable of arranging the particles of an earthy mass into separable layers. We may then regard this change in the older rocks as an imperfect crystallization, and probably induced by electro-chemical agency.
It is also found that all rocks are divided into huge blocks by seams not parallel with the cleavage, and too regular to be considered as fractures. These seams bear an analogy to the secondary faces of crystals, which are never parallel to the cleavage.
2. Concretionary Formations.—There exist in many rocks concretions which differ from the mass of the rocks. In most of the tertiary clays there are small concretionary nodules, which contain more calcareous matter than the mass of clay around them. In the coal formation, the nodular iron ore consists of concretionary masses. In the chalk formation, nodules of flint abound, and generally in layers. In many of these cases, particularly in the clays and coal, the nodules have an organic nucleus, and, although concretionary, they retain the marks of stratification of the adjacent rocks. Hence they could not have been deposited in the form of nodules. There must therefore have been in the rock, though in the solid state, such motion among the molecules that particles of a particular mineral have separated from the mass and rearranged themselves in concretionary layers, yet so gradually as not to disturb the lines of original stratification.
