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.