Essay on the Theory of the Earth

by baron Georges Cuvier

Hitherto we have only spoken of the proximate influence of rocks upon plants; but it cannot be denied, that the remote effects which they produce, (inasmuch as vegetable soil is derived from them, and, therefore, the qualities of this soil depend in a great measure upon their nature,) are of greater importance.

It is from the rocks which constitute the crust of the earth, that the principal portion of productive soil is derived. Although other substances belonging to the animal and vegetable kingdoms, are necessary for the nourishment of plants, a soil consisting chiefly of inorganic particles is still more necessary, both for sustaining their roots, and for receiving, retaining, and partly also preparing nutrition for them; for, according to accurate observations, some inorganic substances exert an influence upon the decomposition of animal and vegetable remains. These effects vary much according to differences in the aggregation and chemical nature of the inorganic parts; of which circumstances, however, the different qualities of rocks are the ultimate cause.

Two kinds of productive soil may be distinguished with regard to their origin. The soil has either originated in the place in which it now is from the subjacent rock, or it has been transported to the places in which it is now found by some power, especially by that of water. The first kind may be named untransported, the second transported soil. To the first kind of soil is to be referred a great part of the soil which covers the summits and declivities of mountains, and to the other, the soil which fills the bottoms of valleys, as well as a great part of the loose soil of extensive strata in hilly countries and plains. Untransported soil is generally thinner than the transported; and of the two the latter is that which most frequently occurs in low land. The first kind of soil, the untransported, is found to be more or less similar, in its principal constituent parts, to the rocks from which it has originated; in the other kind, the transported soil, on the contrary, the parts which were originally in connection, have been variously separated and mixed, by the agency of the powers by which its transportation was effected.

The quantity and quality of the soil derived from the disintegration of rocks, must depend upon the nature of these rocks; its quality being determined by the constituent parts of the rock from which it originated, and its quantity being proportioned to the greater or less degree in which the rock may resist decomposition.

The disintegration of rocks, and their conversion into loose earth, are partly mechanical, and partly chemical. The principal mechanical powers, by which disintegration is effected, are, 1st, The weight of the loosened parts; 2d, Water, not merely in its liquid and mobile state, but also, and that chiefly, in the state of ice; 3d, The roots of vegetables in general, and especially of trees. These powers usually act more or less in conjunction, and the effects produced by this union are in many cases almost incredible.

The disintegration of rocks commences in those parts where the power of cohesion is least energetic. Rents take place owing to the unequal attraction of parts, and also in the direction of planes, in which heterogeneous parts are in contact; and in this manner the original structure of rocks determines the first steps of their disintegration. Water, which enters into the minute fissures of rocks, by the power of capillary attraction, is expanded by congelation, and thus overcomes the cohesion of parts, and produces rents. The roots of trees acting as wedges, produce the same effect in a wonderful degree, a phenomenon which has been so well illustrated by Annæus Seneca, in his Natural Questions. “Let us consider,” says he, “how great a power is exerted by the most minute seeds, which, although at first small as they are, can scarcely find a place in the crevices of rocks, yet at length grow to such a size as to rend asunder vast rocks, overturning crags and cliffs, by the power of their very minute and delicate roots.” The parts of rocks loosened by these powers, are entirely separated, and are carried to a great or less distance, by streams of water, and in the higher regions, by the power of winds. In cliffs and precipices which have been formed by the splitting of masses of rock, effected in the manner above described, the loosened parts often lose their stability; and, following the direction of gravity, fall to the ground, an effect which has also been described by Seneca in another place. “Nor is it alone probable,” says he, “that rocks are split asunder by their mere weight, but also when streams of water are carried over them, the continual moisture works into the joinings of the rock, and daily takes away a portion of the connecting matter, and, if I may so speak, abrades the skin by which it is contained. At length, in the course of ages, this gradual detrition so much diminishes the supporting parts, that they can no longer sustain the weight. Then masses of vast size fall down, and the rock tumbling from its ancient seat, overwhelms whatever lies below.” The cohesion of some rocks, especially argillaceous ones, is so slight, and their porosity so great, that their smallest parts imbibe water, and are sensibly softened by it, an effect which is much assisted by the freezing of the water. This mechanical change is experienced by the different varieties of common clay, slate-clay, and some other rocks.

Chemical powers often act in conjunction with mechanical ones, in breaking down rocks, the former, the chemical, frequently finishing what had been begun by the latter. Mechanical powers only changing the aggregation of rocks, may break down their parts to a certain size, according to their different nature; chemical powers, again, which change the nature of substances, destroy the connection of the minute parts of rocks. When chemical is preceded by mechanical action, it is much assisted by it. The latter has a much more general effect, as all rocks are subjected to its influence; chemical decomposition, on the other hand, acts only upon some rocks, and in these only upon certain parts. The chemical decomposition of rocks is chiefly effected by the oxygen of atmospheric air and of water; but we are also persuaded, that certain cryptogamic plants, intimately attached to the surface of stones, Lichens namely, assist in their destruction.

The oxygen of air and water can only affect the constituent parts of rocks, which have a great affinity to it, such as the iron and sulphur forming pyrites, oxydulous iron, oxydulous manganese, or the same substances mixed with earth or carbonic acid, charcoal and bitumen. Very solid and compact masses of rock, such as greenstone, which are not easily affected by other means, are sometimes corroded by the chemical change of the pyrites contained in them, by which it is converted into a hydrate of iron[414]. In certain other rocks, which are also readily broken down by mechanical agents, clay-slate for instance, the disintegration is much accelerated by the decomposition of the pyrites. The oxydulous iron of felspar is commonly converted by decomposition into a hydrate or ochre. The carbonate of iron, as well as of manganese, which sometimes occur in rocks, in limestone rock for example, are deprived of carbonic acid by the oxidation of their bases. Charcoal and bitumen, which are sometimes contained in rocks, limestone and argillaceous ones for example, are dissipated by the contact of air, so that rocks which were originally of a dark colour, lose it, and become whitish. Water, as a chemical agent, contributes so much to the decomposition of certain rocks, that, either in a pure state, or in combination with carbonic acid, it dissolves their parts, of which gypsum and limestone afford examples. In certain other minerals, in felspar for instance, a separation of the constituent parts, produced by the contact of air and water, is observed, the proximate cause of which has not hitherto been discovered. The mass is decomposed, its lamellar structure is converted into an earthy nature, the alkali contained in the felspar is extracted by the water, a mineral is produced, to which the Chinese have given the name of Kaolin, and which is adapted for the manufacture of porcelain. Granite and gneiss occur in some districts, the felspar of which is decomposed in this manner through the whole mass,—a circumstance which must be of great importance in regard to the formation of productive soil.

Cryptogamic plants covering the surface of rocks, and thriving well in this situation, where more perfect vegetables could not grow, seem also destined to promote the chemical decomposition of rocks, an effect which they produce both directly and indirectly. As they imbibe the water of the atmosphere, and retain it like a sponge, they keep up a constant application of this substance to the rock, and in this manner contribute indirectly to its decomposition. There are some cryptogamic plants also, which consume certain portions of the rocks with which they are in contact, corrode their surface, and destroy the cohesion of its parts, effects which may chiefly be seen in certain cryptogamic plants attached to calcareous rocks. In this manner one sort of vegetation prepares a place for another, and the most imperfect vegetables are subservient to the growth of the more perfect.

After premising thus much, we shall now proceed to the examination of the principal rocks, in so far as regards their connection with the formation of productive soil, beginning with those which resist decomposition in the highest degree, and ending with those which are the most conducive to the formation of loose earth and soil.