Part 41

4. Action of Currents.

No one doubts that currents, near coasts, heap up upon the beach, at the mouth of rivers and harbours, pebbles, sand, gravel, mud, or other transportable matters, whether these currents constantly exist, or simply result from the momentary action of a predominating wind; but this action, although already limited to the mobile matters which form the bottom of the sea only in some parts, whether this action, I say, extends to a great depth, that is to several hundred yards, is a question not yet resolved. In the first place, the observation made by mariners, that, in the most violent tempests, the sea is only agitated towards the coasts, or on shallows, and that bodies, sunk to a great depth, (and still what is this depth in comparison with that of the sea,) do not feel the motions of its surface, nor that of currents; and, secondly, reasoning, and even calculation, according to Messieurs La Place and Poisson, concur to shew, that the violent motions of the waters of the sea are not propagated to a great depth. It is therefore probable, that all the transportable matters, which are at this depth, must remain nearly in the position in which they are, since our Continents have assumed their present configuration, unless phenomena and motions of the sea take place at the bottom, of which we are ignorant, and which are foreign to the subject which at present occupies our attention.

But if we have no perfectly certain ideas regarding the propagation of the motions of the sea in depth, we can assert, that, whatever that extent and that power may be, the submarine currents no more abrade the rocks than rivers do the surface of the land. This proof is always derived from the same kind of fact, namely, from the vegetable and animal bodies which constantly cover the rocks, and which are found, at all times, by means of various sorts of dredge-fishing. In fact, no one has remarked, that the places in which oysters, mussels, corals and sponges are fished, are more sheltered from currents than others; nor that these places, after violent tempests, have been deprived, and consequently, as it were, despoiled of those productions, which, by covering the rocks, demonstrate that they preserve the integrity of their surface. Many of these bodies, however, as sponges, fuci and confervæ, contract but a feeble adherence to the bodies upon which they are placed.

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It therefore appears, if not completely proved, at least extremely probable, from the facts and reasonings which we have related,

1. That the presently existing waters, that is to say, in the state of purity in which we are acquainted with them, have no erosive action upon rocks, whatever be the nature of these rocks, when, 1st, The rocks are completely solid, and when they are neither friable nor disintegrated; 2d, When these waters act by themselves, that is to say, when their action is not complicated with the really erosive action of solid bodies, such as pebbles, sand, and perhaps even pieces of ice.

2. That water, sometimes acquiring, on account of its quality and velocity, a great transporting power, may remove masses, already detached, and of great size, according to its degree of velocity, and the bulk of its mass, and so far as it preserves this same power.

3. That the presently existing waters may have attacked, undermined, and caused to fall down, portions of solid and steep rocks, by mixing with beds of clay, marl, and sand, interposed between their solid strata; that they may also, in their rapid falls, have scooped pretty deep ravines in very inclined deposites, consisting of disintegrated rocks; but that these waters could not have scooped out, either by a violent action, or by a slow one, however long continued, any of those long and broad longitudinal depressions, which are named valleys, or of those narrow openings, with almost vertical walls, which are named gorges or ravines.

4. That, even when the deposites, which border these valleys or these ravines, are composed of transportable matter, the waters which at present flow in them could not have scooped them out, even supposing them to have been much larger in some than they now are; the declivity of the present deposite not being sufficiently great to give to these masses of water the rapidity necessary for producing this effect, and a power sufficient for carrying off the moveable matters which filled the valley or gorge.

5. Lastly, that the present running waters, so far from having contributed to form the numerous valleys, glens, gorges and ravines, continually tend to fill them up, and rather to level the surface of the globe than to furrow it, more deeply than it is.

Vid. Brongniart sur l’Eau.

NOTE

On the Connection of Geology with Agriculture and Planting.

That all sorts of soils are not equally adapted to all productions, is a remark of Virgil’s, the truth of which becomes obvious, when we consider many facts ascertained in Agriculture and Forestry. If, therefore, as the poet advises, our object be to determine what each particular region can produce, and what it cannot, our attention ought in the first place to be directed to the physical circumstances which exert their influence over vegetation.

All plants that are the subject of cultivation are fixed in the ground. By one of their parts, through which they derive their principal nourishment, they penetrate into the soil, which serves them as a basis, and affords them the means of procuring subsistence; by the other part they raise themselves into the atmosphere, which is not only necessary in itself for their existence, but is also the medium through which they derive the warming and vivifying influence of the solar rays. Hence we can understand how much the existence of plants must be influenced by differences in the condition of the soil and air.

The superficial crust of the globe is formed of soil capable of producing vegetables. This productive soil, however, is not everywhere continuous, being interrupted on the one hand by the watery covering of the earth, and on the other by perennial snow and bare rock. Where soil does occur, it separates the solid mass of the earth from the atmosphere, and is the porous medium through which the gaseous and watery parts of the latter may act in a greater or less degree upon the former. It is very seldom that strata of vegetable soil lie beneath strata of other matters; and where they occur in this position, the overlying strata are either of volcanic or of alluvial origin. Of the former case, a very remarkable example occurs in the Isle of Bourbon, in which large tracts covered with vegetables and even trees, have been laid waste and overwhelmed by streams of lava; and large rivers in their overflowings occasionally leave deposits of various characters, over the productive soil containing remains of formerly existing plants.

Productive soil, as well in regard to its situation as to its constitution, depends upon the nature and condition of the rocks which form the solid mass of the earth. It is always of secondary formation, compared with the rock on which it rests, its principal parts usually originating from the decomposition of this rock. While the forms of the surface of the solid mass of the earth, have much influence upon the action of the atmosphere, they also in some degree modify that of climate. From these circumstances it would appear that the solid substrata of productive soil exert an influence in various ways upon vegetables; whence it follows that, in order to obtain a more intimate knowledge of the conditions which operate upon their existence, it is necessary to call in geology to our assistance.

Although the scientific study of agriculture has made great progress in our times, the relations which exist between the constitution of the solid crust of the earth, and the formation and nature of vegetable soil, present a wide field for investigation. Geologists have hitherto too much neglected the examination of the productive covering of the earth, and those who have treated scientifically of agriculture and forestry have usually looked upon the vegetable soil in its own simple capacity, without regard to its foundation and origin. To point out the way by which we are to proceed in our investigation of the relations which exist between the solid crust of the earth and the productive soil which covers it, is the principal object of the following observations.

Bare rocks cannot be made subservient to the purposes of agriculture. Lichens indeed, cover the surface of rocks, deriving their chief nutriment from the atmosphere; mosses draw the water necessary for their subsistence from the fissures of stones; the roots of grasses seek in the chinks of rocks for particles of earth sufficient for their sustenance; various shrubs and trees penetrate here and there into rocky masses by their roots (having the powerful and continued action of living wedges), where the cohesion of the parts is smallest, in order to prepare a fixed seat for themselves, and be secure from the pernicious effects of the atmosphere. The surface of the earth is always sterile, however, when it shows a continuity of naked rock, uncovered by vegetable mould. The cultivation of fields and woods, and even the rearing of cattle, cannot therefore find scope in regions which are entirely rocky. Abrupt and precipitous mountains being generally in this condition are usually barren; but in plains and on declivities, a bare rocky surface is much less frequently the cause of sterility than an unfavourable proportion of mould. Some rocky and moderately elevated regions also occur, more or less destitute of vegetable mould, whose sterility depends upon volcanic causes. Iceland, for example, affords cases of this description. In many parts of Sweden, as in Westrogothia, in Scotland, &c., there occur many elevated regions, in which gneiss and granite predominating, exclude to a great extent all kinds of vegetation excepting lichens. In the same districts we sometimes meet with pastures and corn-fields interrupted here and there by bare rocks rising but little above the surface, by which the value of the ground is much diminished, and great impediments opposed to its cultivation.

As bare rocks are incapable of all cultivation, their distance from the under surface of vegetable mould must also be of great importance. In the plains of the north of Germany, for example, this distance is often so great that a rocky surface is never found, while, on the contrary, in other countries, especially such as are mountainous, the roots of plants not unfrequently touch the subjacent rock; the variation between these extremes being of all degrees. The effect of the distance of the surface of the solid rock from the under surface of productive soil may be both direct and indirect, and may vary much, not only with reference to the species of rock, but also to the vegetables.

The surface of the solid strata of the earth has a direct influence upon the cultivation of plants, because it terminates the extension of their roots, and limits the volume of the soil necessary for their sustenance. As the length and direction of the roots vary exceedingly in different species, the difference of effect with regard to their growth, and the approximation of the rock to the under surface of the soil, must in general be so much the less prejudicial in proportion as the roots decline from the perpendicular; whence it follows, that certain grasses, and some small pasture plants, may grow in very thin layers of soil, where the larger grasses and pasture plants with longer roots, could not find subsistence; and that shrubs and trees, with long perpendicular roots, cannot survive in many places, where others with more horizontal roots may thrive. These inferences are proved to be correct by observations in agriculture and forestry generally known.

Mountainous regions, which are not so elevated but that corn might grow sufficiently well in them, in so far as depends upon the conditions of the air or climate, are yet frequently not adapted for its cultivation, on account of the too near approach of the rock to the surface, or shallowness of the soil, and produce nothing but grasses, and some other pasture plants, among which, however, there is the greatest difference in this respect. Trifolium montanum, for example, can support itself on rocky mountains, where T. pratense could not grow. Hedysarum onobrychis grows luxuriantly on the sunny declivities of calcareous mountains, where Medicago sativa (Lucern) does not find a suitable station. The cultivation of this excellent pasture plant in some mountainous regions, especially where the rocks are calcareous, has not proved so advantageous as might have been expected, because the plants have died out in the course of a few years; whereas, in proper places, where its very long roots find a sufficient depth of soil, they usually last for a great length of time.

The vicinity of the rock to the under surface of the vegetable mould, or the shallowness of the soil, seems to be the principal cause why the Beech grows better on many calcareous mountains than the Oak, which, on the other hand, finds a fitter station on mountains in which sandstone predominates, where the soil is usually deeper. It would seem to be for a similar reason that the Beech grows in many rocky districts, for example, on the Hartz Mountains, at pretty considerable heights, especially on the sides of valleys which run to the south, while these places do not admit the Oak, which is found in the middle provinces of Sweden and Norway; while the Beech, on the other hand, grows only in the southern parts. From the deficiency of soil, the Upper Hartz can produce neither the Pinus pinea, nor P. sylvestris; the horizontal roots, however, of the Abies, or Norway Spruce, are content with the small portion of earth which covers the greywacke and slate, although they cannot find sufficient hold to protect its lofty trunks from being thrown down by the tempest. In some parts of the Forest of Thuringia, where the covering of loose earth is deeper than in the Hartz, the Pinus picea, or pitch pine, grows luxuriantly. The common fir, Pinus sylvestris, which attains a great height in proper soil, on the contrary, is stunted and distorted on rocky mountains, where the roots soon come in contact with the rock. It there loses the character of a tree, and assumes that of a shrub, as in place of a single upright stem, several branches shoot out, and these, not unfrequently, are creeping or bent.

The different conditions of rocks, especially their structure and their state of cohesion, are of some importance in producing these effects; for the surface of rocks must be detrimental or impervious to the roots of plants, in proportion to the compactness of their structure, and the cohesion of their parts. Schistose rocks, for example, afford a more easy passage to roots, than granular crystalline ones; pure quartz resists the roots of plants in the highest degree; sandstone much less; and pure limestone, on account of its comparatively small number of fissures, is much less favourable to vegetation than marl, chalk, or slightly cohering calcareous rocks, the masses of which are usually split in all directions.

The direction and inclination of the strata have also some influence in this matter; for, in proportion as the principal fissures of the strata are, from their direction or inclination, more readily presented to the roots of vegetables, the less prejudicial will their surface be to vegetation. Horizontal strata, therefore, are the least favourable to vegetation, perpendicular ones the most. In the inclination of strata intermediate in some degree between these positions, the roots of vegetables will find a greater obstacle on the side of a hill in which the surface of a stratum is opposed to them, than on the other, in which the principal fissures of the strata are open. The effects of this circumstance may frequently be observed in mountainous tracts having two principal inclinations, the state of vegetation, and especially the growth of wood, being more prosperous on the one of these declivities than on the other.

The surface of the solid strata of the earth may also have an indirect influence upon the cultivation of vegetables. The various inclinations of this surface deserve first to be considered, being of the greatest effect with regard to fixing the fertile soil. The horizontal position of a rocky surface is in the highest degree favourable to the stability of vegetable earth; and the greater its angle of inclination, the greater is the danger of its losing the soil upon it. In a highly inclined plane, the imperfect support of the centre of gravity is the sole cause of the loss of earth; in a less inclined plane the diminution of soil is usually caused by water, which produces this effect in a greater or less degree, according to the difference of inclination. In both these modes, by which a removal of soil is produced, the effect may be modified by a difference in the condition of the loose earth, as not only its stability as to situation, but also its resistance to the power of water, vary according to the size, figure, and cohesion of the parts, as well as their adhesion to the surface of the rock. Sandy loose soils, for example, are more liable to transposition than marly or loamy ones; and these, again, are more easily moved than such as are clayey and adhesive.

Whatever be the nature of the soil, a small degree of inclination in the solid rock is sufficient to favour its denudation by the removal of the former; and the inclinations of the surfaces of rocks having a covering of earth and vegetation, are in reality much less considerable than we usually suppose them to be, judging merely by the eye. The celebrated Humboldt has published observations on this subject. According to his measurements, a slope of even fifteen degrees appears steep, and a declivity of thirty-seven degrees is so abrupt, that if it be covered with a dense sward, it can scarcely be climbed. The inclination of the pastures of the Alps seldom exceeds an angle of ten or fifteen degrees, and a slope of twenty degrees is pretty steep. At an inclination of forty degrees, the surface of the rock is sometimes covered with earth bearing a sward, but at a greater inclination the rocks are usually destitute of soil and vegetation. In the Upper Hartz, the most common inclination of the declivities of the mountains is twenty-five degrees; nor does it usually exceed thirty-three, at which inclination the beech and spruce grow. The greatest declivities at which ground can be advantageously cultivated have an inclination of thirty degrees.

The roots of vegetables, especially of grasses, shrubs, and trees, are of much importance in supporting the earth upon the declivities of rocks. Care must therefore be taken that the declivities of mountains which are covered with turf or wood, be not altogether deprived of these coverings, as sometimes happens in consequence of loosening the turf for agricultural purposes, or of incautiously extirpating the wood. In Norway, near Roraas, there occur mountains, destitute of all vegetation, that had formerly been covered with woods, but where now, from the deficiency of soil, no seeds could take root. The same is the case in many parts of the Alps, where, from the irregular long-continued removal of the timber, the sides of mountains which were formerly covered with thick woods, now show nothing but naked rocks. For this reason, in mountainous countries with very steep declivities, the breeding of cattle and planting of woods are often more advantageous than agriculture. In France the greatest inclination of the public roads is limited by law to an angle of four degrees and forty-six minutes: a similar restriction with regard to agriculture might not be without benefit in certain mountainous countries.