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.
The inclinations of the surface of the solid crust of the earth vary much, according to the different qualities of the rocks; some having a tendency to form abrupt precipices, others, again, to produce gentle declivities. For this reason, mountains consisting of quartz or porphyry, for example, very frequently present surfaces destitute of vegetation; while, on the other hand, those of granite, slate or sandstone, are more frequently adapted for agriculture and planting. In the northern parts of Scotland, quartz rocks, destitute of all vegetation, rise in the midst of mountains covered with gramineous plants, and sometimes wood. In the most fertile part of the south of Norway porphyritic mountains rise from a calcareous and schistose base, with lofty, rugged, and bare cliffs. In the southern parts of the Tyrol the rocky sterility of the abrupt and lofty porphyritic mountains presents a striking contrast to the fertility of the neighbouring limestone mountains, which are covered with vines, walnuts and chesnuts.
The surface of the solid strata of the earth has also an indirect influence upon the cultivation of plants, in so far as the water which the vegetable mould acquires from the atmosphere, is retained in the soil, or is drawn off by the subjacent rock. Different rocks produce very different effects in this respect, depending as well upon their constitution as their structure. The component parts of rocks imbibe water in different modes and degrees; and different sorts of rocks not only attract water with different celerity, but also imbibe different quantities of it. The latter difference depends chiefly upon the various substances of which rocks are composed, partly, also, upon their porosity. Siliceous rocks attract water in the lowest degree, argillaceous ones in the highest, and calcareous rocks appear to have an intermediate action in this respect. Compact and granular crystalline rocks attract water in a smaller degree, and more slowly; friable or crumbled rocks imbibe it in greater quantity, and with more celerity than those which are not disintegrated. The condition of rocks with regard to the attraction of water, affects, in a different manner, the humidity of soil; for, by this attraction, moisture may as well be abstracted from, as imparted to, the loose earth or soil by which rocks are covered. Part of the moisture which vegetable earth or soil derives from the atmosphere passes into the subjacent mass of rock, but this may again be compensated by evaporation; on which account the soil of such rocks as have but a small attraction for water usually dries up more readily than soils whose solid substratum attracts and retains the moisture in a greater degree.
It is probable that the structure of rocks has also a greater, and not less, diversified influence upon the humidity of productive soil. Solid rocks, which are not traversed by numerous perpendicular fissures penetrating to a considerable depth, allow the water to remain in the soil; but columnar and schistose rocks, with perpendicular fissures, and strata declined from the horizontal position, draw off the water from the soil covering their surface, into lower places, where it often re-appears under the form of springs. In these circumstances, we find a partial explanation of the great difference between the humidity of soil covering a surface of solid granite, and that lying upon limestone, which is intersected by numerous fissures. Granitic mountains are often furnished with marshes, whereas, on the other hand, the dryness of the soil upon calcareous mountains is generally excessive[411], the cause of which phenomenon is, in a great measure, to be attributed to the circumstances above mentioned. Columella observes, that silex having a moderate covering of earth, preserves to the latter its humidity; and Palladius repeats the remark. In districts which consist of quartzose rocks, not less than of granitic ones, the surface is often covered with marshes. Porphyritic rocks, on the contrary, which have a remarkable segregation of parts, as well as columnar basalt, let off the water to lower places. Springs are very frequently found at the bottom of basaltic mountains; for the atmospheric waters penetrate by the perpendicular fissures to the strata on which the basalt rests, and appear at the place where the two rocks meet.
The effect of different rocks upon the preservation and diminution of the moisture of fertile soil, influences vegetation in various degrees. The retentive power of the surface of rocks is of the greatest importance, where the soil consists chiefly of sand, through which the water percolates, and passes off entirely, unless it meets with a stratum of such a nature, as to obstruct its passage, or comes upon a surface of solid rock. The cause of the sterility of sandy plains is not merely their sandy nature, but also the great depth of the mass or rock capable of retaining the water. The same sand, when covering mountains consisting of sandstone, has a much less degree of sterility than in those plains, because the surface of the subjacent rock impedes the progress of the water, and consequently retains it in the soil[412]. It has been sufficiently proved by experiments, that plants can grow in pure sand, when furnished with the necessary quantity of water. A subjacent rocky surface has an entirely different effect upon soil which is very retentive of moisture, upon a clay soil for example, as, in that case, the humidity is increased to a prejudicial degree. In land of this nature, a substratum of rock having the property of drawing off the water would be useful.
The different conditions of rocks with regard to caloric, may have some indirect influence upon the vigour of plants. Heat, whether imparted to the vegetable soil by the sun’s rays, or generated by various chemical processes in the earth itself, penetrates to the surface of the subjacent rocks, and is more or less drawn from it in a longer or shorter time. Columella observes, that rocks in the upper part of the soil are prejudicial to vines and trees, but in the lower part cool them. The heat of soil will be more or less drawn from it, according to the greater or less conducting power of the subjacent rock. Compact crystalline rocks are probably better conductors of caloric than those which are of looser texture; siliceous rocks than argillaceous and calcareous ones. The influence of the subjacent rock must be greater in this respect, in proportion to the thinness of the superincumbent soil. The effect of the abduction of caloric is more particularly sensible, where the roots of cultivated plants touch the rock, a circumstance which we often see in vineyards. The vine frequently thrives remarkably on the declivities of mountains, in which it sends its roots among fragments of stones. Experience shows, that the quality of wine is influenced by the different conditions of the stones, among which vines are planted. Albertus Magnus has observed, that the vine thrives well in earth which is mixed with fragments of black roofing slate; and Humboldt remarks, that the vines which grow upon the mountains of the valley of the Rhine, consisting of black clay-slate, afford an excellent wine. At the Cape of Good Hope, also, the vine thrives well in a soil produced by the decomposition of clay-slate, and mixed with fragments of it[413]. It is probable, that the adaptation of this sort of soil to the cultivation of the vine, depends upon its slow conducting power, and upon its rapidly imbibing the rays of the sun, on account of its dark colour, and thus increasing the heat of the ground.
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.
In the first class, we place those rocks which experience no chemical decomposition, in so far as regards their principal mass, and whose cohesion of parts is so great that mechanical powers can only open their natural fissures to a greater extent, and thus break them down into fragments. Of this kind are vitreous lava, pure quartz, compact quartz, flinty slate, and porphyry with a siliceous basis. On mountains consisting of these rocks, scarcely any productive soil is found, and frequently none at all. They are usually characterized by sterile rocks and cliffs, the bases of which are covered with innumerable rough fragments of stones, retaining their sharp edges for a great length of time, the heaps of which seldom produce any thing else than mosses, which frequently cover the interstices of fragments, occasionally a few grasses, and sometimes a solitary shrub or tree. Examples, Bennevis, Paps of Jura, and Morven Hills. Of all rocks, vitreous volcanic productions are the least capable of contributing to the formation of productive soil. Their dark coloured tracts descend from volcanic mountains to the valleys in frightful sterility, the chinks of their rugged masses scarcely affording sufficient water for the roots of mosses[415]. To the second class we refer compact limestone, a rock which contributes extensively to the formation of the solid crust of the globe. In so far as regards its principal constituent parts, it is not affected by atmospheric water or air; but, as its parts have but comparatively little cohesion, and are usually separated in a considerable degree by minute fissures, they are more liable to be broken down and crumbled by mechanical powers, than those of the rocks belonging to the first class. In districts where the fundamental rock is limestone, the layers of loose original soil or subsoil are thin, and filled with numerous fragments. As the soil arising from the disintegration of limestone contains a great proportion of calcareous matter, it is neither favourable to the growth of plants in general, nor to that of the greater number of vegetables which are the object of cultivation. Soil of this kind is too hot, dry and stony; hence the reason why districts, in which pure limestone rocks predominate, are often sterile. The case is different, however, where a portion of clay enters as an ingredient into the composition of calcareous rocks, for here the soil is usually very productive; or, where rocks of a different nature alternate with masses of pure limestone, having a greater capability than it of contributing to the formation of productive soil. When water, containing carbonic acid, passes through limestone rocks, it dissolves portions of it, and deposits them in other places, by which the decomposition of the limestone and the formation of loose earth may be in some measure accelerated.
To the third class belong chalk and gypsum; which, in so far as regards their decomposition by chemical means, are of a similar nature with compact limestone; but possessing a much slighter cohesion of parts, are more liable to be broken down by mechanical means. Water also dissolves gypsum, and thus assists in its disintegration. The soil arising from these rocks resembles that produced by compact limestone, which explains the want of fertility, observable in certain gypseous tracts of the North of Germany, and in the chalk districts of France. The fertility which we see in certain places where chalk is the fundamental rock, as in the Isle of Wight, Island of Rugen, &c. is to be attributed as well to argillaceous and marly strata alternating with the chalk, as to the greater humidity of the atmosphere, by which the dryness and heat of the soil are diminished.
In the fourth class we place certain rocks, composed of different minerals, but compact in appearance, which, although they resist mechanical disintegration, are yet subject to chemical action, and are, by means of it, converted into a loose, compound productive soil. Of this kind are basalt, and some other rocks very nearly allied to it.
To the fifth class we refer those rocks which have a crystalline, granular, or slaty texture. The mutual adhesion of the heterogeneous parts, of which they consist, being, in general, inconsiderable, they are easily broken down by mechanical means, and thus contribute in a high degree to the formation of productive soil. The felspar contained in these rocks, on account of the chemical decomposition which it readily undergoes, has a great effect not only upon the quantity, but also the fertility of the soil produced. The quartz, on the contrary, as well as the mica and hornblende, long resist chemical decomposition; they are, however, useful in this respect, that the argillaceous soil arising from the felspar, has its tenacity diminished; and is consequently rendered better adapted for vegetation, by being intermixed with them. Granite and gneiss, of all truly granular crystalline rocks, afford the deepest and most fertile soil, aptly compounded of different substances, sufficiently loose in its aggregation, and capable of retaining the necessary moisture. Soil arising from the disintegration of granite is unfavourable to vegetation only, where the rock abounds much in quartz, and where the superfluous water cannot run off, and so gives rise to marshes, which produce only vegetables of inferior quality; of which we have examples in the granite districts of Aberdeen. In such places as these, peat is easily generated, which, although of great use, is yet much less advantageous than wood. Syenite, which abounds much in hornblende, is inferior to granite, with respect to the production of fertile soil; and primitive greenstone, which resists disintegration and decomposition in the highest degree, occupies the last place in this class. In the series of slaty crystalline rocks, mica-slate is next to gneiss: but on account of the small proportion of felspar which enters into its composition, it does not afford so productive a soil.
In the sixth class may be placed the slaty rocks, whether simple, or intimately compounded, which do not readily undergo chemical decomposition, but which easily separate at their natural fissures, and are mechanically resolved into an earthy mass, forming a paste with water, circumstances which are observed chiefly in clay-slate, a rock of much importance in the formation of productive soil, usually passing into a clayey sort of earth.
To the seventh class belong the conglomerated rocks, whose parts indeed undergo very little, if any, chemical change, but are easily separated by mechanical means, and are thus converted into a gravelly, sandy, or earthy mass. Of this kind are greywacke, old red sandstone, and sandstones of various kinds. Much diversity is exhibited by these rocks, with regard to the facility with which they undergo disintegration, as well as the nature of the soil arising from them; circumstances which chiefly depend upon the nature of the cement, and its relation to the parts cemented. The disintegration of these rocks is the more easily effected that the cement is abundant, and less intimately connected with the other parts, that is, the more they depart from a crystalline nature; on which account greywacke is less easily converted into soil, than the common varieties of sandstone. By the decomposition of greywacke, a loose and fertile soil is formed, containing particles of quartz and clay in due proportion; on the other hand, by the decomposition of red sandstone, a soil is frequently produced, abounding in argillaceous particles impregnated with iron, and therefore stiff and cold. The variegated sandstone, with a marly cement, not unfrequently affords a pretty fertile soil; the quadersandstein, on the contrary, commonly presents a sandy and arid soil.
Lastly, in the eighth class we shall place those rocks, whether simple or intimately compounded, whose nature is so loose, or whose parts are so separated, that they fall with great facility into an earthy mass, and are also in part mechanically reduced by water. To this class belong the different varieties of marl, slate-clay, basaltic and volcanic tuffa. These rocks, many of which are extensively diffused, are of much importance in the formation of productive soil, although the quality of the earth produced by them varies much, according to their different natures. Slate-clay affords an argillaceous soil; in earth produced by the decomposition of marl, the clay is diminished in proportion to the greater abundance of the calcareous or sandy parts; while a mixed and very fertile soil is usually generated from basaltic and volcanic tufas.
The various relations which exist in the stratification and position of rocks, have much influence in producing a diversity in the soil formed immediately from their decomposition. This diversity cannot be so great when different rocks of various ages occur in a determinate order in horizontal strata; in which case, the uppermost bed may exhibit a great extent of surface of the same nature. When, on the other hand, strata of rocks of different natures, forms, and dimensions, placed at different angles of inclination, and in different directions, appear at the surface, it will easily be understood how it may happen that the soil produced by their decomposition may occur of very different qualities, in places not very distant from each other. The manner in which the soil is influenced by a difference in the arrangement and position of the strata, will become evident, on comparing districts in which one particular sort of rock lies beneath the surface in horizontal strata, with others in which the solid substratum is composed of various rocks differing in their inclination towards the horizon. In districts of the former kind, the qualities of the soil vary in general but little; in such as are of the latter kind, on the contrary, they are often found extremely different. The great diversity of soil seen in England, as well as in Germany, may, in fact, be partly explained by the circumstance, that, in those countries, the nature and position of the strata vary every where. On the other hand, the great similarity which pervades the soil of Southern Russia, is without doubt produced by a uniformity in the position and inclination of the limestone which lies immediately under the soil.
The nature of the principal mass of the strata usually exerts a great degree of influence over the qualities of the soil. When the solid substratum is sandstone, its effect upon the soil is, in general, as evidently seen, though not perhaps in an equal degree, as when it is marl. Exceptions, however, to this rule sometimes occur; as, for instance, when the principal mass of a rock which resists disintegration in a high degree contains beds that are easily reduced to earth. This is the case with the shell-limestone (muschelkalkstein) of Germany, the mountains of which are not unfrequently covered with a clayey soil, which has not been produced by the decomposition of the principal strata themselves, but by that of the slate-clay and argillaceous marl alternating with them.
Hitherto we have considered untransported soil, or that produced from the disintegration or decomposition of the subjacent rocks in the places where it occurs; we have now to examine the relations which exist between the subjacent rock, and the transported soil lying upon it. The nature of the rock does not indeed influence, excepting in a more remote degree, the transported soil, which has been carried to a greater or less distance from the places of its production, by the agency of moving powers, and again deposited of various forms and compositions. However, it may often be plainly seen, that the materials of this soil have been derived from particular rocks, and that these rocks have exerted some degree of influence over the formation and distribution of the transported soil. The examination of these relations is of great importance, because it is with secondary or transported soil that agriculture is principally concerned. The varieties of transported soil depend chiefly upon three circumstances: 1st, The nature of the rocks from which they are derived; 2dly, The quality and effect of the moving powers; 3dly, The changes which they may have undergone after their formation.
The origin of the materials which enter into the composition of transported soil, has been already considered. From their difference may be easily explained why soil generated from the debris of primitive crystalline rocks has different qualities from soil which has been derived from strata of sandstone or marl.
The principal powers which contribute to the transportation of soil, are, The weight of loose masses, ice, and water. The weight of loose masses is a cause of transportation which we frequently see in operation. By it the huge cones of debris at the base and upon the declivities of precipices and mountains, are gradually carried off toward the bottom of the valleys; a phenomenon which can scarcely any where be better seen than in the valleys of the Alps, where mountains sometimes occur evidently consisting of debris, and clothed with trees and shrubs, or covered with pastures, the masses of which are gradually moved, as upon inclined planes, by the action of the water which percolates through them.
Ice effects the transportation of rocks and debris, with a power which nothing can resist. This is no where more conspicuous than among the glaciers of the Alps, by the falling of which great heaps of stones and rubbish are produced. The transportation of large stones by means of ice may also be seen in our mountain torrents in winter. Huge masses of stone, scattered over the plains of the north of Germany and the islands of Denmark, and often very prejudicial to agriculture, whose northern origin appears to be established, may have been carried by the same powerful agent from Finland, Sweden and Norway, into those countries, at a time when the plains of northern Germany, with the other flat districts along the shores of the Baltic, were still covered by the waves of the ocean.
In the formation of transported soil, water usually exerts a great degree of power. By means of it, not only are vast masses transported to the greatest distances, but their parts are at the same time crumbled down and mingled together. To these operations are to be attributed the various terminations of different soils at horizontal distances, as well as the different alternations of their strata at vertical ones. The power of water in the formation of transported soil varies, not only according to the different inclinations of its channel, but also in regard to the form, size, and weight of the parts carried off by it; for which reason, in the formation of such soils, the same phenomena take place on a large scale, that we see on a smaller, in performing the operations of breaking and washing the ores of metals. For the same reason that, in these processes, the larger particles subside, while the smaller are propelled, from which again the heavier particles of ore are sooner deposited than the lighter; in plains in the vicinity of a mountain, covered with transported soil, stones and debris are usually seen first, then earth, clay, and sand mixed together, and farther on, finer sand, with strata of clay.
Transported or secondary soil, produced by water, according to the mode of its formation, is divided into four classes, viz.—1. Soil of Valleys; 2. River Soil; 3. Lake Soil; 4. Marine Soil.
1. Soil of Valleys.—It is washed down by rain and snow water, and partly also produced by rivulets, which carry off the loose parts from the declivities of mountains to the plains. The nature of this soil in general clearly shews the nearness of its origin. Its depth is always greatest in the bottom of the valley, and gradually diminishes toward the declivities of the mountains. The curvature of the different strata is usually accommodated to the irregularity of its external form, so that when a section is made of them, they exhibit a series of parallel curved lines.
2. River Soil, or the soil found in the beds and banks of rivers, and which is produced by the continual propelling power of large rivers. To this class belong two different kinds; 1st, Soil containing pebbles of various sizes, produced by the power of torrents in the vicinity of mountains; and, 2d, Earth or mud, deposited in the beds of rivers, in places at a distance from mountains. A peculiarity of river soil in general is, that it is much extended in length, while its breadth is comparatively but small. The different layers have neither so much irregularity as in the preceding kind, nor are they so precise in arrangement as in the following.
3. Lake Soil, deposited at the bottom of still water. To this class is to be referred the soil in the bottoms of valleys, which had formerly been lakes, either separate or connected with rivers. The horizontal dimensions of this kind of soil are often more or less equal. Sometimes, indeed, the length is greater than the breadth; not, however, in the same degree as in soil deposited in the bed of rivers. The surface is usually plane, and the different strata alternate in a parallel manner.
4. Marine Soil, that is to say, the mud of the ancient ocean. It is the greatest of all in its extent, both in a horizontal and a vertical direction. Its surface is more or less undulated, very seldom even. Its masses are both very thick and very uniform in composition. Different and alternating strata, however, do occur, whose forms and dimensions are usually more or less regular, and which are not unfrequently undulated.
Soil, after being formed, is acted upon by natural powers in various ways. The atmosphere is perpetually modifying it; rivers, waves, and winds, act here and there upon its surface, and alter its external form; water introduces into it the substances which it holds in solution. The different constituent parts of soil act upon each other chemically, and in this manner new decompositions and mixtures are produced; and this chemical change is increased by the action of vegetables, as well as of bodies deriving their origin from both organic kingdoms.
From what has been said of the relations existing between the masses of which the solid crust of the globe is composed, and the loose earth or soil by which it is covered, it appears evident enough (Hausmann concludes) that they have great influence over its formation and nature, and therefore upon the more perfect vegetables, and especially those which are the objects of cultivation; and that although the fertility of the soil is much increased by these vegetables themselves, yet the first foundation of their vigour is derived from the disintegration and decomposition of rocks. If this be correct, the constitution of the solid crust of the earth has a much more extended influence. For, by preparing a habitation for the greater and most important parts of plants, it also exerts a high degree of influence upon the animals which derive their sustenance from them, and, at the same time, affords the means of subsistence to man[416].
As the Irish Elk, the Fossil Elephant or Mammoth, and the Mastodon, are among the most remarkable of the fossil and extinct species of quadrupeds mentioned in the preceding pages of this work, we, with the view of farther gratifying the curiosity of our readers, now lay before them the following additional details from the writings of Cuvier, Goldfuss, and others.
(Noticed at p. 286.)
One of the most magnificent of the bisulcated animals met with in a fossil state in the British Islands is the Elk of Ireland, the Cervus megaceros. Bones and horns of vast size of this species are almost daily dug out of the bogs and marl pits of Ireland. Similar remains have been met with in alluvial strata in Britain, and also in the Isle of Man.
“So frequently do these remains,” Mr Hart remarks, “occur in most parts of Ireland, that there are very few of the peasantry who are not, either from personal observation or report, acquainted with them by the familiar name of the horns of the ‘old deer.’ Indeed in some parts of the country they have been found so often, that far from being regarded as objects of any extraordinary interest, they have been either thrown aside as lumber, or applied to the commonest economical uses[418].
“I have made diligent but fruitless search for an account of the particular time when any of these remains were first discovered. As they generally occur in marl, it is most likely that they did not begin to attract attention until the advanced state of agriculture had created an increased demand for that mineral as a manure. We can very easily imagine the astonishment which the appearance of horns so large, and of such strange form, must have excited in the minds of those who discovered them for the first time, and how readily they obtained a place in the hall of some adjoining mansion, where they were deposited as an ornament of great curiosity, from the contrast which they formed with the horns of the species of deer known at present. In this way we may account for the preservation of so many specimens as are found in the possession of the gentry in different parts of this country.
“Very lately an entire skeleton of the Irish Elk was dug up in that country. The following statement of the circumstances under which the bones were found, with their geological position, was laid before the Dublin Society, in a letter from Archdean Maunsell to the Right Hon. George Knox.