So the annual overflowings of the Nile, the Ganges, and the river of Amazons, gradually deposit accumulations of soil over surfaces of great extent;—and so also the bottoms of most lakes are covered with thick beds of sand, gravel, and clay, which have been conveyed into them from the higher grounds by the rivers through which they are fed.
To these and similar agencies, a large portion of the existing dry land of the globe has been, and is still exposed. Hence in many places, the rocks, and the soils naturally derived from them, are buried beneath accumulated heaps or layers of sand, gravel, and clay, which have been brought from a greater or less distance, and which have not unfrequently been derived from rocks of a totally different kind from those of the district in which they are now found. On these accumulations of transported materials, a soil is produced which often has no relation in its characters to the rocks which cover the country, and the nature of which a familiar acquaintance with these rocks would not enable us to predict.
To this cause is due that discordance between the first indications of geology, as to the origin of soils from the rocks on which they rest, and the actually observed character of those soils in certain districts—of which discordance mention has been made as likely to awaken doubt and distrust in the mind of the less instructed student in regard to the predictions of agricultural geology. There are several circumstances, however, by which the careful observer is materially aided in endeavouring to understand what the nature of the soils is likely to be, and how they ought to be treated, even when the subjacent rocks are thus overlaid by masses of drifted materials. Thus—
1. It not unfrequently happens, that the materials brought from a distance are more or less mixed up with the fragments and decayed matter of the rocks which are native to the spot, so that though modified in quality, the soil, nevertheless, retains the general characters of that which is formed on other spots from the decay of these rocks alone.
2. Where the formation is extensive, or covers a large area, as the new red sandstones and coal measures do in this country,—the mountain limestones in Ireland, and the granites in the north of Scotland—the transported sand, gravel, or clay, strewed over one part of the formation, has not unfrequently been derived from the rocks of another part of the same formation, so that, after all, the soils may be said to be produced from the rocks on which they rest, and may be judged of from the known constitution of these rocks.
3. Or if not from the rocks of the same formation, they have most frequently been derived from those of a neighbouring formation—from rocks which are to be found at no great distance, and generally on higher ground. Thus the ruins of the millstone grit rocks are often spread over the surface of the coal measures—of these, again, over the magnesian limestone,—of the latter, over the new red sandstone, and so on. The effect of this kind of transport upon the soils, is merely to overlap, as it were, the edges of one formation with the proper soils of the formations that adjoin it in the particular direction from which the drifted materials are known to have come.
It appears, therefore, that the occurrence on certain spots, or tracts of country, of soils that have no apparent relation to the rocks on which they immediately rest, tends in no way to throw doubt upon, to discredit or to disprove, the conclusions drawn from the more general facts and principles of geology. It is still generally true that soils are derived from the rocks on which they rest. The exceptions are local, and the difficulties which these local exceptions present, require only from the agricultural geologist a more careful study of the structure of each district, before he pronounces a decided opinion as to the degree of fertility it either naturally possesses, or by skilful cultivation may be made to attain.
Geological maps point out with more or less precision the extent of country over which the chalk, the red sandstone, the granites, &c., are found immediately beneath the loose materials on the surface; and these maps are of great value in indicating also the general quality of the soils over the same districts. It may be true, that here and there the natural soils are masked or buried by transported materials, yet the political economist may, nevertheless, with safety estimate the general agricultural capabilities and resources of a country by the study of its geological structure—the capitalist judge in what part of it he is likely to meet with an agreeable investment—and the practical farmer in what country he may expect to find land that will best reward his labours—that will admit of the kind of culture to which he is most accustomed, or, by the application of better methods, will manifest the greatest agricultural improvement.
SECTION III.—OF THE PHYSICAL CHARACTERS
OF SOILS.
The influence of climate on the fertility of a soil is often very great. This influence depends very much upon what are called the physical properties of soils.