Section Ii.—of the Superficial Accumulations of Transported Materials

ON DIFFERENT PARTS OF THE EARTH’S SURFACE.

It is necessary to guard the reader against disappointment, when he proceeds to examine the existing relation between the soils and the rocks on which they lie, or to infer the quality of the soil from the known nature of the rock in conformity with what has been above laid down,—by explaining another class of geological appearances which present themselves not only in our own country but in almost every other part of the globe.

The unlearned reader of the preceding section and chapter may say—I know excellent land resting upon the granites, fine turnip soils on the Oxford or London clays, tracts of fertile fields on the coal measures, and poor, gravelly farms on the boasted new red sandstone: I have no faith in theory—I can have none in theories which are so obviously contradicted by natural appearances. Such, it is to be feared, is the hasty mode of reasoning among too many locally excellent practical men, familiar, it may be, with many useful and important facts, but untaught to look through and beyond isolated facts to the principles on which they depend.

By locally excellent, I mean those who are the best possible farmers of their own district and after their own way, but who would fail in other districts requiring other methods. To the possessor of agricultural principles the modifications required by difference of crop, soil, and climate, readily suggest themselves, where the mere practical man is bewildered, disheartened, and in despair.

Every one who has lived long, on the more exposed shores of our island, has seen, that when the weather is dry, and the sea winds blow strong, the sands of the beach are carried inland and spread over the soil, sometimes to a considerable distance from the coast. In some countries this sand-drift takes place to a very great extent, and gradually swallows up large tracts of fertile land.

Again, most people are familiar with the fact, that during periods of long continued rain, when the rivers are flooded and overflow their banks, they not unfrequently bear with them loads of sand and gravel, which they carry far and wide, and strew at intervals over the surface soil.

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.