2. The trap rocks, comprising the greenstones and basalts, consist essentially[15] of felspar and hornblende or augite. In contrasting the trap rocks with the granites, it may be stated generally, that while the granites consist of felspar and quartz, the traps consist of felspar and hornblende (or augite). In the traps, both the felspar and the hornblende are reduced, by the action of the weather to a more or less fine powder, affording materials for a soil; in the granites the felspar is the principal source of all the earthy matter they are capable of yielding. If we compare together, therefore, the chemical composition of the two minerals (hornblende and felspar), we shall see in what respect these two varieties of soil ought to differ. Thus they consist of
| Felspar. | Hornblende. | |
| Silica, | 65 | 42 |
| Alumina, | 18 | 14 |
| Potash and soda, | 17 | trace. |
| Lime, | trace. | 12 |
| Magnesia, | ” | 14 |
| Oxide of iron, | ” | 14½ |
| Oxide of manganese, | ” | ½ |
| 100 | 97 |
Thus theory shews, that while granite soils may be eminently unfruitful, trap soils may be eminently fertile. And such is actually the result of observation and experience in every part of the globe. Unproductive granite soils cover nearly the whole of Scotland north of the Grampians, and large tracts of land in Devon and Cornwall, and on the east and west of Ireland; while fertile trap soils extend over thousands of square miles in the lowlands of Scotland, and in the north of Ireland; and where in Cornwall they occasionally mix with the granite soils, they are found to redeem them from their natural barrenness.
While such is the general rule in regard to these two classes of soils, it happens on some spots that the presence of other minerals in the granites, or of hornblende or mica in larger quantity than usual, give rise to a granitic soil of average fertility, as is the case in the Scilly isles; while, in like manner, the trap rocks are sometimes, as in parts of the isle of Skye, so peculiar in constitution as to condemn the land to almost hopeless infertility.
In some districts the decayed traps are dug up, and applied with advantage, as a top-dressing, to other kinds of land; and as by admixture with the decayed trap, the granitic soils are known to be improved in quality, so an admixture of decayed granite with many trap soils, were it readily accessible, might add to their fertility also.
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[16] 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.
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.