Chapter XXIX: Section I: Soils of the Granites and Trap Rocks

1. The _granites_ consist of a mixture, in different proportions, of three minerals, known by the names of _quartz_, _felspar_, and _mica_. The latter, however, is generally present in such small quantity, that in our general description it may be safely left out of view. Granites, therefore, consist chiefly of quartz and felspar, in proportions which vary very much, but the former, on an average, constitutes perhaps from one-third to one-half of the whole.

_Quartz_ has already been described—(see p. 51)—as the substance of flint, the silica of the chemist. When the granite decays, this portion of it forms a more or less coarse siliceous sand.

_Felspar_ is a white, greenish, or flesh-coloured mineral, often more or less earthy in its appearance, but generally hard and brittle, and sometimes glassy. It is scratched by, and thus is readily distinguished from, quartz. When it decays, it forms an exceedingly fine clay.

A remarkable difference appears thus to exist, in chemical constitution, between these two minerals—a difference which must affect also the soils produced from them. A granite soil, in addition to the siliceous sand, will consist chiefly of silica, alumina, and potash; a hornblende soil, in addition to silica and alumina, of much lime, magnesia, and oxide of iron—of nearly 2½ cwt. of each of these latter for every ton of decayed rock. A hornblende soil, therefore, contains more of those inorganic constituents which the plants require for their healthy sustenance, and therefore will prove more generally productive than a soil of decayed felspar. But when the two are mixed, as in the greenstones, the soil must be still more favourable to vegetable life. The potash and soda, of which the hornblende is nearly destitute, the felspar is able abundantly to supply; while, by the hornblende are yielded lime and magnesia, which are known to exercise a remarkable influence on the progress of vegetation.

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

[15] The reader is referred for more _precise_ information to the author’s “LECTURES,” pp. 377 to 390.

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.