The friability and minute division of the soil to which Mr Cruickshanks refers, existing around the bulbs of trees, can only be of utility where the soil is too adhesive. Light soil is often injured by being cropped by plants which tend greatly to reduce adhesion—what the farmer styles being _driven_: besides, all luxuriant annual crops render adhesive soils friable; and, remaining for a time under natural grass, gives what is termed a turfiness to soils, which continues for several years, and which renders both adhesive and light soils more productive, preventing the adhesive from sinking down into mortar under cultivation, and the light from losing all adhesion or granular arrangement. There is, no doubt, a disposition to accumulate vegetable deposit in forests, from the moistness, coolness of the ground, and shade, not tending so much as the sunshine and exposure of open country to dissipate or volatilize the residuum of the decayed leaves and roots. In a lower latitude, beyond the line of peat formation, this will have some influence to increase the depth and richness of the vegetable mould; but, in Scotland, where cold till bottom prevails, more injury will result from forest tending to throw the debri of vegetation into combinations unfavourable to the nourishment of plants (such as peat {321} and compounds in which iron forms a part), than advantage, from the dead vegetable matter not being so much dissipated by aration and exposure to the sun. We have often observed the effect of remaining for a length of time in a state of considerable dryness, dissipating the vegetable part of the soil, in some of the old infield clays, where the crown of the large ridges are raised up a foot or two above the original surface-level. At the crown of the ridge, the vegetable clay mould often only extends down about nine inches from the surface, the subsoil immediately under being nearly void of vegetable matter, and extremely close tenacious clay,—a solid foot of it, though of equal moistness, being nearly double the weight of the same bulk of the vegetable clay mould above it. From this clay, almost purely mineral, being a little above the original surface-level, there can be no doubt, that at one time it consisted of the vegetable surface mould of the country, heaped up by repeated ploughings, and that it has gradually lost the vegetable part. The depth of vegetable soil, near the furrows of the ridges, is generally found to be greater than at the ridge crown.