Under the Scotch fir, for example, the pasture is not worth 6d. more per acre than before it was planted—under the beech and spruce, it is worth even less than before, though the spruce affords excellent shelter;—under ash, it gradually acquires an increased value of 2s. or 3s. per acre. In oak copses, it becomes worth 5s. or 6s., but only during the last eight years (of the twenty-four), before it is cut down. But under the larch, after the first thirty years, when the thinnings are all cut, land not worth originally more than 1s. per acre, becomes worth 8s. to 10s. per acre for permanent pasture.[20] [20] The result of trials made on the _mica slate_ and _gneiss soils_ (see page 100) of the Duke of Atholl. The cause of this improvement is to be found in the nature of the soil, which gradually accumulates beneath the trees by the shedding of their leaves. The shelter from the sun and rain which the foliage affords, prevents the vegetable matter which falls from being so speedily decomposed, or from being so much washed away, and thus permits it to collect in larger quantities in a given time, than where no such cover exists. The more complete the shelter, therefore, the more rapid will the accumulation of soil be in so far as it depends upon this cause. But the quantity of leaves which annually falls has also much influence upon the extent to which the soil is capable of being improved by any given species of tree, as well as the degree of rapidity with which those leaves, under ordinary circumstances, undergo decay. The broad membranous leaf of the beech and oak decay more quickly than the needle-shaped leaves of the pine tribes, and this circumstance may assist in rendering the larch more valuable as a permanent improver. We should expect likewise that the quantity and quality of the inorganic matter contained in the leaves,—brought up year by year from the roots, and strewed afterwards uniformly over the surface where the leaves are shed,—would materially affect the value of the soil they form. The leaves of the oak contain about 5 per cent. of saline and earthy matter, and those of the Scotch fir less than 2 per cent.; so that, supposing the actual weight of leaves which falls from each kind of tree to be equal, we should expect a greater depth of soil to be formed in the same time by the oak than by the Scotch fir. I am not aware of any experiments on the quantity of ash left by the leaves of the larch.