We have mentioned the formation of dust, and the action of wind upon it, as a cause tending to level the surface of the land. It may appear to many that this cause is too insignificant to be noticed among those which modify the earth’s surface. In reality, however, owing to its continuous action, and to its always acting (in the main) in one direction, this cause is much more important than might be supposed. We overlook its action as actually going on around us, because in a few years, or in a few generations, it produces no change that can be readily noticed. But in long periods of time it changes very markedly the level of lower lands, and that too even in cities, where means exist for removing the accumulations of dust which are continually collecting on the surface of the earth. We know that the remains of old Roman roads, walls, houses, and so forth, in this country, are found, not at the present level of the surface, but several feet—in some cases many yards—below this level. The same holds elsewhere, under the same conditions—that is, where we know quite certainly that the substances thus found underground were originally on the surface, and that there has been neither any disturbance causing them to be engulfed, nor any deposition of scoriæ, volcanic dust, or other products of subterranean disturbance. We cannot hesitate to regard this burying of old buildings as due to the continual deposition of dust, which eventually becomes compacted into solid earth. We know, moreover, that the formation of dust is in the main due to rain converting the surface layers of the earth into mud, which on drying requires but the frictional action of heavy winds to rise in clouds of dust. In some soils this process goes on more rapidly than in others, as every one who has travelled much afoot is aware. There are parts of England, for instance, where, even in the driest summer, the daily deposition of dust on dry and breezy days is but slight, others where in such weather a dust layer at least a quarter of an inch in thickness is deposited in the course of a day. If we assumed, which would scarcely seem an exaggerated estimate, that in the course of a single year a layer of dust averaging an inch in thickness is deposited over the lower levels of the surface of the land, we should find that the average depth of the layer formed in the last thousand years would amount to no less than eighty-three feet. Of course in inhabited places the deposition of dust is checked, though not so much as most persons imagine. There is not probably in this country a single building five hundred years old, originally built at a moderately low level, the position of whose foundation does not attest the constant gathering of matter upon the surface. The actual amount by which the lower levels are raised and the higher levels diminished in the course of a thousand years may be very much less, but that it must amount to many feet can scarcely be questioned.