THE LEVELLING POWER OF RAIN. 379 



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 scoriae, 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 com- 

 pacted 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 



