[Illustration: +Fig.+ 7.—Diagram to illustrate the effect of an obstacle on the transportation and deposition of sand. The direction of the wind is indicated by the upper arrow. The lower arrows represent the direction of eddies in the air occasioned by the obstacle. If the surface in which the obstacle was set was originally flat (dotted line), the sand would tend to be piled up on either side at a little distance from the obstacle, but more to leeward. At the same time, depressions would be hollowed out near the obstacle itself (see full line). (After Cornish.)] Much of the dust transported by the wind is carried out over seas or lakes and falls into them. By this means, sedimentation is doubtless going on at the bottom of the whole ocean, and at the bottoms of all lakes. While means of determining the amount of dust blown into the sea are not at hand, it is safe to say that, were such determinations possible, the result, if stated in terms of weight, would be surprising. =Transportation and deposition of sand.=—In its transportation by the wind, sand is not commonly lifted far above the surface of the land, and its movement is therefore more generally interfered with by surface obstacles than is the movement of dust. A shrub, a tree, a fence, a building, or even a stone may occasion the lodgment of sand in considerable quantity, though it has little effect on the lodgment of dust. The effect of obstacles is illustrated by Fig. 7 (see also Fig. 5). If the obstacle which occasions the lodgment of sand presents a surface which the wind cannot penetrate, such as a wall, sand is dropped abundantly on its windward side as well as on the leeward; but if it be penetrable, like an open fence, the lodgment takes place chiefly on its leeward side. In cultivated regions cases are known where, in a few weeks of dry weather, sand has been drifted into lanes in the lee of hedges to the depth of two or three feet, making them nearly impassable to vehicles. =Formation of dunes.=—In contrast with dust deposited from the atmosphere, wind-blown sand is commonly aggregated into mounds and ridges in the process of lodgment. These mounds and ridges are dunes. Once a dune is started, it occasions the further lodgment of sand, and is a cause of its own growth. Dunes sometimes reach heights of 200 or 300 feet, but they are much more commonly no more than 10 or 20 feet in height. On plane surfaces, there is a limit in height above which they do not rise, though the limit is different under different conditions. The velocity of wind at the bottom of the air is not so great as that higher up, and as a dune is built up, a level is presently reached where the stronger upper winds sweep away as much sand as is brought to the top. The very even crests of many dune ridges are probably to be accounted for in this way. Wind-blown or eolian sand, not piled up in heaps or ridges, is somewhat widespread, but does not constitute dunes.