FORMATION OF DUNES—SAND BANKS. 565 
hours’ duration, sand ridges may be observed on the north side 
of the fences, like the snow wreaths deposited by a drifting 
wind in winter. Some of the particles are carried back by con- 
trary winds, but most of them lodge on or behind the dunes, or 
in the moist soil near the lake, or are entangled by vegetables, 
and tend permanently to elevate the level. Like effects are 
produced by constant sea-winds, and dunes will generally be 
formed on all low coasts where such prevail, whether in tide- 
less or in tidal waters. 
Jobard thus describes the modus operandi, under ordinary 
circumstances, at the mouths of the Nile, where a tide can 
scarcely be detected: “When a wave breaks, it deposits an 
almost imperceptible line of fine sand. The next wave brings 
also its contribution, and shoves the preceding line a little 
higher. As soon as the particles are fairly out of the reach of 
the water they are dried by the heat of the burning sun, and 
immediately seized by the wind and rolled or borne farther in- 
land. The gravel is not thrown out by the waves, but rolls 
backwards and forwards until it is worn down to the state of 
fine sand, when it, in its turn, is cast upon the land and taken 
up by the wind.” * This description applies only to the com- 
mon every-day action of wind and water; but just in propor- 
tion to the increasing force of the wind and the waves, there is 
an increase in the quantity of sand, and in the magnitude of 
the particles carried off from the beach by it, and, of course, 
every storm in a landward direction adds sensibly to the accu- 
mulation upon the shore. 
Sand Banks. 
Although dunes, properly so called, are found only on dry 
land and above ordinary high-water mark, and owe their eleva- 
tion and structure to the action of the wind, yet, upon many 
shelving coasts, accumulations of sand much resembling dunes 
are formed under water at some distance from the shore by the 
* STARING, De Boden van Nederland, i., p. 827, note. 
