566 SAND BANKS. 
oscillations of the waves, and are well known by the name of 
sand banks. They are usually rather ridges than banks, of 
moderate inclination, and with the steepest slope seawards,* 
and their form differs little from that of dunes except in this 
last particular and in being lower and more continuous. Upon 
the western coast of the island of Amrum, for example, there 
are three rows of such banks, the summits of which are at a 
distance of perhaps a couple of miles from each other ; so that, 
including the width of the banks themselves, the spaces between 
them, and the breadth of the zone of dunes upon the land, the 
belt of moving sands on that coast is probably not less than 
eight miles wide. 
Under ordinary circumstances, sand banks are always rolling 
landwards, and they compose the magazine from which the 
material for the dunes is derived.t The dunes, in fact, are but 
aquatic sand banks transferred to dry land. The laws of their 
formation are closely analogous, because the action of the two 
fluids, by which they are respectively accumulated and built up, 
is very similar when brought to bear upon loose particles of 
solid matter. It would, indeed, seem that the slow and com- 
paratively regular movements of the heavy, unelastic water 
ought to affect such particles very differently from the sudden 
and fitful impulses of the light and elastic air. But the velo- 
city of the wind currents gives them a mechanical force approx- 
imating to that of the slower waves, and, however difficult it 
may be to explain all the phenomena that characterize the 
* Kou, Jnseln und Marschen Schleswig Holsteins, ii., p. 33. From a draw- 
ing in ANDRESEN, Om Klitformationen, p. 24, it would appear that on the 
Schleswig coast the surf-formed banks have the steepest slope landwards, 
those farther from the shore, as stated in the text. 
+ Sand banks sometimes connect themselves with the coast at both ends, 
and thus cut off a portion of the sea. In this case, as well as when salt water 
is enclosed by sea-dikes, the water thus separated from the ocean gradually 
becomes fresh, or at least brackish. The Haffs, or large expanses of fresh 
water in Eastern Prussia—which are divided from the Baltic by narrow sand 
banks called Nehrungen, or, at sheltered points of the coast, by fluviatile 
deposits called Werders—all have one or more open passages, through which 
the water of the rivers that supply them at last finds its way to the sea. 
oe ee ee 
