DUNES OF AMERICA 
469 
name of sand banks. They are usually rather ridges than 
banks, of moderate inclination, and with the steepest slope sea¬ 
ward ; and their form difiers from that of dunes only 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 roll¬ 
ing landward, and they compose the magazine from which 
the material for the dunes is derived. 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 par¬ 
ticles of solid matter. It would, indeed, seem that the slow 
and comparatively 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 velocity of the wind currents gives them a mechanical 
force approximating to that of the slower waves, and, however 
difficult it may be to explain all the phenomena that charac¬ 
terize the structure of the dunes, observation has proved that 
it is nearly identical with that of submerged sand banks. The 
differences of form are generally ascribable to the greater num¬ 
ber and variety of surface accidents of the ground on which 
the sand hills of the land are built up, and to the more frequent 
changes, and wider variety of direction, in the courses of the 
wind. 
Dunes on the Coast of America. 
Upon the Atlantic coast of the United States, the preva¬ 
lence of western or off-shore winds is unfavorable to the forma¬ 
tion of dunes, and, though marine currents lodge vast quail- 
