OF SAND FORMATIONS ON MARINE COASTS. 25 



the wind carries it further inland. On tlie coasts in question, the 

 westerly winds are the prevailing, and therefore the sand wanders in 

 an easterh' direction. 



Because of their extreme shiftiness of soil, the dunes do not attain 

 any considerable elevation. The sand deposited by the wind on the 

 summit of the hill is always in a state of unstable equilibrium. It has 

 a constant tendency to be precipitated down the other side, and the 

 higher the summit, the greater is this tendency, so that the dune arrives 

 at last to a point when no further accumulation is possible. The dune, 

 however, still continues to grow, extending its base and generally 

 increasing in dimensions, but does not increase in elevation. 



The size and the height of the dune depends on the distance from the 

 sea and on the strength of the wind. In some cases it has been observed 

 that during a strong wind a dune has decreased in height 5 em., while 

 other dunes have increased 25 cm. Every dune has one side placed 

 against the prevailing wind. This front side has a lower grade than 

 that on the lee side, which is always more abrupt. As long as the same 

 wind prevails, and as long as the wind carries only so much sand as 

 it is able to take away from the top of the dune, so long will the dune 

 retain its position and form, just as a whirlpool in a river is constant, 

 so long as the river maintains the same velocity and volume. Because 

 the sand grains cannot be lifted to any greater height in the air, the 

 dunes, when they have reached a certain elevation, would present to the 

 sand grains an almost insurmountable obstacle, but they have very seldom 

 time to cohere. The wind modifies its work incessantly and the height 

 of the dune is very soon reduced by stronger winds. 



The transporting power of the air is, as already mentioned, small 

 compared with that of water, because of its lightness and want of 

 cohesion. The size of the particles has, therefore, a great influence 

 not only on the degree to which the sand is liable to drift, but also 

 on the extent in which it may manifest properties relative to the 

 texture of the soil, among others that of retaining moisture, which is 

 so important to vegetation. 



The amount of sand transportation is greatest, other things being 

 ec[ual, where there is no cover of vegetation to keep down the sands, 

 and the deposits made are most extensive in the direction of the pre- 

 vailing currents. The coarser dune sand particles are pushed along the 

 ground, while the finer form clouds of dust in the air, and settle rapidly 

 or slowly near to or remote from the source of supply according to the 

 force of the wind and the size of the particles suspended. 



Library Publications. 2 



