442 SAND-PLAINS OF BELGIUM. [April 



accumulates ou these, the slope becomes more steep ; and it 

 becomes so more especially on the windward side of the bank, 

 where it may take an inclination of from 5° to 15° at the base, 

 and much steeper towards the summit ; while to leeward the 

 moving sand, being sheltered from the propelling power of 

 the wind, comes under the sole influence of gravitation and the 

 vis inertia with which it is borne forward, the result being that it 

 forms a slope approximating to the angle of stability. Wessely 

 found by measurement the slope to leeward of dunes on the 

 Banat sand -waste to be from 27° to 32°, on an average 30°. 

 Hayard found it in dunes on the Baltic from 26j° to 31^°. Forch- 

 hammer found it in Denmark, 30°. 



Krause makes mention, in a valuable work on the subject, of 45°, 

 and Brementier, of from 50° to 60°. Wessely alleges these state- 

 ments must be erroneous, and this from the inchnation having been 

 estimated by the eye, not determined by measurement. My recol- 

 lection of what I saw of dunes on the Flats, between Table Bay 

 and False Bay, at the Cape of Good Hope, makes me hesitate to 

 consider the statements erroneous. Blessen gives 36 as the 

 constant slope of dunes on their leeward side, but this, says Wessely, 

 was apparently not the result of measurement, but of opinion ; and 

 he states that observation has proved that a slope always appears 

 greater by the eye than measurement proves it to be. 



The slope is oftentimes found exceptionally great, more especially 

 when the dunes have been more or less cousoHdated by rain, or 

 dew, or spray. Hagen, by measurement of a dune on the Baltic, 

 20 feet high, sheltered from the wind by a dense bush, and over- 

 grown with mosses and ferns, found the slope to leeward 41°; and 

 Wessely mentions that he has also seen exceptional cases on the 

 Banat, but they proved only of temporary duration ; for with 

 the next dry and warm weather, the sand dried and slipped down 

 till it reached the normal slope of 30°. 



But for the partial arrest of the drift by the formation of dunes, 

 and by the shelter which they afford, the drift might spread to 

 an indefinite extent over the fertile land to leeward ; the whole 

 progress of the operation may be traced within a limited range in 

 strand dunes ; and it may be seen that the dunes grow in height 

 wherever their summit is not pressed onward, the only limit being 

 the supply of sand. Wliile the arrest of growth in height is brought 

 about by the quantity carried onward becoming equal to the ac- 

 cessions made, the dunes in any district will be found to be of 

 greater or less height according as the sand layers from which they 

 are formed are of greater or less thickness. Thus those on the 

 strand may become like hills, and they are only prevented from 



