FORMATION OF DUNES. 373 



which is able to live in water less salt than that ol 

 the ocean. 



The encroachments of the sea on dry land are not 

 confined to inundations ; it sometimes bores beneath 

 the sand, which it throws up from its bosom. The 

 dunes (so called from the Celtic word dun, which sig- 

 nifies an elevated spot), or sandhills, are formed on the 

 seashore, as in the African desert, by the action of 

 the wind on the loose sand. They present a gentle 

 inclination towards the sea, whilst on the land-side 

 they are terminated by an abrupt declivity. Their 

 height is generally from 15 to 20 yards, but in rare 

 instances they attain a height of 80 yards, which may 

 be considered the extreme limit. 



For the rapid formation of dunes it is necessary 

 that the sea should leave a large space bare, which 

 subsequently it will cover with its waters and the 

 sand it carries with it. These conditions are best 

 fulfilled on those coasts where the daily ebb-and-flow 

 of the tide leaves a large extent of sand exposed to the 

 drying action of the sun and wind. Another condition 

 on which the increase of the dunes depends, is that 

 the sea-winds blow more frequently than those from 

 the land — otherwise the work of one day will be 

 undone by that of another. 



The formation of dunes is sufficiently simple. Tho 

 wind from the sea blowing over the sandy waste. 



