SAND-BANKS AND SAND-DUNES. 107 



Pyrenees, and moving forward at the rate of sixty or seventy 

 feet every year, have buried several villages, which were 

 well known in the Middle Ages. In some places their pro- 

 gress is arrested by quite small running streams, the sand, 

 as it drifts into the water, being carried back into the sea ; 

 but, on the other hand, they have proved more than a match 

 for the river Ad our, which they have turned nearly a mile 

 and a quarter out of its original course. (Fig. n, p. 38.) 



In the outer Hebrides, the encroachment of the sands 

 has been checked by planting them with the sea-reed or 

 mat-grass, whose tough roots are often twenty feet long, and 

 serve to bind the sand together. 



In Ceylon, where the rivers flow rapidly down from lofty 

 hills, they reach the coast heavily laden with sand and mud, 

 which, instead of being carried any distance out to sea, are 

 heaped in bars along the shore by the currents of the Bay 

 of Bengal. The bars extend north and south, and at length 

 attain such dimensions that the rivers, being unable to force 

 their way through, are obliged to flow behind them in search 

 of a fresh outlet.* Long embankments, from a mile to three 

 miles broad, and forty miles long, have thus gradually accu- 

 mulated, and having first been in some degree consolidated 

 by the growth of an ipomoea or convolvulus, which sends 

 out roots from every joint, the soil has then been fertilised 

 by glassworts, saltworts, and other sand-loving plants, until 

 at last it has become capable of supporting plantations of 

 cocoanut trees. 



* These barriers grow especially fast on the east of the island, to which 

 large quantities of sand are brought by the southern current from the 

 Coromandel coast. 



