542 FOEMATION OF DUlfES. 



gradually rise to tlie height of thirty, fifty, sixty or a hundred 

 feet, and sometimes even much higher. Strong winds, instead 

 of adding to their elevation, sweep off loose particles from their 

 surface, and these, with others blown over or between them, build 

 up a second row of dunes, and so on according to the character 

 of the wiud, the supply and consistence of the sand, and the face 

 of the country. In this way is formed a belt of sand-dunes, ir- 

 regularly dispersed, varying much in height and dimensions, 

 and sometimes many miles in breadth. On the Island of Sylt, 

 in the German Sea, where there are several rows, the width of 

 the belt is from half a mile to a mile. There are similar ranges 

 on the coast of HoUand, exceeding two miles in breadth, while 

 at the mouths of the Kile they form a zone not less than ten 

 miles wide. 



The base of some of the dunes in the Delta of the Kile is 

 reached by the river during the annual inundation, and the infil- 

 tration of the water, which contains Hme, has converted the lower 

 strata into a silicious limestone, or rather a calcareous sandstone, 

 and thus afforded an opportunity of studying the structm*e of 

 that rock in a locality where its origin and mode of aggregation 

 and sohdification are known. 



The tide, though a usual, is by no means a necessary, condition 

 for the accumulations of sand out of which dunes are formed. 

 The Baltic and the Mediterranean are almost tideless seas, but 

 there are vast ranges of dunes on the Russian and Prussian coasts 

 of the Baltic, and at the mouths of the Kile and many other 

 points on the shores of the Mediterranean. The vast shoals in 

 the latter sea, known to the ancients as the Greater and Lesser 

 Syrtis, are of marine origin. They are still filling up with sand, 

 washed up from greater depths or sometimes drifted from the 

 coast in small quantities, and wiU probably be converted, at some 

 future period, into dry land covered with sand-hills. There are 



French capitalists would lose the money they had invested in that great un- 

 dertaking. 



Ponds of water are often found in the depressions between the sand-hills of 

 the dune chains in the North American desert. 



The best water supplied to the city of Amsterdam is rain-water absorbed 

 by the coast-dunes and purified by those natural filters, from which large 

 quantities are conveyed to the city. 



