HYDEOGRAPHY. 



465 



piles, and covered with fascines and rush mats. The wood is concealed beneath an 

 armour of flat-headed nails, so that teredo worms may find no crevice to lodge in. 

 The ballast consists of huge blocks of granite, brought at great expense from the 

 interior or from Norway. As a rule these dykes are about 30 feet in height, and 

 between 1Ô0 and 350 feet in thickness. Their construction was all the more 



Fig. 260. — The Zuiderpoldeer now. 

 Scale I : 76,000. 



1 Mile. 



difficult as, owing to the depth of the alluvial deposits, it is quite impossible to 

 secure solid foundations in Holland. 



The dykes are as a rule maintained by the landed proprietors. The expense 

 is very considerable, especially when projecting groins have to be added as a 

 defence against undermining currents. When a dyke, in spite of all, threatens to 

 105 



