400 CONSTRUCTION OF DIKES. 



The superior strata of tlie lowlands upon and near the coast 

 are, as we have seen, principally composed of soil brought down 

 by the great rivers I have mentioned, and either directly depos- 

 ited by them upon the sands of the bottom, or carried out to sea 

 by their currents, and then, after a shorter or longer exposure to 

 the chemical and mechanical action of salt-water and marine cur- 

 rents, restored again to the land by tidal overflow and subsidence 

 from the waters in which it was suspended. At a very remote 

 period the coast-flats were, at many points, raised so high by suc- 

 cessive alluvions or tidal deposits as to be above ordinary high- 

 water level, but they were still liable to occasional inundation 

 from river-floods, and from the sea-water also, when heavy or 

 long-continued west winds drove it landwards. The extraordi- 

 nary fertility of this soil and its security as a retreat from hostile 

 violence attracted to it a considerable population, while its want 

 of protection against inundation exposed it to the devastations of 

 which the chroniclers of the Middle Ages have left such highly 

 colored pictures. The first permanent dwellings on the coast- 

 flats were erected upon artificial mounds, and many similar pre- 

 carious habitations still exist on the unwalled islands and shores 

 beyond the chain of dikes. River embankments, which, as is 

 f amiharly known, have from the earliest antiquity been employed 

 in many countries where sea-dikes are unknown, were probably 

 the first works of this character constructed in the Low Countries, 

 and when two neighboring streams of fresh water had been em- 

 banked, the next step in the process would naturally be to connect 

 the river-walls together by a transverse dike or raised causeway, 

 which would serve as a means of communication between different 

 hamlets and at the same time secure the intermediate ground both 

 against the backwater of river-floods and against overflow by the 

 sea. The oldest true sea-dikes described in historical records, 

 however, are those enclosing islands in the estuaries of the great 

 rivers, and it is not impossible that the double character they 

 possess as a security against maritime floods and as a military 

 rampart, led to their adoption upon those islands before similar 

 constructions had been attempted upon the mainland. 



At some points of the coast, various contrivances, such as piers, 

 piles, and, in fact, obstnictions of all sorts to the ebb of the cur- 

 rent, are employed to facilitate the deposit of shme, before a 



