412 CONSTRUCTION OF DIKES. 
professional treatises, and I shall content myself with present- 
ing such information as can be gathered from works of a more 
popular character. 
The superior strata of the 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 de- 
posited by them upon the sands of the bottom, or carried out 
to sea by their currents, and then, after a shorter or longer ex- 
posure to the chemical and mechanical action of salt-water and 
marine currents, 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 successive alluvious 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 seawater 
also, when heavy or long-continued west winds drove it land- 
wards. The extraordinary 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 ex- 
posed 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 arti- 
ficial mounds, and many similar precarious habitations still 
exist on the unwalled islands and shores beyond the chain of 
dikes. River embankments, which, as is familiarly known, 
have from the earliest antiquity been employed in many coun- 
tries 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 con- 
nect the river-walls together by a transverse dike or raised 
causeway, which would serve as a means of communication be- 
tween different hamlets and at the same time secure the inter- 
mediate ground both against the backwater of river-floods and 
against overflow by the sea. The oldest true sea-dikes describ- 
ed in historical records, however, are those enclosing islands in 
