ISLANDS ENLARGED BY DIKING. 
339 
is composed of the same constituents. First comes a stratum 
of sand, with marine shells, or the shells of mollusks living in 
brackish water. If there he tides, and, of course, flowing and 
ebbing currents, mud is let fall upon the sand only after the 
latter has been raised above low-water mark ; for then only, 
at the change from flood to ebb, is the water still enough to 
form a deposit of so light a material. Where mud is found at 
greater depths, as, for example, in a large proportion of the Ij, 
it is a proof that at this point there was never any consid¬ 
erable tidal flow or other current. * * * The powerful 
tidal currents, flowing and ebbing twice a day, drift sand with 
them. They scoop out the bottom at one point, raise it at 
another, and the sandbanks in the current are continually 
shifting. As soon as a bank raises itself above low-water 
mark, flags and reeds establish themselves upon it. The me¬ 
chanical resistance of these plants checks the retreat of the 
high water and favors the deposit of the earth suspended in it, 
and the formation of land goes on with surprising rapidity. 
When it has risen to high-water level, it is soon covered with 
grasses, and becomes what is called schor in Zeeland, kwelder 
in Friesland. Such grounds are the foundation or starting 
point of the process of diking. When they are once elevated 
to the flood-tide level, no more mud is deposited upon them 
except by extraordinary high tides. Their further rise is, 
accordingly, very slow, and it is seldom advantageous to delay 
longer the operation of diking.” * 
The formation of new banks by the sea is constantly going 
on at points favorable for the deposit of sand and earth, and 
hence opportunity is continually afforded for enclosure of new 
land outside of that already diked in, the coast is fast advanc¬ 
ing seaward, and every new embankment increases the se¬ 
curity of former enclosures. The province of Zeeland consists 
of islands washed by the sea on their western coasts, and sep¬ 
arated by the many channels through which the Schelde and 
some other rivers find their way to the ocean. In the twelfth 
* Voormaals en Thans , pp. 150, 151. 
