688 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [6] 
tion of the flats nearly 2 meters higher, and over the southern portion, 
out from the mouth of the Elbe, nearly 3 meters higher than at the 
ebb. The tide generally attains three-fourths of its entire height about 
three hours after turning. In this short time immense masses of water 
move towards the coast, and in many places currents are formed as 
swift as the current of the Rhine between Coblenz and Bonn, the rate 
of which is from 1.5 to 2 meters per second. Yet the ebb-currents 
are nearly everywhere stronger than the flood-currents, since they not. 
only carry off the sea-water which has been brought in, but also the 
fresh water from the land, which was checked in its flow during the 
flood. Hence the ebb-currents bring about much greater changes in the 
soil of the sea-flats than the flood-currents, and they displace and trans- 
port the constituents of the flats in the most powerful manner, wherever 
great fresh-water streams enter the sea, as at the mouths of the Eider, 
Elbe, Weser, and Ems. Here the floating buoys and the implanted 
buoy-stakes (Baken), which indicate navigable water for vessels, are 
changed nearly every year because of the changes in the channels. 
The principal ingredient of the bottom of this changeful sea is quartz. 
sand. In many places there are accumulations of mud, which is very 
slimy and sticky, and contains much organic matter. This mud is found 
along the shores of the mainland and on the east side of the island of 
Sylt, principally at those points where, after the changing of the currents, — 
the water does not flow fast enough to carry away all of the muddy ma- 
terial which was deposited upon the bottom as the tide ran out. Along 
the slopes which lie between those portions of the flats, which the tide 
leaves dry, and the deep channels are long dry stretches of soil where 
the ground is covered with coarse sand, small and large stones, and 
shells. At such places colonies of oysters, so-called oyster-beds, are 
found, along with many other sea-animals. 
Diagram of a cross-section of a deep channel in the sea-flats, upon the left bank 
of which lies an oyster-bed. Upon both sides are flats which are left dry by the ebb- 
tide. (The breadth of the channel is drawn upon a much smaller seale than the depth.) 
