OYSTER CULTURE IN GERMANY. 647 



the uncovered worms, mussels, and crabs, before they 

 withdraw into the earth. When the flats, at the time of 

 the lowest ebb, are lying, dry and silent, above the water, 

 one can already hear in the distant depths the roar of the 

 incoming flood. First it comes in slowly, then faster and 

 faster, and finally more slowly again, until at the full flood 

 the water stands over the northern portion of the flats 

 nearly two meters higher, and over the southern portion, 

 out from the mouth of the Elbe, nearly three 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 transport 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 im- 

 planted 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 change- 

 ful sea is quartz sand. In many places there are accumu- 

 lations of mud, which is very slimy and sticky, and contains 

 much organic matter. This mud is found along the shores 



