Oyster Culture in Europe and Japan 83 



The difficulties that have been enumerated are met by 

 converting the ground into a series of basins by means 

 of walls or dikes. These ponds are variable in form 

 and in area, but are usually rectangles of two or three 

 hundred square yards. The walls are often simple and 

 inexpensive. Very shallow basins, like those shown in 

 Figure 20, are easily constructed. To form the wall, 

 parallel lines of planks are held in position on edge by 

 means of wooden pegs. Parallel lines of stakes are also 

 sometimes employed, the space between being filled with 

 soil. If tide currents are strong, stones are used with 

 the other filling material, in order to give sufficient 

 strength to the wall. In some localities it has been profit- 

 able to build walls of solid masonry, but this construction 

 is usually resorted to only when a large area, to be 

 flooded to a considerable depth, is to be shut off from the 

 sea. The height of the wall varies from a few inches 

 to several feet. At some favorable point in the wall, 

 a gate is constructed that, when closed, may retain the 

 water that enters at high tide. 



The bottom of the basin must now be prepared. In 

 the better parks, the center is made a little higher than 

 the margins, where trenches are often dug, in order that 

 sediment, settling on the bottom, may be carried 

 into them. When the gate of the full basin is opened, 

 the current leaving the inclosure by these is sufficient to 

 carry away much of the soft silt. After the bottom has 

 been roughly shaped, it is sometimes hardened by pound- 

 ing, and is covered by a layer of clay or sand and gravel. 

 If the soil in the walls is pervious, these also are lined 

 with clay in order that water may be retained. 



It is interesting to observe that bottoms, originally of 

 the softest mud, are easily reclaimed and made hard and 



