330 BOARD OF AGRICULTURE. 



geological period. They are either the settled scourings of 

 the ocean, or of both fresh and salt water; and resemble 

 quite frequently, certain layers of sand, or loam, or marl," 

 found in the sea-bottom-lands of preceding geological periods. 

 In some instances they consist of nothing but layers of fine 

 sand, terminating in more or less extensive sand-bars of a 

 coarser material, — as, for instance, along the coast of the 

 Baltic Sea. 



As they are usually quite deficient in most of the essential 

 constituents of a good agricultural soil, they remain unpro- 

 ductive. Formations of this kind are, in a large degree, the 

 consequence of a low tide, which prevents the temporary 

 existence of basins behind the bar, and thereby excludes the 

 chance of retaining the finer floating material of the sea-water. 

 These sand-deposits become, occasionally, the habitations of 

 numerous mollusks, and are thereby enriched by shells and 

 organic matter, changing subsequently into sandy marls. 

 They are found in some localities along the western coast of 

 Sweden, where they are highly valued for agricultural pur- 

 poses. The most fertile sea-marshes, as a general rule, 

 however, are found where tide-water and rivers have had an 

 equally important share in their formation. The well-known 

 sea-marshes at the mouths of the rivers Me use, Scheldt, 

 Rhine, Jahde, etc., belong to that class {Polders). The soil 

 differs widely in these localities. It is known to vary from 

 10 to 20 per cent, of sand, and 75 to 85 per cent, of clay, 

 with 5 per cent, of humus, — to from 10 to 15 per cent, of 

 clay, with 75 to 85 per cent, of sand. In some instances it 

 consists in the main of a calcareous clay. Most of these lands 

 are remarkably productive ; even those which contain from 

 70 to 80 per cent, of sand pay well if subjected to a rational 

 mode of cultivation. Success in the latter case is considered 

 certain, wherever, at a few feet depth, a clayish deposit 

 prevents an unusual waste of moisture and plant-food ; or 

 where a judicious system of ditches secures a constant but 

 moderate supply of fresh water. 



Most noteworthy among the European sea-marshes are 

 those of Belgium. They form several distinct belts, of a 

 varying character, as far as the extremes of their respective 

 soils are concerned, and are, in many places, in a high state 



