Salt marshes are formed of the silty deposits of rivers and of the sea. The former bring down vegetable mould and fine earth from the uplands, and the latter contribute sea weeds and grasses, sand and shells, and millions of animalculæ which, born for life in salt water only, die, and are deposited with the other matters, at those points where, from admixture with the fresh flow of the rivers, the water ceases to be suitable for their support. It is estimated that these animalculæ alone are the chief cause of the obstructions at the mouths of the rivers of Holland, which retard their flow, and cause them to spread over the flat country adjoining their banks. It is less important, however, for the purposes of this chapter, to consider the manner in which salt marshes are formed, than to discuss the means by which they may be reclaimed and made available for the uses of agriculture. The improvement may be conveniently considered under three heads:— First—The exclusion of the sea water. Second—The removal of the causes of inundation from the upland. Third—The removal of the rain-fall and water of filtration. *The Exclusion of the Sea* is of the first importance, because not only does it saturate the land with water,—but this water, being salt, renders it unfertile for the plants of ordinary cultivation, and causes it to produce others which are of little, or no value. The only means by which the sea may be kept out is, by building such dykes or embankments as shut out the highest tides, and, on shores which are exposed to the action of the waves, will resist their force. Ordinarily, the best, because the cheapest, material of which these embankments can be made, is the soil of the marsh itself. This is rarely,—almost never,—a pure peat, such as is found in upland swamps; it contains a large proportion of sand, blue clay, muscle mud, or other earthy deposits, which give it great weight and tenacity, and render it excellent for forming the body of the dyke. On lands which are overflowed to a considerable extent at each high tide, (twice a day,) it will be necessary to adopt more expensive, and more effective measures, but on ordinary salt meadows, which are deeply covered only at the spring tides, (occurring every month,) the following plan will be found practical and economical.