LAGOON, or LAG UNE. Lagoons are sheets of water formed either by the encroachments of rivers or seas upon the land, or by the separation of a portion of the sea by the intervention of a bank. there are fluvial and marine lagoon& When the land on either aide of a river's course is lower than the immediate banks of the etream, and the river, in the season of the floods, either overflows its banks or in part breaks them down, the water inundates the low land; and if on the subsidence of the flood the water again flows back into the channel, the lagoon is merely temporary, and is simply an inuudation. This, according to circumstances, may be a benefit to the country- or a disaster. Inundations are a benefit when they bring with them and deposit a rich vegetable humus, which, ou being cultivated, yields abundant crops ; to such deposita Lower Egypt owes its great fertility. Inundations of this kind either diminish or increase anuually, for, by repeated deposits, the soil becomes raised; and unless the bed of the river rise in proportion, the water is eventually kept within its channel; but if, on the contrary, the bed rise, the inundation gains every year in extent of surface what it loses in depth of water. Should the inundation however, instead of a prolific mud, bring nothing but sand and stones, then, as has been frequently experienced in Italy, the inundations are a cruel disaster, for they condemn rich land, to eternal barrenness and sterility. If the configuration of the Land and other circumstances prevent the water of the inundated parts from flowing back on the subsidence of the flood, then a permanent lagoon is formed, and the land thus laid under water can only be recovered at a great expense, even if that be possible. These lagoons are generally fatal to
the neighbourhood, for the water in them, being stagnant, gives rise to unwholesome miasmata, producing agues and other malignant dis orders. Such lagoons are not absolutely confined to the lower parts of water-courses, though it is in such places that they are most frequeutly met with. Fluvial lagoon* are sometimes formed by infiltration; a remarkable instance of which is the marshy lagoon of Ybora, ou the Parana, in South America.
Marine lagoons are much more common than those on the borders of rivets. They are formed sometimes by the encroachments of the sea, and sometimes by the throwing up of a bar or bank, which eventually divides off a portion of the sea altogether, or leaves merely a small opening. In Europe there are many marine lagoons ; the Adriatic, on its north and north-western parts particularly, is full of them. The Znytler Zoo is a vast lagoon. There are also two very large ones known by the names of the Frisch° Half and the Curische Hatt at the south-east angle of the Baltic Sea. In the Sea of Azoff there is the Sivaach or Putrid Sea. On the east coast of South America there are some very large lagoons, and they abound at the bottom of the Mexican Gulf. Marinelagoon's can never be useful unless when sufficiently large and deep to admit of being navigated, in which case they form secure harbours. When shallow, they give out foetid exhalations like fluvial lagoons, as is too well known in Venice, which is built on the GO islands of the lagoon at the extremity of the gulf ; though lu this case much of the evil arises undoubtedly from the circumstance of the lagoon being the receptacle of all the filth of the city.