When the dense foliage hinders the action of the sun and the refreshing breeze, and thus retards the decomposition of the vegetable masses, where the ground, flat and without any declivity, allows the accumulation of water, and the more since the heaped-up bodies of dead plants continually increase the barriers to the efflux, and the humus formed greedily sucks up the moisture--there are formed the most extensive swamps. By the progressive action of the remains of vegetation the ground becomes elevated, and such spongy, semi-fluid masses often lie, at length, far above the level of the surrounding plain, the sun’s heat never sufficing, even when storms remove the protecting roof, to dry up the marsh, or to restrain its increase. Such a swamp rises twelve feet above the surrounding plains in Virginia, between the towns of Suffolk and Walden, and is called by the inhabitants “the Great Dismal,” giving origin to considerable rivers and supplying them with water. The North American cypress (Cupressus disticha) it is which with its delicate but dense foliage gives rise to the formation of these structures. It is the same tree which forms the terrible evil-renowned cypress swamps of Louisiana, on the banks of the Red River and the Mississippi. Gigantic trunks of unprecedented mightiness crowd together, interweaving their branches and spreading an obscure twilight in the brightest day. The soil consists merely of half-decayed blocks piled one upon another, alternating with a fathomless mud, in which the voracious alligators and snapping-turtles wallow, the sole lords of this hell, steaming up almost beneath the tropical sun--thus in the height of summer; in the spring the thick, miry floods of the issuing streams impetuously overflow this malignant vegetation for many miles. Thus these cypress-swamps, of which Seatsfield has given us such a vivid picture, correspond in inland countries to the mangrove-woods which border the mouths of almost all the tropical rivers. Composed of a very few species of plants, among which the mangrove-tree is the most common, they are especially striking from the great number of strong roots springing out high up the stem, and bearing this aloft above the surface. The peculiar habitation of this plant is the _brackish water_, which consists, at the ebb, of the fresh water of the river, which is dislodged by the sea-water at the flood. The numerous roots often form a so thickly entangled mass that the interspaces may be stopped up by the falling leaves, collecting thus a soil for a new vegetation, beneath which, at different hours of the day, roll the waves of the river and the sea. But more frequently the roots merely operate to retard the flow of the water and to retain in their interlacements the vegetable and animal bodies driven down the river, which then decay here in contact with sea-water and its salts. In these regions the terrible sulphureted hydrogen gas is developed so