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316 east wind carries before it, in dense whirling masses, large bodies of sand. For many centuries a line of movable hills has been thus gradually formed behind the city. These hillocks, improperly called medanos, are continually augmented by fresh additions, and are ever changing, according to the caprice of the wind, their place and figure. Some rise in the air like pyramids, from the top of which small portions of sand are constantly flying off like a never-failing bank of fog. The great number of these medanos, many of which attain a height of from fourteen to more than thirty feet, threatens to bury the town; but, as the danger is still distant, and in hot countries one's existence hangs merely by a thread, the inhabitants leave to their posterity the task of providing against that emergency. Another disadvantage still more serious is, that the medanos hinder the rain-water from flowing away. Small lakes are thus formed at the bottom of these sand-hills; and the parched-up ground is gradually converted into a fenny marsh, from which arise the most pernicious exhalations. A thick layer of mud fertilizes the sand, and all the noxious plants which abound in low, moist grounds are here produced in countless profusion. During the rainy season this rank vegetation spreads and grows round all the margin of the ponds. The mangroves shoot their branches down to the ground. They take root there, produce new trunks, and soon form impenetrable thickets—haunts of numberless reptiles of every kind. A thick crust of greenish scum carpets the surface of the water. The fermentation which sets in on the return of hot weather in these frightful marshes disperses deleterious miasmas abroad, and removes to a distance the swarms of musquitoes. For three months of the