Floating-islands are formed either by the aggregation of driftwood in the creeks and bays of tropical rivers and the deposition thereon of soil and vegetable matter, or by the detachment of portions of a river-bank or lake-shore, on which the interlacing roots of plants constitute a foundation sufficiently strong to support soil whereon herbage, and occasionally even trees, are able to grow. Such islands are sometimes seen 50 or 100 miles distant from the mouth of the large rivers of America, Asia, and Africa. Portions of the alluvial soil from river-deltas, held together by the roots of mangroves and other trees, are sometimes detached by hurricanes or typhoons and then swept out to sea; such islands have been met with in the Philippines, in the seas of the East Indies, and in the Pacific. A floating-island is mentioned by Herodotus as existing in Egypt. Others were known to Roman writers: Seneca speaks of them as being in the Vadimonian, Cutilian, and Stationian lakes in Italy; to these Pliny adds the islands floating on lakes near Cæcubum, Reate, and Modena, and two others in Asia Minor. Those on Lake Vadimona were, according to the Younger Pliny, capable of supporting sheep. Varenius, in the middle of the 17th century, cites a lake in Honduras with floating-islands. Passing over others in Prussia, Italy, and South America, we come to Great Britain, where we find it recorded that Loch Lomond long possessed a floating-island, which has now, however, disappeared or become attached to one of the stationary islands of the loch. In Ireland large masses of peat float about some of the bogs—the Bog of Allen, for instance. In England, in Lake Derwentwater, there is an instance of an island which appears and disappears from time to time in the same spot. Perhaps the most satisfactory of the many theories which have been proposed to account for this phenomenon is that which attributes its rising from the bottom of the lake, where it ordinarily rests, to the permeation of its mass by marsh-gas during hot weather, the upward motion being assisted by the growth of buoyant water-plants on its surface. See Symons, Floating Island in Derwentwater (1888).
Between 1696 and 1829, similar islands were observed at irregular intervals, generally, however, after great droughts and violent storms, in Lake Rålång in the Swedish province of Småland. Oceanic floating-islands sometimes perform important service in the transportation of vegetable seeds from place to place, also in the distribution of animal species, by carrying insects, land mollusca, and small mammalia, more rarely reptiles. Darwin met with islands floating on Lake Tagua-Cagua in Chili which passed from side to side of the lake and carried 'cattle and horses as passengers.' Between 1863 and 1881 the rivers of the central
Nile system were rendered almost unnavigable owing to the accumulations of floating vegetable matter getting set fast across the channels, practically forming dams, which in some instances gave rise to serious inundations. The vale of Cashmere contains many lakes, which frequently overflow and drown the surrounding country. This has taught the inhabitants to construct floating-gardens for the cultivation of their vegetables. The gardens are in reality portions of the marshy ground made to float artificially by cutting through the roots of the reeds and other plants about two feet below the surface. The Chinese, too, devote considerable attention to this style of horticulture, but more by way of ornamentation. Floating-gardens, or chinampas, also existed in Mexico before the Spanish conquest. Clavigero describes them as formed of wicker-work, the stems of water-plants, and mud, the largest sometimes having on them a tree or a hut. Both flowers and vegetables were grown on them.