But though the Low German tribes, including probably the Anglo-Saxons, have used peat as fuel from time immemorial, it appears not to have been known to the High Germans until a recent period. At least, I can find neither in Old nor in Middle High German lexicons and glossaries any word signifying peat. Zurb indeed is found in Graff as an Old High German word, but only in the sense of grass-turf, or greensward. Peat bogs of vast extent occur in many High German localities, but the former abundance of wood in the same regions rendered the use of peat unnecessary.] and of smaller animals, insects, and birds, in destroying the woods. [Footnote: See Chapter II., post.] Bogs generally originate in the checking of watercourses by the falling of timber or of earth and rocks, or by artificial obstructions across their channels. If the impediment is sufficient to retain a permanent accumulation of water behind it, the trees whose roots are overflowed soon perish, and then by their fall increase the obstruction, and, of course, occasion a still wider spread of the stagnating stream. This process goes on until the water finds a new outlet, at a higher level, not liable to similar interruption. The fallen trees not completely covered by water are soon overgrown with mosses; aquatic and semiaquatic plants propagate themselves, and spread until they more or less completely fill up the space occupied by the water, and the surface is gradually converted from a pond to a quaking morass. The morass is slowly solidified by vegetable production and deposit, then very often restored to the forest condition by the growth of black ashes, cedars, or, in southern latitudes, cypresses, and other trees suited to such a soil, and thus the interrupted harmony of nature is at last reestablished. [Footnote: "Aquatic plants have a utility in raising the level of marshy grounds, which renders them very valuable, and may well be called a geological function. The engineer drains ponds at a great expense by lowering the surface of the water; nature attains the same end, gratuitously, by raising the level of the soil without depressing that of the water; but she proceeds more slowly. There are, in the Landes, marshes where this natural filling has a thickness of four metres, and some of them, at first lower than the sea, have been thus raised and drained so as to grow summer crops, such, for example, as maize."--Boitel, Mise en valeur des Terres pauvres, p. 227. The bogs of Denmark--the examination of which by Steenstrap and Vaupell has presented such curious results with respect to the natural succession of forest trees--appear to have gone through this gradual process of drying, and the birch, which grow freely in very wet soils, has contributed very effectually by its annual deposits to raise the surface above the water level, and thus to prepare the ground for the oak.--Vaupell, Bogens Indvandring, pp. 39, 40.