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