tation of the mosses, such as
the _Sphagnum_, can fix itself on the shore.

As soon as the _Sphagnum_ mat has begun its growth, the strength given
by its interlaced fibres enables it to extend off from the shore and
float upon the water. In this way it may rapidly enlarge, if not
broken up by the waves, so that its front advances into the lake at
the rate of several inches each year. While growing outwardly it
thickens, so that the bottom of the mass gradually works down toward
the floor of the basin. At the same time the lower part of the sheet,
decaying, contributes a shower of soft peat mud to the floor of the
lake. In this way, growing at its edge, deepening, and contributing to
an upgrowth from the bottom, a few centuries may serve entirely to
fill a deep basin with peaty accumulation. In general, however, the
surface of the bog closes over the lake before the accumulation has
completely filled the shoreward portions of the area. In these
conditions we have what is familiarly known as a quaking bog, which
can be swayed up and down by a person who quickly stoops and rises
while standing on the surface. In this state the tough and thick sheet
of growing plants is sufficient to uphold a considerable weight, but
so elastic that the underlying water can be thrown into waves. Long
before the bog has completely filled the lake with the peaty
accumulations the growth of trees is apt to take place on its surface,
which often reduces the area to the appearance of a very level wet
wood.

[Illustration: Fig. 17.--Diagram showing beginning of peat bog: A,
lake; B, lilies and rushes; C, lake bog; D, climbing bog.]

Climbing and lake bogs in the United States occupy a total area of
more than fifty thousand square miles. In all North America the total
area is probably more than twice as great. Similar deposits are
exceedingly common in the Eurasian continent and in southern
Patagonia. It is probable that the total amount of these fields in
different parts of the world exceeds half a million square miles.
These two groups of fresh-water swamps have an interest, for the
reason that when reduced to cultivation by drainage and by subsequent
removal of the excess of peaty matter, by burning or by natural decay,
afford very rich soil. The fairest fields of northern Europe,
particularly in Great Britain and Ireland, have been thus won to
tillage. In the first centuries of our era a large part of
England--perhaps as much as one tenth