of the ground now tilled in that
country--was occupied by these lands, which retained water in such
measure as to make them unfit for tillage, the greater portion of this
area being in the condition of thin climbing bog. For many centuries
much of the energy of the people was devoted to the reclamation of
these valuable lands. This task of winning the swamp lands to
agriculture has been more completely accomplished in England than
elsewhere, but it has gone far on the continent of Europe,
particularly in Germany. In the United States, owing to the fact that
lands have been cheap, little of this work of swamp-draining has as
yet been accomplished. It is likely that the next great field of
improvement to be cultivated by the enterprising people will be found
in these excessively humid lands, from which the food-giving resources
for the support of many million people can be won.

[Illustration: Fig. 18.--Diagram showing development of swamp: A,
remains of lake; B, surface growth; c, peat.]

The group of marine marshes differs in many important regards from
those which are formed in fresh water. Where the tide visits any
coast line, and in sheltered positions along that shore, a number of
plants, mostly belonging to the group of grasses, species which have
become accustomed to having their roots bathed by salt water, begin
the formation of a spongy mat, which resembles that composed of
_Sphagnum_, only it is much more solid. This mat of the marine marshes
soon attains a thickness of a foot or more, the upper or growing
surface lying in a position where it is covered for two or three hours
at each visit of the tide. Growing rapidly outward from the shore, and
having a strength which enables it to resist in a tolerably effective
manner waves not more than two or three feet high, this accumulation
makes head against the sea. To a certain extent the waves undermine
the front of the sheet and break up masses of it, which they
distribute over the shallow bottom below the level at which these
plants can grow. In this deeper water, also, other marine animals and
plants are continually developing, and their remains are added to the
accumulations which are ever shallowing the water, thus permitting a
further extension of the level, higher-lying marsh. This process
continues until the growth has gone as far as the scouring action of
the tidal currents will permit. In the end the bay, originally of
wide-open water, is only such at hi