ins the form of an irregular
rolling plain common to sea floors, and also in regions where the work
done by glaciers has confused the drainage which the antecedent
streams may have developed. In an old, well-elaborated river system
swamps are commonly absent, or, if they occur, are due to local
accidents of an unimportant nature.

For our purpose swamps may be divided into three groups--climbing
bogs, lake bogs, and marine marshes. The first two of these groups
depend on the movements of the rain water over the land; the third on
the action of the tides. Beginning our account with the first and most
exceptional of these groups, we note the following features in their
interesting history:

Wherever in a humid region, on a gentle slope--say with an inclination
not exceeding ten feet to the mile--the soil is possessed by any
species of plants whose stems grow closely together, so that from
their decayed parts a spongelike mass is produced, we have the
conditions which favour the development of climbing bogs. Beginning
usually in the shores of a pool, these plants, necessarily of a
water-loving species, retain so much moisture in the spongy mass
which they form that they gradually extend up the slope. Thus
extending the margin of their field, and at the same time thickening
the deposit which they form, these plants may build a climbing bog
over the surface until steeps are attained where the inclination is so
great that the necessary amount of water can not be held in the spongy
mass, or where, even if so held, the whole coating will in time slip
down in the manner of an avalanche.

The greater part of the climbing bogs of the world are limited to the
moist and cool regions of high latitudes, where species of moss
belonging to the genus _Sphagnum_ plentifully flourish. These plants
can only grow where they are continuously supplied with a bath of
water about their roots. They develop in lake bogs as far south as
Mexico, but in the climbing form they are hardly traceable south of
New England, and are nowhere extensively developed within the limits
of the United States. In more northern parts of this continent, and in
northwestern Europe, particularly in the moist climate of Ireland,
climbing bogs occupy great areas, and hold up their lakes of
interstitially contained water over the slopes of hills, where the
surface rises at the rate of thirty feet or more to the mile. So long
as the deposit of decayed vegetable matter w