s which are required by plants,
the very large surface which they expose to decay provides the soil
with a continuous enrichment. In a cubic foot of pebbly glacial earth
we often find that the mass offers several hundred times as much
surface to the action of decay as is afforded by the underlying solid
bed rock from which a soil of immediate derivation has to win its
mineral supply. Where the pebbly glacial waste is provided with a
mixture of vegetable matter, the process of decay commonly goes
forward with considerable rapidity. If the supply of such matter is
large, such as may be produced by ploughing in barnyard manure or
green crops, the nutritive value of the earth may be brought to a very
high point.

It is a familiar experience in regions where glacial soils exist that
the earth beneath the swamps when drained is found to be
extraordinarily well suited for farming purposes. On inspecting the
pebbles from such places, we observe that they are remarkably decayed.
Where the masses contain large quantities of feldspar, as is the case
in the greater part of our granitic and other crystalline rocks, this
material in its decomposition is converted into kaolin or feldspar
clay, and gives the stones a peculiar white appearance, which marks
the decomposition, and indicates the process by which a great variety
of valuable soil ingredients are brought into a state where they may
be available for plants.

In certain parts of the glacial areas, particularly in the region near
the margin of the ice sheet, where the glacier remained in one
position for a considerable time, we find extensive deposits of
silicious sand, formed of the materials which settled from the
under-ice stream, near where they escaped from the glacial cavern.
These kames and sand plains, because of the silicious nature of their
materials and the very porous nature of the soil which they afford,
are commonly sterile, or at most render a profit to the tiller by dint
of exceeding care. Thus in Massachusetts, although the first settlers
seized upon these grounds, and planted their villages upon them
because the forests there were scanty and the ground free from
encumbering boulders, were soon driven to betake themselves to those
areas where the drift was less silicious, and where the pebbles
afforded a share of clay. Very extensive fields of this sandy nature
in southeastern New England have never been brought under tillage.
Thus on the island of Martha's V