zing than that derived from
the mud of the great river.

The group of glaciated soils differs in many ways from either of those
mentioned. In it we find the mineral matter to have been broken up,
transported, and accumulated without the influence of those conditions
which ordinarily serve to mix rock _debris_ with organic matter during
the process by which it is broken into bits. When vegetation came to
preoccupy the fields made desolate by glacial action, it found in most
places more than sufficient material to form soils, but the greater
part of the matter was in the condition of pebbles of very hard rock
and sand grains, fragments of silex. Fortunately, the broken-up state
of this material, by exposing a great surface of the rocky matter to
decay, has enabled the plants to convert a portion of the mass into
earth fit for the uses of their roots. But as the time which has
elapsed since the disappearance of the glaciers is much less than that
occupied in the formation of ordinary soil, this decay has in most
cases not yet gone very far, so that in a cubic foot of glaciated
waste the amount of material available for plants is often only a
fraction of that held in the soils of immediate derivation.

In the greater portion of the fields occupied by glacial waste the
processes which lead to the introduction of organic matter into the
earth have not gone far enough to set in effective work the great
laboratory which has to operate in order to give fertile soil. The
pebbles hinder the penetration of the roots as well as the movement of
insects and other animals. There has not been time enough for the
overturning of trees to bring about a certain admixture of vegetable
matter with the soil--in a word, the process of soil-making, though
the first condition, that of broken-up rock, has been accomplished,
is as yet very incomplete. It needs, indeed, care in the introduction
of organic matter for its completion.

It is characteristic of glacial soils that they are indefinitely deep.
This often is a disadvantageous feature, for the reason that the soil
water may pass so far down into the earth that the roots are often
deprived of the moisture which they need, and which in ordinary soils
is retained near the surface by the hard underlayer. On the other
hand, where the glacial waste is made up of pebbles formed from rocks
of varied chemical composition, which contain a considerable share of
lime, potash, soda, and other substance