t a small part of
the land surface. It would be possible, indeed, to make yet another
division, including those areas which when emerging from the sea were
covered with fine, uncemented detritus ready at once to serve the
purposes of a soil. Only here and there, and but seldom, do we find
soils of this nature.

It is characteristic of soils belonging to the group to which we have
given the title of immediate derivation that they have accumulated
slowly, that they move very gradually down the slopes on which they
lie, and that in all cases they represent, with a part of their mass
at least, levels of rock which have disappeared from the region which
they occupied. The additions made to their mass are from below, and
that mass is constantly shrinking, generally at a pretty rapid rate,
by the mineral matter which is dissolved and goes away with the spring
water. They also are characteristically thin on steep slopes,
thickening toward the base of the incline, where the diminished grade
permits the soil to move slowly, and therefore to accumulate.

In alluvial soils we find accumulations which are characterized by
growth on their upper surfaces, and by the distant transportation of
the materials of which they are composed. In these deposits the
outleaching removes vast amounts of the materials, but so long as the
floods from time to time visit their surfaces the growth of the
deposits is continued. This growth rarely takes place from the waste
of the bed rocks on which the alluvium lies. It is characteristic of
alluvial soils that they are generally made up of _debris_ derived
from fields where the materials have undergone the change which we
have noted in the last paragraph; therefore these latter deposits have
throughout the character which renders the mineral materials easily
dissolved. Moreover, the mass as it is constructed is commonly mingled
with a great deal of organic waste, which serves to promote its
fertility. On these accounts alluvial grounds, though they vary
considerably in fertility, commonly afford the most fruitful fields of
any region. They have, moreover, the signal advantage that they often
may be refreshed by allowing the flood waters to visit them, an
action which but for the interference of man commonly takes place once
each year. Thus in the valley of the Nile there are fields which have
been giving rich grain harvests probably for more than four thousand
years, without any other effective fertili