.

Where a region has an earth which has recently passed from beneath the
sea or a great lake, the surface is commonly covered by incoherent
detritus which has escaped consolidation into hard rock by the fact
that it has not been buried and thus brought into the laboratory of
the earth's crust. When such a region becomes dry land, the materials
are immediately ready to enter into the state of soil. They commonly
contain a good deal of waste derived from the organic life which
dwelt upon the sea bottom and was embedded in the strata as they were
formed. Where these accumulations are made in a lake, the land
vegetation at once possesses the field, even a single year being
sufficient for it to effect its establishment. Where the lands emerge
from the sea, it requires a few years for the salt water to drain away
so that the earth can be fit for the uses of plants. In a general way
these sea-bottom soils resemble those formed in the alluvial plains.
They are, however, commonly more sandy, and their substances less
penetrated by that decay which goes on very freely in the atmosphere
because of the abundant supply of oxygen, and but slowly on the sea
floor. Moreover, the marine deposits are generally made up in large
part of silicious sand, a material which is produced in large
quantities by the disruption of the rocks along the sea coast. The
largest single field of these ocean-bottom soils of North America is
found in the lowland region of the southern United States, a wide belt
of country extending along the coast from the Rio Grande to New York.
Although the streams have channelled shallow valleys in the beds of
this region, the larger part of its surface still has the peculiar
features of form and composition which were impressed upon it when it
lay below the surface of the sea.

Local variations in the character of the soil covering are exceedingly
numerous, and these differences of condition profoundly affect the
estate of man. We shall therefore consider some of the more important
of these conditions, with special reference to their origin.

The most important and distinctly marked variation in the fertility of
soils is that which is produced by differences in the rainfall. No
parts of the earth are entirely lacking in rain, but over considerable
areas the precipitation does not exceed half a foot a year. In such
realms the soil is sterile, and the natural coating of vegetation
limited to those plants which can subs