the superficial detrital
layer, we find normal fruitful soils, though in their more arid
conditions they may be fit for but few species of plants. When, by
increasing aridity, we pass to conditions where there is no tolerably
permanent store of water in the _debris_, the material ceases to have
the qualities of a soil, and becomes mere rock waste. At the other
extreme of the scale we pass to conditions where the water is
steadfastly maintained in the interstices of the detritus, and there
again the characteristic of the soil and its fitness for the uses of
land vegetation likewise disappear. In a word, true soil conditions
demand the presence of moisture, but that in insufficient quantities,
to keep the pores of the earth continually filled; where they are thus
filled, we have the condition of swamps. Between these extremes the
level at which the water stands in the soil in average seasons is
continually varying. In rainy weather it may rise quite to the
surface; in a dry season it may sink far down. As this water rises and
falls, it not only moves, as before noted, the soluble mineral
materials, but it draws the air into and expels it from the earth with
each movement. This atmospheric circulation of the soil, as has been
proved by experiment, is of great importance in maintaining its
fertility; the successive charges of air supply the needs of the
microscopic underground creatures which play a large part in enriching
the soil, and the direct effect of the oxygen in promoting decay is
likewise considerable. A part of the work which is accomplished by
overturning the earth in tillage consists in this introduction of the
air into the pores of the soil, where it serves to advance the actions
which bring mineral matters into solution.

[Illustration: _Mountain gorge, Himalayas, India. Note the difference
in the slope of the eroded rocks and the effect of erosion upon them;
also the talus slopes at the base of the cliffs which the torrent is
cutting away. On the left of the foreground there is a little bench
showing a recent higher line of the water._]

In the original conditions of any country which is the seat of
considerable rainfall, and where the river system is not so far
developed as to provide channels for the ready exit of the waters, we
commonly find very extensive swamps; these conditions of bad drainage
almost invariably exist where a region has recently been elevated
above the level of the sea, and still reta