the
point where only the strongest roots--those belonging to our
tap-rooted trees--can penetrate through it to the bed rocks.

Almost all forests are from time to time visited by winds which uproot
the trees. When they are thus rent from the earth, the underground
branches often form a disk containing a thick tangle of stones and
earth, and having a diameter of ten or fifteen feet. The writer has
frequently observed a hundred cubic feet of soil matter, some of it
taken from the depth of a yard or more, thus uplifted into the air. In
the path of a hurricane or tornado we may sometimes find thousands of
acres which have been subjected to this rude overturning--a natural
ploughing. As the roots rot away, the _debris_ which they held falls
outside of the pit, thus forming a little hillock along the side of
the cavity. After a time the thrusting action of other roots and the
slow motion of the soil down the slope restore the surface from its
hillocky character to its original smoothness; but in many cases the
naturalist who has learned to discern with his feet may note these
irregularities long after it has been recovered with the forest.

Great as is the effect of plants on the soil, that influence is almost
equalled by the action of the animals which have the habit of entering
the earth, finding there a temporary abiding place. The number of
these ground forms is surprisingly great. It includes, indeed, a host
of creatures which are efficient agents in enriching the earth. The
species of earthworms, some of which occupy forested districts as well
as the fields, have the habit of passing the soil material through
their bodies, extracting from the mass such nutriment as it may
contain. In this manner the particles of mineral matter become
pulverized, and in a measure affected by chemical changes in the
bodies of the creatures, and are thus better fitted to afford plant
food. Sometimes the amount of the earth which the creatures take in in
moving through their burrows and void upon the surface is sufficient
to form annually a layer on the surface of the ground having a depth
of one twentieth of an inch or more. It thus may well happen that the
soil to the depth of two or three feet is completely overturned in the
course of a few hundred years. As the particles which the creatures
devour are rather small, the tendency is to accumulate the finer
portions of the soil near the surface of the earth, where by solution
they may c