about its mouth. The result is the overturning of a considerable amount of the earth, and a consequent commingling of the material with vegetable matter. When brought to the surface and left exposed to the action of frost, the breaking up of the material is greatly favored, and thus the formation of the soil is facilitated.
Considerable as is the effect of burrowing mammals, the principal overturning of the earth in our primeval forests is accomplished by the invertebrate animals. Where the woods are not very dense, and particularly where the soil is somewhat sandy, our largest species of ants are very effective agents in working over the soil. Their burrows extend to the depth of some feet below the surface, and each hill brings to the air several cubic feet of excavated matter, which, as slight inspection shows, is much commingled with vegetable matter. Wherever these ant-hills abound they commonly exist to the number of a score or more on each acre, and the occupants of each hill, in many cases, bring as much as a cubic foot of matter to the surface in the course of a single year. The action of rain constantly operates to diffuse this material on every side of the hill. We may often observe a thin layer of sediment extending for a considerable distance from the elevation.
As is well known to all those who have inspected the soil within virgin forests, the earth is occupied by a host of larval insects, principally belonging to the group of beetles, but including also many orthopterous insects. These creatures, in the course of their life underground, displace a good deal of soil, a portion of which is thrown upon the surface, the greater part, however, being merely dislodged beneath the surface. The effect, however, is to commingle and to break up the soil, and thus favor its comminution. Although the roots of trees do by far the larger part of the rending which is accomplished in the soil-layer, they do not bring about much commingling of the soil. The thrusts which they apply to it shear the materials about, and so, to a certain extent, mix them, but by far the larger part of the commingling is effected by the animal life which dwells beneath the forest-bed.
Where the woods are wet and favor the development of the crayfish, the effect of this group of animals on the overturning of the soil is extremely great. It probably exceeds that which is accomplished in our ordinary fields by the action of the earth-worms. A single cray-fish will often bring in the course of a single season's activity not less than half a cubic foot of earthy matter to the surface. In certain districts where these animals abound, there appear to be not less than a thousand to each acre of surface. If such be their number, it is evident that not less than five hundred cubic feet of matter is brought to the surface from a considerable depth in the course of a year. As this matter is generally of a rather fine nature and easily dissolved in water, it rapidly washes away and forms a thin sheet on the surface.