Page:Popular Science Monthly Volume 34.djvu/50

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too compact for the uses of this creature, come within a short distance of the surface, they avoid the ground.

The geological effects of this creature in the district which it inhabits are considerable. In a region extensively occupied by them, they turn over the earth, to the depth of some feet, with amazing rapidity. On selected areas chosen to represent the work done by these animals I found that the number of hillocks varied from fifty to two hundred to the acre, and that the heaps contained an average of rather more than one fourth of a cubic foot of sand when reduced to the measure of compactness which it occupied when in its original place. I came to the conclusion that it would be safe to estimate that in each year this soil matter thrown up by the gophers on the surface of an acre amounted to an average of fifty cubic feet, the greater portion of which was uplifted from a depth of a foot or more below the surface. At this rate they would completely overturn the soil, for the depth of a foot or more, in about eight hundred years. In addition to the effect produced by the process of throwing out the earth upon the surface, these creatures accomplish a vastly greater amount of subsoiling by continually ascending and descending in the earth, pushing the earth behind them as they go. I am inclined to think that they displace vertically, about the amount of a foot or more, all the sands to the depth of about three feet in the course of less than a century. The result is, that in the regions they occupy there is no distinct soil coating whatsoever; the thin layer of half decayed vegetable matter, rarely exceeding an inch or two in thickness, lies immediately upon the sands, which are scarcely commingled with the humus material. Although the rapid decay of vegetation in the warm climate of the South may in part account for this peculiarity, I think that it in the main is due to the action of these creatures. This view is supported by the observation as to the character of the soil in places where the gophers have not done their underground work. Thus, where the ground is too wet for their occupancy, we commonly find a thick coating of vegetable matter and a soil which is charged with humus to a considerable distance from the surface. The wet "hammock" or "hummock" lands, which exist as occasional patches in the sandy districts where these animals are plenty, apparently owe in good part their more normal character of soil to the exemption from the overturning of the superficial materials which these creatures effect.

There is another important way in which these gophers have influenced the geological conditions of the districts they inhabit. The sands through which they make their burrows have evidently been deposited by water-action. They were probably originally in the stratified form, but so thoroughly have they been overturned