Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/303

Rh L1FE.] of the existing sea that we encounter analogies to the older formations. And thus we reach by another and a new approach the conclusion which on very different grounds has been arrived at, viz., that the present continental ridges have existed from the remotest times, and that the marine strata which constitute so large a portion of their mass have been accumulated not as deep water formations, but in com- paratively shallow water along their ﬂanks. Section III. —Life. Among the agents by which geological changes are carried on upon the surface of the globe living organisms must be enumerated. Both plants and animals co-operate with the inorganic agents in promoting the degradation of the land ; and in some cases, on the other hand, they protect rocks from decay. Again by the accumulation of their remains they form extensive formations both upon the land and in the sea. Their operations may hence be described as alike destructive, conservative, and reproductive. Under this heading also we may notice the inﬂuence of man as a geological agent. I. ])EsTnUcT1vE AcrIoN.—Plants aid in the general pro- gress of disintegration in various ways. 1. By keeping the surfaces of rocks moist, and thus promoting both the mechanical an(l chemical dissolution of the rocks. This action is especially shown by liverworts, mosses, and other plants which only thrive in copious moisture. 2. By producing through their decay carbonic and other acids, which, with decaying organic matter taken up by passing moisture, become potent in effecting the chemical decomposition of rocks, and in promoting the disintegration of soils. 3. By inserting their roots or branches between joints of rock, which are thereby loosened, so that large slices may be eventually wedged off. On the sides of wooded hills and cliffs this process may often be seen ; even among old ruins an occasional sapling ash or elm may be found to have cast its roots round a portion of the masonry and to be slowly detaching it from the rest of the wall. 4. By attracting rain, as thick woods, forests, and mosses do, and thus accelerating the general scouring of a country by running water. The indiscriminate de- struction of the woods in the Levanthas been assigned with much plausibility as the main cause of the present desicca- tion of that region. 5. By promoting the decay of dis- eased and dead plants and animals, as when fungi over- spread a damp rotting tree or the carcase of a dead animal. Of the destructive inﬂuences of animal life numerous illustrations might be given. 1. The composition and arrangement of soil are affected. 1Vor1ns are continually engaged in bringing up the lower portions of the soil to the surface, and thus increase its fertility and its capability of being washed away by rain. Burrowing animals, by throw- ing up the soil and subsoil, expose these to be dried and blown away by the wind. At the same time their subter- ranean passages serve to drain off the superﬁcial water and to injure the stability of the surface of the ground above them. In Britain the mole and rabbit are familiar ex- amples. In North America the prairie dog has undermined extensive tracts of pasture land in the west. In Cape Jolony wide areas of open country seem to be in a constant state of eruption from the burrowing operations of multi- tudes of Bcztltyergi and C'ILr_z/soc]:loris—small mole-like animals which bring up the soil and bury the grassy vegeta- tion under it. 2. The ﬂow of streams is sometimes inter- fered with, or even diverted, by the operations of animals. Thus the beaver, by constructing dams, checks the current of water-courses, intercepts ﬂoating materials, and sometimes even diverts the water into new channels. This action is typically displayed in Canada and other parts of North America. The embankments of the Mississippi are some- times weakened to such an extent by the burrowings of the GEOLOGY 289 cray-ﬁsh as to give way and allow the river to inundate the surrounding country. Similar results have happened in Europe from the subterranean operations of rats. 3. Some Mollusca (1’/Lolas, b'cu:iccwa, Teredo, &c.) bore into stone or wood, and by the number of contiguous perforations greatly weaken the material. Pieces of drift-wood are soon riddled with long holes by the teredo ; while wooden piers, and the bottom of wooden ships, are often rapidly perfor- ated. The saxicavous shells, by piercing rocks and leaving open cavities for rain and sea water to ﬁll, promote the decay of the stone. 4. Many animals exercise a ruin- ously destructive inﬂuence upon vegetation. Of the many insect plagues of that kind it will be enough to enumerate the locust, phylloxera, and Colorado beetle. The pasture in some parts of the south of Scotland has in recent years been much damaged by mice, which have increased in numbers owing to the indiscriminate shooting and trapping of owls, hawks, and other predaceous creatures. Grass- hoppers cause the destruction of vegetation in some parts of Wyoming and other western territories of the United States. The way in which animals destroy each other, often on a great scale, may likewise be included among the geological operations now under description. II. CONSERVATIVE AcT1oN.——This is admirably shown by many kinds of vegetation. 1. The formation of a stratum of turf protects the soil and rocks from being rapidly disintegrated and washed away by atmospheric action. Hence the surface of a district so protected is denuded with extreme slowness except along the lines of its water-courses. 2. Many plants, even without forming a layer of turf, serve by their roots or branches to protect the loose sand or soil on which they grow from being removed by wind. The common sand-carex and other arenaceous plants bind the loose sand-dunes of our coasts, and give them a permanence which would at once be destroyed were the sand laid bare again to storms. In North America the sandy tracts of the western territories are in many places protected by plants known as sage-brush and grease-wood. The growth of shrubs and brushwood along the course of a stream not only keeps the alluvial banks from being so easily undermined and removed as would otherwise be the case, but serves to arrest the sedi- ment in ﬂoods, ﬁltering the water, and thereby adding to the height of the flood plain. On some parts of the west coast of France extensive ranges of sand-hills have been gradually planted with pine woods which, while preventing the destructive inland march of the sand, also yield a large revenue in timber, and have so improved the climate as to make these districts a resort for pulmonary invalids. In tropical countries the mangrove grows along the margin of the sea, and not only protects the land, but adds to its breadth, by forming and increasing an alluvial belt along the coast. 3. Some marine plants likewise afford protec- tion to shore rocks. This is done by the calcareous nulli- pores, which form upon them a hard incrustation ; likewise by the tangles and smaller fuci which grow abundantly on the littoral zone and break the force of the waves, or diminish the effects of ground swell. 4. Forests and brush- wood protect the soil, especially on slopes, from being washed away by rain. This is shown by the disastrous results of the thoughtless destruction of such woods. According to Reclus (La T erre, p. 410), in the three centuries from 1471 to 1776, the “vigueries,” or provostry-districts of the French Alps, lost a third, a half, and even three—fourths of their cultivated ground, and the population has diminished in somewhat similar proportions. From 1836 to 1866 the departments of Hautes and Basses Alpes lost 25,000 111- habitants, or nearly one-tenth of their population—a diminution which has with plausibility been assigned to the reckless removal of the pine forests, w;1{ereby the steep "' 37