Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/278

Rh 264 III. LIFE.-—The part taken by plants and animals in preserving, destroying, or reproducing geological formations. The words destructive, reproductive, and conservative, employed in describing the operations of the epigene agents, do not necessarily imply that anything useful to man is destroyed, reproduced, or preserved. On the contrary, the destructive action of the atmosphere may turn barren rock into rich soil, while its reproductive effects sometimes turn rich land into barren desert. Again, the conservative inﬂuence of vegetation has sometimes for centuries retained as barren morass what might otherwise have become rich meadow or luxuriant woodland. The terms, therefore, are used in a strictly geological sense, to denote the removal and re-deposition of material, and its agency in preserving what lies beneath it. Section I.-Air. Its composition having been already treated of (ante, . 220), we shall consider here (1) the motions, and the geological action of the air, which arises partly from its composition, and partly from its movements. I. MOVEMI-:."rs or THE AIR. These are due to differences in the pressure or density of the atmosphere, the law being that the air always moves from where the pressure is high to where it is low. Atmospheric pressure is understood to be determined by two causes, temperature and aqueous vapour. 1. Temperature.——Varm air, being less dense than cold air, ascends, while the latter ﬂows in to take its place. The unequal heating of the earth's surface, by causing upward currents from the warmed portions, produces horizontal cur- rents from the surrounding cooler regions inwards to the central ascending mass of heated air. To this cause the trade winds and the familiar land and sea breezes are due. 2. Aqueous l'apour.—In proportion as the quantity of watery vapour increases, the density of the air lessens. Consequently moist air tends to rise as warmed air does, with a corresponding but often very violent inﬂow of the drier and consequently heavier air from the surrounding tracts. The ascent of the moist air lessens the atmospheric pressure, which is indicated by the fall of the barometer. When the up-streaming vapour rises into the higher regions of the atmosphere, it expands and cools, condensing into visible form, and descending in copious showers to the earth. Unequal and rapid heating of the air, or accumulation of aqueous vapour in the air, and possibly some other influ- ences not yet properly understood, give rise to extreme dis- turbances of pressure, and consequently to storms and hurricanes. For instance, the barometer sometimes indi- cates in tropical storms a fall of an inch and a half in an hour, showing that somewhere about a twentieth part of the whole mass of the atmosphere has in that short space of time been displaced over a certain area of the earth’s sur- face. No such sudden change can occur without resulting in the most destructive tempest or tornado. In Britain the tenth of an inch of barometric fall in an hour is regarded as a large amount, such as only accompanies great storms.‘ Vhen the pressure of the air at one place is shown by the barometer to differ from that at a neighbour- ing locality at the same time, the wind will be observed to move on the whole from the area of high to the area of low pressure; and if the difference be great or sudden, the movement of the air may rise to the force of a hurricane until the equilibrium of pressure is restored. The meteorological conditions of the atmosphere do not 1 Buchan’s Meteorology, p. 266. GEOLOGY [I1I. DYNA MICA L. belong to the scope of this article (see A'rMosrm-:m-:, CLIMATE, MErEoRoLoG'). The reader, however, may note as of interest from a geological point of view tl1e ascertained velocity and pressure exercised by the air in motion across the surface of the earth as expressed in the subjoined table :— Velocity in miles Pressure in pounds per hour. 1 er square foot. ('alm .............................. .. Light breeze ..................... .. 14 1 Strong breeze .................... .. 4'3 9 Strong gale ....................... .. 71) 25 Ilurricanc ....................... .. 6-1 36 II. GEOLOGICAL INFLUENCES or THE Am. The paramount importance of the atmosphere as the vehicle for the circulation of temperature and inoisture over the globe, and consequently as powerfully influ- encing the distribution of chmate and the growth of plants and animals, must be fully recognized by the geologist. Attention will be conﬁned at present to the direct changes produced 011 the surface of the earth by the air—(l) on land, and on water. 1. Its Inﬂuence on Lam]. I. DESTRUCTIVE INrLUE.'cEs.—’1‘liese are either (a) chemical or (,8) mechanical, though in nature the two kinds of action are often inseparably interwoven. (a) Under the denomination of chemical changes we in- clude the oxidation of these minerals which can contain more oxygen, as in the peroxidation and precipitation of protosalts of iron ; likewise the absorption of carbonic acid by rocks, and the production of alkaline and earthy carbon- ates and bicarbonates, which still further promote the process of decomposition. In the one case the active agent of change is the oxygen of the air, or rather of the aqueous vapour in the air, for perfectly dry air seems to have little or no oxidizing effect. A familiar illustration is afforded by the rust, or oxide, which forms on iron when exposed to moisture, though this iron may be kept long bright if allowed to remain screened from moist air. In the other case, the active agent is the carbonic acid of the air, though here again it appears to be requisite that moisture should intervene as the medium of introducing the acid to the sub- stance which is to be altered by it. The occurrence of sulphuric and nitric acids in the air, especially noticeable in large towns, likewise leads to considerable corrosion of metallic surfaces, as well as of stones and lime. The mortar of walls may often be observed to be slowly swelling o11t and dropping oﬂ’, owing to the conversion of the lime into sulphate. Great injury is likewise done from a similar cause to marble monuments in exposed graveyards. As a rule, the changes effected by the air lead to many subsequent transformations. For example, the oxidation of the bisulphide of iron produces sulphuric acid, which de- composes silicates, carbonates, and other compounds with which it comes in contact. These changes, however, are more appropriately noticed under the head of rain (p. 267). Among the more recognizable mec/zanical changes of a destructive kind, brought about by the atmosphere, we may notice the following inﬂuences :— 1. Expansion and ('ontraclion.—The effect of heat is to expand rocks, of cold to contract them. Strictly speaking, these results on the surface of the earth are due, not to the air, but to the heat-rays of the sun which reach the rocks through the air. In countries with a great annual range of temperature considerable difficulty is sometimes experi- enced in selecting building materials liable to be little affected by the alternate expansion and contraction which prevents the joints of masonry from remaining close and