All those disturbances known as earthquakes, all those elevations andsubsidences which they severally produce, all those accumulated effects ofmany such elevations and subsidences exhibited in ocean-basins, islands,continents, table-lands, mountain-chains, and all those formations whichare distinguished as volcanic, geologists now regard as modifications ofthe Earth's crust caused by the actions and reactions of its interior. Evensupposing that volcanic eruptions, extrusions of igneous rock, and upheavedmountain-chains, could be otherwise satisfactorily accounted for, it wouldbe impossible otherwise to account for those wide-spread elevations and depressionswhence continents and oceans result. Such phenomena as the fusion or agglutinationof sedimentary deposits, the warming of springs, the sublimation of metalsinto the fissures where we find them as ores, may be regarded as positiveresults of the residuary heat of the Earth's interior; while fractures ofstrata and alterations of level are its negative results, since they ensueon its escape. The original cause of all these effects is still, however,as it has been from the first, the gravitating movement of the Earth's mattertowards the Earth's centre; seeing that to this is due both the eternal heatitself and the collapse which takes place as it is radiated into space.
To the question -- Under what forms previously existed the force whichworks out the geological changes classed as aqueous, the answer is less obvious.
The effects of rain, of rivers, of winds, of waves, of marine currents, donot manifestly proceed from one general source. Analysis, nevertheless, provesthat they have a common genesis. If we ask, -- Whence comes the power ofthe river-current, bearing sediment down to the sea? the reply is, -- Thegravitation of water throughout the tract which this river drains. If weask, -- How came the water to be dispersed over this tract? the reply is,-- It fell in the shape of rain. If we ask, -- How came the rain to be inthat position whence it fell? the reply is, -- The vapour from which it wascondensed was drifted there by the winds. If we ask, -- How came this vapourto be at that height? the reply is, -- It was raised by evaporation. Andif we ask, -- What force thus raised it? the reply is, -- The Sun's heat.
Just that amount of gravitative force which the Sun's heat overcame in raisingthe molecules of water, is given out again in the fall of those moleculesto the same level. Hence the denudations effected by rain and rivers, duringthe descent of this condensed vapour to the level of the sea, are indirectlydue to the radiated energy of the Sun. Similarly with the winds that transportthe vapours hither and thither. Consequent as atmospheric currents are ondifferences of temperature (either general, as between the equatorial andpolar regions, or special as between tracts of the Earth's surface havingunlike physical characters) all such currents are due to that source fromwhich the irregularly distributed heat proceeds. And if the winds thus originate,so too do the waves raised by them on the sea's surface. Whence it followsthat whatever changes waves produce -- the wearing away of cliffs, the breakingdown of rocks into shingle, sand, and mud -- are also traceable to the solarrays as their primary cause. The same may be said of ocean-currents. Generatedas the larger ones are by the excess of heat which the ocean in tropicalclimates acquires from the Sun; and determined as the smaller ones are inpart by local shapes of land; it follows that the distribution of sedimentand other geological processes which these marine currents effect, are affiliableupon the energy the Sun radiates. The only aqueous agency otherwise originatingis that of the tides -- an agency which, equally with the others, is traceableto unexpended celestial motion. But making allowance for the changes thisworks, we conclude that the slow wearing down of continents and gradual fillingup of seas, effected by rain, rivers, winds, waves, and ocean-streams, arethe indirect effects of solar heat.
Thus we see that while the geological changes classed as igneous, arisefrom the still-progressing motion of the Earth's substance to its centreof gravity; the antagonistic changes classed as aqueous, arise from the still-progressingmotion of the Sun's substance towards its centre of gravity. §70. That the forces exhibited in vital actions, vegetal and animal,are similarly derived, is an obvious deduction from the facts of organicchemistry. Let us note first the physiological generalizations; and thenthe generalizations which they necessitate.
Plant-life is all directly or indirectly dependent on the heat and lightof the Sun-directly dependent in the immense majority of plants, and indirectlydependent in plants which, as the fungi, flourish in the dark: since these,growing at the expense of decaying organic matter, mediately draw their forcesfrom the same original source. Each plant owes the carbon and hydrogen ofwhich it mainly consists, to the carbon dioxide and water contained in thesurrounding air and earth. These must, however, be decomposed before theircarbon and hydrogen can be assimilated. To overcome the affinities whichhold their elements together requires the expenditure of energy; and thisenergy is supplied by the Sun. When, under fit conditions, plants are exposedto the solar rays, they give off oxygen and accumulate carbon and hydrogen.
