The Birth-Time of the World and Other Scientific Essays Part 4

Indeed, it is almost impossible to deal with the subject without entering upon controversial matters. In the following pages I shall endeavour to keep to broad issues which are, at the present day, either conceded by the greater number of authorities on the subject, or are, from their strictly quant.i.tative character, not open to controversy.

It is evident, in the first place, that denudation--or the wearing away of the land surfaces of the earth--is mainly a result of the circulation of water from the ocean to the land, and back again to the ocean. An action entirely conditioned by solar heat, and without which it would completely cease and further change upon the land come to an end.

To what actions, then, is so great a potency of the

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circulating water to be traced? Broadly speaking, we may cla.s.sify them as mechanical and chemical. The first involves the separation of rock ma.s.ses into smaller fragments of all sizes, down to the finest dust. The second involves the actual solution in the water of the rock const.i.tuents, which may be regarded as the final act of disintegration. The rivers bear the burden both of the comminuted and the dissolved materials to the sea. The mud and sand carried by their currents, or gradually pushed along their beds, represent the former; the invisible dissolved matter, only to be demonstrated to the eye by evaporation of the water or by chemical precipitation, represents the latter.

The results of these actions, integrated over geological time, are enormous. The entire bulk of the sedimentary rocks, such as sandstones, slates, shales, conglomerates, limestones, etc., and the salt content of the ocean, are due to the combined activity of mechanical and solvent denudation. We shall, later on, make an estimate of the magnitude of the quant.i.ties actually involved.

In the Swiss valleys we see torrents of muddy water hurrying along, and if we follow them up, we trace them to glaciers high among the mountains. From beneath the foot of the glacier, we find, the torrent has birth. The first debris given to the river is derived from the wearing of the rocky bed along which the glacier moves. The river of ice bequeaths to the river of water--of which it is the parent--the spoils which it has won from the rocks

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The work of mechanical disintegration is, however, not restricted to the glacier's bed. It proceeds everywhere over the surface of the rocks. It is aided by the most diverse actions. For instance, the freezing and expansion of water in the c.h.i.n.ks and cracks in those alpine heights where between sunrise and sunset the heat of summer reigns, and between sunset and sunrise the cold of winter.

Again, under these conditions the mere change of surface temperature from night to day severely stresses the surface layers of the rocks, and, on the same principles as we explain the fracture of an unequally heated gla.s.s vessel, the rocks cleave off in slabs which slip down the steeps of the mountain and collect as screes in the valley. At lower levels the expansive force of vegetable growth is not unimportant, as all will admit who have seen the strong roots of the pines penetrating the crannies of the rocks. Nor does the river which flows in the bed of the valley act as a carrier only. Listening carefully we may detect beneath the roar of the alpine torrent the crunching and knocking of descending boulders. And in the potholes scooped by its whirling waters we recognise the abrasive action of the suspended sand upon the river bed.

A view from an Alpine summit reveals a scene of remarkable desolation (Pl. V, p. 40). Screes lie piled against the steep slopes. Cliffs stand shattered and ready to fall in ruins. And here the forces at work readily reveal themselves. An occasional wreath of white smoke among

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the far-off peaks, followed by a rumbling reverberation, marks the fall of an avalanche. Water everywhere trickles through the shaly _debris_ scattered around. In the full suns.h.i.+ne the rocks are almost too hot to bear touching. A few hours later the cold is deadly, and all becomes a frozen silence. In such scenes of desolation and destruction, detrital sediments are actively being generated. As we descend into the valley we hear the deep voice of the torrents which are continually hurrying the disintegrated rocks to the ocean.

A remarkable demonstration of the activity of mechanical denudation is shown by the phenomenon of "earth pillars." The photograph (Pl. IV.) of the earth pillars of the Val d'Herens (Switzerland) shows the peculiar appearance these objects present. They arise under conditions where large stones or boulders are scattered in a deep deposit of clay, and where much of the denudation is due to water scour. The large boulders not only act as shelter against rain, but they bind and consolidate by their mere weight the clay upon which they rest. Hence the materials underlying the boulders become more resistant, and as the surrounding clays are gradually washed away and carried to the streams, these compacted parts persist, and, finally, stand like walls or pillars above the general level. After a time the great boulders fall off and the underlying clay becomes worn by the rainwash to fantastic spikes and ridges. In the Val d'Herens the earth pillars are formed

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of the deep moraine stuff which thickly overlies the slopes of the valley. The wall of pillars runs across the axis of the valley, down the slope of the hill, and crosses the road, so that it has to be tunnelled to permit the pa.s.sage of traffic. It is not improbable that some additional influence--possibly the presence of lime--has hardened the material forming the pillars, and tended to their preservation.

Denudation has, however, other methods of work than purely mechanical; methods more noiseless and gentle, but not less effective, as the victories of peace ate no less than those of war.

Over the immense tracts of the continents chemical work proceeds relentlessly. The rock in general, more especially the primary igneous rock, is not stable in presence of the atmosphere and of water. Some of the minerals, such as certain silicates and carbonates, dissolve relatively fast, others with extreme slowness. In the process of solution chemical actions are involved; oxidation in presence of the free oxygen of the atmosphere; attack by the feeble acid arising from the solution of carbon dioxide in water; or, again, by the activity of certain acids--humous acids--which originate in the decomposition of vegetable remains. These chemical agents may in some instances, _e.g._ in the case of carbonates such as limestone or dolomite--bring practically the whole rock into solution. In other instances--_e.g._ granites, basalts, etc.--they may remove some of the

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const.i.tuent minerals completely or partially, such as felspar, olivine, augite, and leave more resistant substances to be ultimately washed down as fine sand or mud into the river.

It is often difficult or impossible to appraise the relative efficiency of mechanical and chemical denudation in removing the materials from a certain area. There can be, indeed, little doubt that in mountainous regions the mechanical effects are largely predominant. The silts of glacial rivers are little different from freshly-powdered rock. The water which carries them but little different from the pure rain or snow which falls from the sky. There has not been time for the chemical or solvent actions to take place. Now while gravitational forces favour sudden shock and violent motions in the hills, the effect of these on solvent and chemical denudation is but small. Nor is good drainage favourable to chemical actions, for water is the primary factor in every case. Water takes up and removes soluble combinations of molecules, and penetrates beneath residual insoluble substances.

It carries the oxygen and acids downwards through the soils, and finally conveys the results of its own work to the rivers and streams. The lower mean temperature of the mountains as well as the perfect drainage diminishes chemical activities.

Hence we conclude that the heights are not generally favourable to the purely solvent and chemical actions. It is on the lower-lying land that soils tend to acc.u.mulate,

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and in these the chief solvent and the chief chemical denudation of the Earth are effected.

The solvent and chemical effects which go on in the finely-divided materials of the soils may be observed in the laboratory. They proceed faster than would be antic.i.p.ated. The observation is made by pa.s.sing a measured quant.i.ty of water backwards and forwards for some months through a tube containing a few grammes of powdered rock. Finally the water is a.n.a.lysed, and in this manner the amount of dissolved matter it has taken up is estimated. The rock powder is examined under the microscope in order to determine the size of the grains, and so to calculate the total surface exposed to the action of the water. We must be careful in such experiments to permit free oxidation by the atmosphere. Results obtained in this way of course take no account of the chemical effects of organic acids such as exist in the soils. The quant.i.ties obtained in the laboratory will, therefore, be deficient as compared with the natural results.

In this manner it has been found that fresh basalt exposed to continually moving water will lose about 0.20 gramme per square metre of surface per year. The mineral orthoclase, which enters largely into the const.i.tution of many granites, was found to lose under the same conditions 0.025 gramme. A gla.s.sy lava (obsidian) rich in silica and in the chemical const.i.tuents of an average granite, was more resistant still; losing but 0.013 gramme per square metre per year. Hornblende, a mineral

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abundant in many rocks, lost 0.075 gramme. The mean of the results showed that 0.08 gramme was washed in a year from each square metre. Such results give us some indication of the rate at which the work of solution goes on in the finely divided soils.[1]

It might be urged that, as the mechanical break up of rocks, and the production in this way of large surfaces, must be at the basis of solvent and chemical denudation, these latter activities should be predominant in the mountains. The answer to this is that the soils rarely owe their existence to mechanical actions.

The alluvium of the valleys const.i.tutes only narrow margins to the rivers; the finer _debris_ from the mountains is rapidly brought into the ocean. The soils which cover the greater part of continental areas have had a very different origin.

In any quarry where a section of the soil and of the underlying rock is visible, we may study the mode of formation of soils. Our observations are, we will suppose, pursued in a granite quarry.

We first note that the material of the soil nearest the surface is intermixed with the roots of gra.s.ses, trees, or shrubs.

Examining a handful of this soil, we see glistening flakes of mica which plainly are derived from the original granite. Was.h.i.+ng off the finer particles, we find the largest remaining grains are composed of the all but indestructible quartz.

[1] Proc. Roy. Irish Acad., VIII., Ser. A, p. 21.

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This also is from the granite. Some few of the grains are of chalky-looking felspar; again a granitic mineral. What is the finer silt we have washed off? It, too, is composed of mineral particles to a great extent; rock dust stained with iron oxide and intermixed with organic remains, both animal and vegetable.

But if we make a chemical a.n.a.lysis of the finer silt we find that the composition is by no means that of the granite beneath. The chemist is able to say, from a study of his results, that there has been, in the first place, a large loss of material attending the conversion of the granite to the soil. He finds a concentration of certain of the more resistant substances of the granite arising from the loss of the less resistant. Thus the percentage amount of alumina is increased. The percentage of iron is also increased. But silica and most other substances show a diminished percentage. Notably lime has nearly disappeared. Soda is much reduced; so is magnesia. Potash is not so completely abstracted. Finally, owing to hydration, there is much more combined water in the soil than in the rock. This is a typical result for rocks of this kind.

Deeper in the soil we often observe a change of texture. It has become finer, and at the same time the clay is paler in colour.

This subsoil represents the finer particles carried by rain from above. The change of colour is due to the state of the iron which is less oxidised low down in the soil. Beneath the subsoil the soil grows

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again coa.r.s.er. Finally, we recognise in it fragments of granite which ever grow larger as we descend, till the soil has become replaced by the loose and shattered rock. Beneath this the only sign of weathering apparent in the rock is the rusty hue imparted by the oxidised iron which the percolating rain has leached from iron-bearing minerals.

The soil we have examined has plainly been derived in situ from the underlying rock. It represents the more insoluble residue after water and acids have done their work. Each year there must be a very slow sinking of the surface, but the ablation is infinitesimal.

The depth of such a soil may be considerable. The total surface exposed by the countless grains of which it is composed is enormous. In a cubic foot of average soil the surface area of the grains may be 50,000 square feet or more. Hence a soil only two feet deep may expose 100,000 square feet for each square foot of surface area.

It is true that soils formed in this manner by atmospheric and organic actions take a very long time to grow. It must be remembered, however, that the process is throughout attended by the removal in solution: of chemically altered materials.

Considerations such as the foregoing must convince us that while the acc.u.mulation of the detrital sediments around the continents is largely the result of activities progressing on the steeper slopes of the land, that is,

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among the mountainous regions, the feeding of the salts to the ocean arises from the slower work of meteorological and organic agencies attacking the molecular const.i.tution of the rocks; processes which best proceed where the drainage is sluggish and the quiescent conditions permit of the development of abundant organic growth and decay.

Statistics of the solvent denudation of the continents support this view. Within recent years a very large amount of work has been expended on the chemical investigation of river waters of America and of Europe. F. W. Clarke has, at the expense of much labour, collected and compared these results. They are expressed as so many tonnes removed in solution per square mile per annum.

For North America the result shows 79 tonnes so removed; for Europe 100 tonnes. Now there is a notable difference between the mean elevations of these two continents. North America has a mean elevation of 700 metres over sea level, whereas the mean elevation of Europe is but 300 metres. We see in these figures that the more mountainous land supplies less dissolved matter to the ocean than the land of lower elevation, as our study has led us to expect.

We have now considered the source of the detrital sediments, as well as of the dissolved matter which has given to the ocean, in the course of geological time, its present gigantic load of salts. It is true there are further solvent and chemical effects exerted by the sea water

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upon the sediments discharged into it; but we are justified in concluding that, relatively to the similar actions taking place in the soils, the solvent and chemical work of the ocean is small. The fact is, the deposited detrital sediments around the continents occupy an area small when contrasted with the vast stretches of the land. The area of deposition is much less than that of denudation; probably hardly as much as one twentieth.

And, again, the conditions of aeration and circulation which largely promote chemical and solvent denudation in the soils are relatively limited and ineffective in the detrital oceanic deposits.

The summation of the amounts of dissolved and detrital materials which denudation has brought into the ocean during the long denudative history of the Earth, as we might antic.i.p.ate, reveals quant.i.ties of almost unrealisable greatness. The facts are among the most impressive which geological science has brought to light. Elsewhere in this volume they have been mentioned when discussing the age of the Earth. In the present connection, however, they are deserving of separate consideration.