Chapter 4

The basis of our reasoning is that the ocean owes its saltness mainly if not entirely to the denudative activities we have been considering. We must establish this.

We may, in the first place, say that any other view at once raises the greatest difficulties. The chemical composition of the detrital sediments which are spread over

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the continents and which build up the mountains, differs on the average very considerably from that of the igneous rocks. We know the former have been derived from the latter, and we know that the difference in the composition of the two classes of materials is due to the removal in solution of certain of the constituents of the igneous rocks. But the ocean alone can have received this dissolved matter. We know of no other place in which to look for it. It is true that some part of this dissolved matter has been again rejected by the ocean; thus the formation of limestone is largely due to the abstraction of lime from sea water by organic and other agencies. This, however, in no way relieves us of the necessity of tracing to the ocean the substances dissolved from the igneous rocks. It follows that we have here a very causa for the saltness of the ocean. The view that the ocean "was salt from the first" is without one known fact to support it, and leaves us with the burden of the entire dissolved salts of geological time to dispose of--Where and how?

The argument we have outlined above becomes convincingly strong when examined more closely. For this purpose we first compare the average chemical composition of the sedimentary and the igneous rocks. The following table gives the percentages of the chief chemical constituents: [1]

[1] F. W. Clarke: _A Preliminary Study of Chemical Denudation_, p. 13

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Igneous. Sedimentary. Silica (SiO2) - 59.99 58.51 Alumina (Al2O3) - 15.04 13.07 Ferric oxide (F2O3) - 2.59 3.40 Ferrous oxide (FeO) - 3.34 2.00 Magnesia (MgO) - 3.89 2.52 Lime (CaO) - 4.81 5.42 Soda (Na2O) - 3.41 1.12 Potash (K2O) - 2.95 2.80 Water (H2O) - 1.92 4.28 Carbon dioxide (CO2) - -- 4.93 Minor constituents - 2.06 1.95 100.00 100.00

In the derivation of the sediments from the igneous rocks there is a loss by solution of about 33 per cent; _i.e._ 100 tons of igneous rock yields rather less than 70 tons of sedimentary rock. This involves a concentration in the sediments of the more insoluble constituents. To this rule the lime-content appears to be an exception. It is not so in reality. Its high value in the sediments is due to its restoration from the ocean to the land. The magnesia and potash are, also, largely restored from the ocean; the former in dolomites and magnesian limestones; the latter in glauconite sands. The iron of the sediments shows increased oxidation. The most notable difference in the two analyses appears, however, in the soda percentages. This falls from 3.41 in the igneous rock to 1.12 in the average sediment. Indeed, this

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deficiency of soda in sedimentary rocks is so characteristic of secondary rocks that it may with some safety be applied to discriminate between the two classes of substances in cases where petrological distinctions of other kinds break down.

To what is this so marked deficiency of soda to be ascribed? It is a result of the extreme solubility of the salts of sodium in water. This has not only rendered its deposition by evaporation a relatively rare and unimportant incident of geological history, but also has protected it from abstraction from the ocean by organic agencies. The element sodium has, in fact, accumulated in the ocean during the whole of geological time.

We can use the facts associated with the accumulation of sodium salts in the ocean as a means of obtaining additional support to the view, that the processes of solvent denudation are responsible for the saltness of the ocean. The new evidence may be stated as follows: Estimates of the amounts of sedimentary rock on the continents have repeatedly been made. It is true that these estimates are no more than approximations. But they undoubtedly _are_ approximations, and as such may legitimately be used in our argument; more especially as final agreement tends to check and to support the several estimates which enter into them.

The most recent and probable estimates of the sediments on the land assign an average thickness of one mile of

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secondary rocks over the land area of the world. To this some increase must be made to allow for similar materials concealed in the ocean, principally around the continental margins. If we add 10 per cent. and assign a specific gravity of 2.5 we get as the mass of the sediments 64 x 1016 tonnes. But as this is about 67 per cent. of the parent igneous rock--_i.e._ the average igneous rock from which the sediments are derived--we conclude that the primary denuded rock amounted to a mass of about 95 x 1016 tonnes.

Now from the mean chemical composition of the secondary rocks we calculate that the mass of sediments as above determined contains 0.72 x1016 tonnes of the sodium oxide, Na2O. If to this amount we add the quantity of sodium oxide which must have been given to the ocean in order to account for the sodium salts contained therein, we arrive at a total quantity of oxide of sodium which must be that possessed by the primary rock before denudation began its work upon it. The mass of the ocean being well ascertained, we easily calculate that the sodium in the ocean converted to sodium oxide amounts to 2.1 x 1016 tonnes. Hence between the estimated sediments and the waters of the ocean we can account for 2.82 x 1016 tonnes of soda. When now we put this quantity back into the estimated mass of primary rock we find that it assigns to the primary rock a soda percentage of 3.0. On the average analysis given above this should be 3.41 per cent. The agreement,

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all things considered, more especially the uncertainty in the estimate of the sediments, is plainly in support of the view that oceanic salts are derived from the rocks; if, indeed, it does not render it a certainty.

A leading and fundamental inference in the denudative history of the Earth thus finds support: indeed, we may say, verification. In the light of this fact the whole work of denudation stands revealed. That the ocean began its history as a vast fresh-water envelope of the Globe is a view which accords with the evidence for the primitive high temperature of the Earth. Geological history opened with the condensation of an atmosphere of immense extent, which, after long fluctuations between the states of steam and water, finally settled upon the surface, almost free of matter in solution: an ocean of distilled water. The epoch of denudation then began. It will, probably, continue till the waters, undergoing further loss of thermal energy, suffer yet another change of state, when their circulation will cease and their attack upon the rocks come to an end.

From what has been reviewed above it is evident that the sodium in the ocean is an index of the total activity of denudation integrated over geological time. From this the broad facts of the results of denudation admit of determination with considerable accuracy. We can estimate the amount of rock which has been degraded by solvent and chemical actions, and the amount of sediments which has been derived from it. We are,

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thus, able to amend our estimate of the sediments which, as determined by direct observation, served to support the basis of our argument.

We now go straight to the ocean for the amount of sodium of denudative origin. There may, indeed, have been some primitive sodium dissolved by a more rapid denudation while the Earth's surface was still falling in temperature. It can be shown, however, that this amount was relatively small. Neglecting it we may say with safety that the quantity of sodium carried into the ocean by the rivers must be between 14,000 and 15,000 million million tonnes: _i.e._ 14,500 x 1012 tonnes, say.

Keeping the figures to round numbers we find that this amount of sodium involves the denudation of about 80 x 1016 tonnes of average igneous rock to 53 x 1016 tonnes of average sediment. From these vast quantities we know that the parent rock denuded during geological time amounted to some 300 million cubic kilometres or about seventy million cubic miles. The sediments derived therefrom possessed a bulk of 220 million cubic kilometres or fifty million cubic miles. The area of the land surface of the Globe is 144 million square kilometres. The parent rock would have covered this to a uniform depth of rather more than two kilometres, and the derived sediment to more than 1.5 kilometres, or about one mile deep.

The slow accomplishment of results so vast conveys some idea of the great duration of geological time.

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The foregoing method of investigating the statistics of solvent denudation is capable of affording information not only as to the amount of sediments upon the land, but also as to the quantity which is spread over the floor of the ocean.

We see this when we follow the fate of the 33 per cent. of dissolved salts which has been leached from the parent igneous rock, and the mass of which we calculate from the ascertained mass of the latter, to be 27 x 1016 tonnes. This quantity was at one time or another all in the ocean. But, as we saw above, a certain part of it has been again abstracted from solution, chiefly by organic agencies. Now the abstracted solids have not been altogether retained beneath the ocean. Movements of the land during geological time have resulted in some portion being uplifted along with other sediments. These substances constitute, mainly, the limestones.

We see, then, that the 27 x 1016 tonnes of substances leached from the parent igneous rocks have had a threefold destination. One part is still in solution; a second part has been precipitated to the bottom of the ocean; a third part exists on the land in the form of calcareous rocks.

Observation on the land sediments shows that the calcareous rocks amount to about 5 per cent. of the whole. From this we find that 3 x 1016 tonnes, approximately, of such rocks have been taken from the ocean. This accounts for one of the three classes of material

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into which the original dissolved matter has been divided. Another of the three quantities is easily estimated: the amount of matter still in solution in the ocean. The volume of the ocean is 1,414 million cubic kilometres and its mass is 145 x 1016 tonnes. The dissolved salts in it constitute 3.4 per cent. of its mass; or, rather more than 5 x 1016 tonnes. The limestones on the land and the salts in the sea water together make up about 8 x 1016 tonnes. If we, now, deduct this from the total of 27 x 1016 tonnes, we find that about 19 x 1016 tonnes must exist as precipitated matter on the floor of the ocean.

The area of the ocean is 367 x 1012 square metres, so that if the precipitated sediment possesses an average specific gravity of 2.5, it would cover the entire floor to a uniform depth of 218 metres; that is 715 feet. This assumes that there was uniform deposition of the abstracted matter over the floor of the ocean. Of course, this assumption is not justifiable. It is certain that the rate of deposition on the floor of the sea has varied enormously with various conditions--principally with the depth. Again, it must be remembered that this estimate takes no account of solid materials otherwise brought into the oceanic deposits; _e.g._, by wind-transported dust from the land or volcanic ejectamenta in the ocean depths. It is not probable, however, that any considerable addition to the estimated mean depth of deposit from such sources would be allowable.

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The greatness of the quantities involved in these determinations is almost awe inspiring. Take the case of the dissolved salts in the ocean. They are but a fraction, as we have seen, of the total results of solvent denudation and represent the integration of the minute traces contributed by the river water. Yet the common salt (chloride of sodium) alone, contained in the ocean, would, if abstracted and spread over the dry land as a layer of rock salt having a specific gravity of 2.2, cover the whole to a depth of 107 metres or 354 feet. The total salts in solution in the ocean similarly spread over the land would increase the depth of the layer to 460 feet. After considering what this means we have to remember that this amount of matter now in solution in the seas is, in point of fact, less than a fifth part of the total dissolved from the rocks during geological time.

The transport by denudation of detrital and dissolved matter from the land to the ocean has had a most important influence on the events of geological history. The existing surface features of the earth must have been largely conditioned by the dynamical effects arising therefrom. In dealing with the subject of mountain genesis we will, elsewhere, see that all the great mountain ranges have originated in the accumulation of the detrital sediments near the shore in areas which, in consequence of the load, gradually became depressed and developed into synclines of many thousands of feet in depth. The most impressive surface features of the Globe originated

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in this manner. We will see too that these events were of a rhythmic character; the upraising of the mountains involving intensified mechanical denudation over the elevated area and in this way an accelerated transport of detritus to the sea; the formation of fresh deposits; renewed synclinal sinking of the sea floor, and, finally, the upheaval of a younger mountain range. This extraordinary sequence of events has been determined by the events of detrital denudation acting along with certain general conditions which have all along involved the growth of compressive stresses in the surface crust of the Earth.

The effects of purely solvent denudation are less easily traced, but, very probably, they have been of not less importance. I refer here to the transport from the land to the sea of matter in solution.

Solvent denudation, as observed above, takes place mainly in the soils and in this way over the more level continental areas. It has resulted in the removal from the land and transfer to the ocean of an amount of matter which represents a uniform layer of one half a kilometre; that is of more than 1,600 feet of rock. The continents have, during geological time, been lightened to this extent. On the other hand all this matter has for the greater part escaped the geosynclines and become uniformly diffused throughout the ocean or precipitated over its floor principally on the continental slopes before the great depths are reached. Of this material the ocean

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waters contain in solution an amount sufficient to increase their specific gravity by 2.7 per cent.

Taking the last point first, it is interesting to note the effects upon the bulk of the ocean which has resulted from the matter dissolved in it. From the known density of average sea water we find that 100 ccs. of it weigh just 102.7 grammes. Of this 3.5 per cent. by weight are solids in solution. That is to say, 3.594 grammes. Hence the weight of water present is 99.1 grammes, or a volume of 99.1 ccs. From this we see that the salts present have increased the volume by 0.9 ccs. or 0.9 per cent.

The average depth of the ocean is 2,000 fathoms or 3,700 metres. The increase of depth due to salts dissolved in the ocean has been, therefore, 108 feet or 33.24 metres. This result assumes that there has been no increased elastic compression due to the increased pressure, and no change of compressional elastic properties. We may be sure that the rise on the shore line of the land has not been less than 100 feet.

We see then that as the result of solvent denudation we have to do with a heavier and a deeper ocean, expanded in volume by nearly one per cent. and the floor of which has become raised, on an average, about 700 feet by precipitated sediment.

One of the first conceptions, which the student of geology has to dismiss from his mind, is that of the immobility or rigidity of the Earth's crust. The lane, we live on sways even to the gentle rise and fail of ocean tides

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around the coasts. It suffers its own tidal oscillations due to the moon's attractions. Large tracts of semi-liquid matter underlie it. There is every evidence that the raised features of the Globe are sustained by such pressures acting over other and adjacent areas as serve to keep them in equilibrium against the force of gravity. This state of equilibrium, which was first recognised by Pratt, as part of the dynamics of the Earth's crust, has been named isostasy. The state of the crust is that of "mobile equilibrium."

The transfer of matter from the exposed land surfaces to the sub-oceanic slopes of the continents and the increase in the density of the ocean, must all along have been attended by isostatic readjustment. We cannot take any other view. On the one hand the land was being lightened; on the other the sea was increasing in mass and depth and the flanks of the continents were being loaded with the matter removed from the land and borne in solution to the ocean. How important the resulting movements must have been may be gathered from the fact that the existing land of the Globe stands at a mean elevation of no more than 2,000 feet above sea level. We have seen that solvent denudation removed over 1,600 feet of rock. But we have no evidence that on the whole the elevation of land in the past was ever very different from what it now is.

We have, then, presented to our view the remarkable fact that throughout the past, and acting with extreme

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slowness, the land has steadily been melted down into the sea and as steadily been upraised from the waters. It is possible that the increased bulk of the ocean has led to a certain diminution of the exposed land area. The point is a difficult one. One thing we may without much risk assume. The sub-aereal current of dissolved matter from the land to the ocean was accompanied by a sub-crustal flux from the ocean areas to the land areas; the heated viscous materials creeping from depths far beneath the ocean floor to depths beneath the roots of the mountains which arose around the oceans. Such movements took ages for their accomplishment. Indeed, they have been, probably, continuous all along and are still proceeding. A low degree of viscosity will suffice to permit of movements so slow. Superimposed upon these movements the rhythmic alternations of depression and elevation of the geosynclines probably resulted in releasing the crust from local accumulation of strains arising in the more rigid surface materials. The whole sequence of movements presents an extraordinary picture of pseudo-vitality--reminding us of the circulatory and respiratory systems of a vast organism.

All great results in our universe are founded in motions and forces the most minute. In contemplating the Cause or the Effect we stand equally impressed with the spectacle presented to us. We shall now turn from the great effects of denudation upon the history and evolution of a world and consider for a moment activities

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so minute in detail that their operations will probably for ever elude our bodily senses, but which nevertheless have necessarily affected and modified the great results we have been considering.

The ocean a little way from the land is generally so free from suspended sediments that it has a blackness as of ink. This blackness is due to its absolute freedom from particles reflecting the sun's light. The beautiful blue of the Swiss and Italian lakes is due to the presence of very fine particles carried into them by the rivers; the finest flour of the glaciers, which remain almost indefinitely suspended in the water. But in the ocean it is only in those places where rapid currents running over shallows stir continually the sediments or where the fresh water of a great river is carried far from the land, that the presence of silt is to be observed. The beautiful phenomenon of the coal-black sea is familiar to every yachtsman who has sailed to the west of our Islands.[1]

There is, in fact, a very remarkable difference in the manner of settlement of fine sediments in salt and in fresh water. We are here brought into contact with one of those subtle yet influential natural actions the explanation of which involves scientific advance along many apparently unconnected lines of investigation.

[1] See Tyndall's Voyage to Algeria in _Fragments of Science._ The cause of the blue colour of the lakes has been discussed by various observers, not always with agreement.

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It is easy to observe in the laboratory the fact of the different behaviour of salt and fresh water towards finely divided substances. The nature of the insoluble substance is not important.

We place, in a good light, two glass vessels of equal dimensions; the one filled with sea water, the other with fresh water. Into each we stir the same weight of very finely powdered slate: just so much as will produce a cloudiness. In a few hours we find the sea water limpid. The fresh water is still cloudy, however; and, indeed, may be hardly different in appearance from what it was at starting. In itself this is a most extraordinary experiment. We would have anticipated quite the opposite result owing to the greater density of the sea water.

But a still more interesting experiment remains to be carried out. In the sea water we have many different salts in solution. Let us see if these salts are equally responsible for the result we have obtained. For this purpose we measure out quantities of sodium chloride and magnesium chloride in the proportion in which they exist in sea water: that is about as seven to one. We add such an equal amount of water to each as represents the dilution of these salts in sea water. Then finally we stir a little of the finely powdered slate into each. It will be found that the magnesium chloride, although so much more dilute than the sodium chloride, is considerably more active in clearing out the suspension. We may now try such marine salts as magnesium sulphate,

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or calcium sulphate against sodium chloride; keeping the marine proportions. Again we find that the magnesium and calcium salts are the most effective, although so much more dilute than the sodium salt.

There is no visible clue to the explanation of these results. But we must conclude as most probable that some action is at work in the sea water and in the salt solutions which clumps or flocculates the sediment. For only by the gathering of the particles together in little aggregates can we explain their rapid fall to the bottom. It is not a question of viscosity (_i.e._ of resistance to the motion of the particles), for the salt solutions are rather more viscous than the fresh water. Still more remarkable is the fact that every dissolved substance will not bring about the result. Thus if we dissolve sugar in water we find that, if anything, the silt settles more slowly in the sugar solution than in fresh water.