The Birth-Time of the World and Other Scientific Essays Part 5

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

41

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 cla.s.ses of materials is due to the removal in solution of certain of the const.i.tuents 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 const.i.tuents: [1]

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

42

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 const.i.tuents - 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 const.i.tuents. 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 a.n.a.lyses appears, however, in the soda percentages. This falls from 3.41 in the igneous rock to 1.12 in the average sediment.

Indeed, this

43

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 cla.s.ses 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, acc.u.mulated in the ocean during the whole of geological time.

We can use the facts a.s.sociated with the acc.u.mulation 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 a.s.sign an average thickness of one mile of

44

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, princ.i.p.ally around the continental margins. If we add 10 per cent. and a.s.sign a specific gravity of 2.5 we get as the ma.s.s 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 ma.s.s of about 95 x 1016 tonnes.

Now from the mean chemical composition of the secondary rocks we calculate that the ma.s.s of sediments as above determined contains 0.72 x1016 tonnes of the sodium oxide, Na2O. If to this amount we add the quant.i.ty 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 quant.i.ty of oxide of sodium which must be that possessed by the primary rock before denudation began its work upon it. The ma.s.s 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 quant.i.ty back into the estimated ma.s.s of primary rock we find that it a.s.signs to the primary rock a soda percentage of 3.0. On the average a.n.a.lysis given above this should be 3.41 per cent.

The agreement,

45

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,

46

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 quant.i.ty 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 quant.i.ties 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.

47

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 quant.i.ty 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 ma.s.s of which we calculate from the ascertained ma.s.s of the latter, to be 27 x 1016 tonnes. This quant.i.ty 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 const.i.tute, 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 cla.s.ses of material

48

into which the original dissolved matter has been divided.

Another of the three quant.i.ties 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 ma.s.s is 145 x 1016 tonnes. The dissolved salts in it const.i.tute 3.4 per cent. of its ma.s.s; 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 a.s.sumes that there was uniform deposition of the abstracted matter over the floor of the ocean.

Of course, this a.s.sumption is not justifiable. It is certain that the rate of deposition on the floor of the sea has varied enormously with various conditions--princ.i.p.ally 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.

49

The greatness of the quant.i.ties 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 acc.u.mulation of the detrital sediments near the sh.o.r.e 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

50

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 princ.i.p.ally on the continental slopes before the great depths are reached. Of this material the ocean

51

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