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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.