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