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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.