obably, 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

54

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.

55

It is easy to observe in the laboratory the fact of the different
behaviour of salt and fresh water towards finely d