ns are composed mainly of the internal
casts of the shells of _Foraminifera_, and we have thus another
and a very interesting example how rock-deposits of considerable
extent and of geological importance can be built up by the operation
of the minutest living beings.

As regards _argillaceous_ deposits, containing _alumina_ or _clay_
as their essential ingredient, it cannot be said that any of
these have been actually shown to be of organic origin. A recent
observation by Sir Wyville Thomson would, however, render it not
improbable that some of the great argillaceous accumulations of
past geological periods may be really organic. This distinguished
observer, during the cruise of the Challenger, showed that the
calcareous ooze which has been already spoken of as covering
large areas of the floor of the Atlantic and Pacific at great
depths, and which consists almost wholly of the shells of
_Foraminifera_, gave place at still greater depths to a red ooze
consisting of impalpable clayey mud, coloured by oxide of iron,
and devoid of traces of organic bodies. As the existence of this
widely-diffused red ooze, in mid-ocean, and at such great depths,
cannot be explained on the supposition that it is a sediment
brought down into the sea by rivers, Sir Wyville Thomson came to
the conclusion that it was probably formed by the action of the
sea-water upon the shells of _Foraminifera_. These shells, though
mainly consisting of lime, also contain a certain proportion of
alumina, the former being soluble in the carbonic acid dissolved
in the sea-water, whilst the latter is insoluble. There would
further appear to be grounds for believing that the solvent power
of the sea-water over lime is considerably increased at great
depths. If, therefore, we suppose the shells of _Foraminifera_
to be in course of deposition over the floor of the Pacific, at
certain depths they would remain unchanged, and would accumulate
to form a calcareous ooze; but at greater depths they would be
acted upon by the water, their lime would be dissolved out, their
form would disappear, and we should simply have left the small
amount of alumina which they previously contained. In process
of time this alumina would accumulate to form a bed of clay; and
as this clay had been directly derived from the decomposition
of the shells of animals, it would be fairly entitled to be
considered an organic deposit. Though not finally established,
the hypothesis of Sir Wyvill