ains; and
various other bodies of a more or less similar nature.

[Footnote 3: Lat. _fossus_, dug up.]

Fossilisation.-- The term "fossilisation" is applied to all those
processes through which the remains of organised beings may pass
in being converted into fossils. These processes are numerous
and varied; but there are three principal modes of fossilisation
which alone need be considered here. In the first instance, the
fossil is to all intents and purposes an actual portion of the
original organised being--such as a bone, a shell, or a piece
of wood. In some rare instances, as in the case of the body of
the Mammoth discovered embedded in ice at the mouth of the Lena
in Siberia, the fossil may be preserved almost precisely in its
original condition, and even with its soft parts uninjured. More
commonly, certain changes have taken place in the fossil, the
principal being the more or less total removal of the organic
matter originally present. Thus bones become light and porous
by the removal of their gelatine, so as to cleave to the tongue
on being applied to that organ; whilst shells become fragile, and
lose their primitive colours. In other cases, though practically
the real body it represents, all the cavities of the fossil,
down to its minutest recesses, may have become infiltrated with
mineral matter. It need hardly be added, that it is in the more
modern rocks that we find the fossils, as a rule, least changed
from their former condition; but the original structure is often
more or less completely retained in some of the fossils from
even the most ancient formations.

In the second place, we very frequently meet with fossils in the
state of "casts" or moulds of the original organic body. What
occurs in this case will be readily understood if we imagine any
common bivalve shell, as an Oyster, or Mussel, or Cockle, embedded
in clay or mud. If the clay were sufficiently soft and fluid, the
first thing would be that it would gain access to the interior
of the shell, and would completely fill up the space between the
valves. The pressure, also, of the surrounding matter would insure
that the clay would everywhere adhere closely to the exterior of
the shell. If now we suppose the clay to be in any way hardened
so as to be converted into stone, and if we were to break up the
stone, we should obviously have the following state of parts.
The clay which filled the shell would form an accurate cast of
the _interior_ o