sents the rings of growth and fibrous structure of
recent wood, and which under the microscope exhibits the minutest
vessels which characterise ligneous tissue, together with the even
more minute markings of the vessels (fig. 2). The whole, however,
instead of being composed of the original carbonaceous matter of
the wood, is now converted into flint. The only explanation that
can be given of this by no means rare phenomenon, is that the
wood must have undergone a slow process of decay in water charged
with silica or flint in solution. As each successive particle of
wood was removed by decay, its place was taken by a particle of
flint deposited from the surrounding water, till ultimately the
entire wood was silicified. The process, therefore, resembles
what would take place if we were to pull down a house built of
brick by successive bricks, replacing each brick as removed by
a piece of stone of precisely the same size and form. The result
of this would be that the house would retain its primitive size,
shape, and outline, but it would finally have been converted
from a house of brick into a house of stone. Many other fossils
besides wood--such as shells, corals, sponges, &c.--are often
found silicified; and this may be regarded as the commonest form
of fossilisation by replacement. In other cases, however, though
the principle of the process is the same, the replacing substance
may be iron pyrites, oxide of iron, sulphur, malachite, magnesite,
talc, &c.; but it is rarely that the replacement with these minerals
is so perfect as to preserve the more delicate details of internal
structure.

CHAPTER II.

THE FOSSILIFEROUS ROCKS.

Fossils are found in rocks, though not universally or promiscuously;
and it is therefore necessary that the palaeontologist should
possess some acquaintance with, at any rate, those rocks which
yield organic remains, and which are therefore said to be
"_fossiliferous_." In geological language, all the materials
which enter into the composition of the solid crust of the earth,
be their texture what it may--from the most impalpable mud to
the hardest granite--are termed "rocks;" and for our present
purpose we may divide these into two great groups. In the first
division are the _Igneous Rocks_--such as the lavas and ashes of
volcanoes--which are formed within the body of the earth itself,
and which owe their structure and origin to the action of heat.
The Igneous Rocks are formed primar