They have yet another and fourth title, which is, _Fossiliferous
Rocks_.

Fossils are the hardened remains of animals and vegetables found in
rocks. They are rarely, if ever, seen in unstratified rocks; but many
layers of stratified rocks abound in these remains. Whole skeletons as
well as single bones, whole tree-trunks as well as single leaves, are
found thus embedded in rock-layers, where in ages past the animal or
plant died and found a grave. They exist by thousands in many parts of
the world, varying in size from the huge skeleton of the elephant to
the tiny shell of the microscopic animalcule.

[Illustration: FOSSIL OF CARBONIFEROUS FERN.]

Fossils differ greatly in kind. Sometimes the entire shell or bone is
changed into stone, losing all its animal substance, but retaining its
old outline and its natural markings. Sometimes the fossil is merely
the hardened impress of the outside of a shell or leaf, which has
dented its picture on soft clay, and has itself disappeared, while the
soft clay has become rock, and the indented picture remains fixed
through after-centuries. Sometimes the fossil is the cast of the
inside of a shell; the said shell having been filled with soft mud,
which has taken its exact shape and hardened, while the shell itself
has vanished. The most complete description of fossil is the first of
these three kinds. It is wonderfully shown sometimes in fossil wood,
where all the tiny cells and delicate fibres remain distinctly marked
as of old, only the whole woody substance has changed into hard stone.

[Illustration: FOOTPRINTS FROM TRIASSIC SANDSTONE OF CONNECTICUT.]