[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.]

But although the fossil remains of quadrupeds and other land-animals are found in large quantities, their number is small compared with the enormous number of fossil sea-shells and sea-animals.

[Illustration:  FOSSIL FOOTPRINTS.]

Land-animals can, as a rule, have been so preserved, only when they have been drowned in ponds or rivers, or mired in bogs and swamps, or overtaken by frost, or swept out to sea.

Sea-animals, on the contrary, have been so preserved on land whenever that land has been under the sea; and this appears to have been the case, at one or another past age, with the greater part of our present continents.  These fossil remains of sea-animals are discovered in all quarters of the world, not only on the seashore but also far inland, not only deep down underground but also high up on the tops of lofty mountains—­a plain proof that over the summits of those mountains the ocean must once have rolled, and this not for a brief space only, but through long periods of time.  And not on the mountain-summit only are these fossils known to abound, but sometimes in layer below layer of the mountain, from top to bottom, through thousands of feet of rock.

[Illustration:  FOSSIL SHELLS.]

This may well seem puzzling at first sight.  Fossils of sea-creatures on a mountain-top are startling enough; yet hardly so startling as the thought of fossils inside that mountain.  How could they have found their way thither?

The difficulty soon vanishes, if once we clearly understand that all these thousands of feet of rock were built up slowly, layer after layer, when portions of the land lay deep under the sea.  Thus each separate layer of mud or sand or other material became in its turn the top layer, and was for the time the floor of the ocean, until further droppings of material out of the waters made a fresh layer, covering up the one below.