[Continued.]
The sea is full of life, both of plants and animals. These organisms die,
and their remains necessarily get mixed up with the different materials
laid down upon the sea floor. So that, beside the mere sand and mud,
great numbers of shells, corals, and the harder parts of other sea
creatures must be buried there, as generation after generation comes and
goes.
It often happens that on parts of the sea bed the remains of some of
these animals are so abundant that they themselves form thick and
wide-spread deposits. Oysters, for example, grow thickly together; and
their shells, mingled with those of other similar creatures, form what
are called shell banks. In the Pacific and the Indian Oceans a little
animal, called the coral-polyp, secretes a hard limy skeleton from the
sea water; and as millions of these polyps grow together, they form great
reefs of solid rock, which are sometimes, as in the Great Barrier Reef
of Australia, hundreds of feet thick and a thousand miles long. It is by
means of the growth of these animals that those wonderful rings of coral
rock or coral islands are formed in the middle of the ocean. Again, a
great part of the bed of the Atlantic Ocean is covered with fine mud,
which on examination is found to consist almost wholly of the remains of
very minute animals called foraminifera.
Over the bottom of the sea, therefore, great beds of sand and mud,
mingled with the remains of plants and animals, are always accumulating.
If now this bottom could be raised up above the sea level, even though
the sand and mud should get as dry and hard as any rock among the hills,
you would be able to say with certainty that they had once been under
the sea, because you would find in them the shells and other remains of
marine animals. This raising of the sea bottom has often taken place in
ancient times. You will find most of the rocks of our hills and valleys
to have been originally laid down in the sea, where they were formed
out of sand and mud dropped on the sea floor, just as sand and mud are
carried out to sea and laid down there now. And in these rocks, not
merely near the shore, but far inland, in quarries or ravines, or the
sides and even the tops of the hills, you will be able to pick out the
skeletons and fragments of the various sea creatures which were living in
the old seas.
Since the bottom of the sea forms the great receptacle into which the
mouldered remains of the surface of the land are continually carried, it
is plain that if this state of things were to go on without modification
or hindrance, in the end the whole of the solid land would be worn away,
and its remains would be spread out on the sea floor, leaving one vast
ocean to roll round the globe.
But there is in nature another force which here comes into play to retard
the destruction of the land.