RECORDS LEFT BY THE SEA
We have already spoken of the story which the sea writes in the
annals of geology. It is a story with two plots. In the first place,
the sea is always wearing away the land. In the second place, it is
arranging on its own bed the materials which it takes from the land,
either directly or indirectly. As a sequel to both stories, the
materials all neatly ranged, packed, and folded are revealed when the
sea subsides from them, or when, in process of one of those great
geological changes, the origin of which we have already attempted
to account for, the sea bottom is raised to become the land of a
continent. The first part of the sea's belligerent story is written
so plainly for all eyes to see that one scarcely need dwell on it.
Every strip of coast around these islands bears witness to it.
Off Shetland masses of rock twelve or thirteen tons in weight have
been cut out from the cliff seventy feet above the smooth-water
level. The sea's battering-rams are the masses of shingle, gravel,
and loose blocks of stone which it carries with it; but it has
subtler methods in the corrosive action of its salts, for just as it
rusts or wears away iron, so its salts and acids must eat their way
into many rocks.
But, after all, the coast-line of the world is a small fraction of
the whole land surface of the globe; and a smaller fraction of the
sea's own wide area. On that area are flung all the records and
treasures which the sea has wrested from the land. The rivers, as
we have already several times repeated, are the chief carriers of
deposits to the sea. By their deltas they may be known. The deltas of
the Ganges and Brahmaputra cover an area as large as that of England
and Wales. The delta has been bored through to a depth of nearly five
hundred feet, and has been found to consist of numerous alternations
of fine clays, marls, and sands or sandstones, with occasional layers
of gravel. In all this accumulation of sediment there are no traces
of marine animals; but land plants and the plants and animals of the
river and of the surrounding land have been discovered in quantity.
The sea most often destroys land; but it sometimes deposits beaches;
and, we might almost say, silts up the land. At Romney Marsh, for
example, a tract of eighty square miles which was marsh in Julius
Cæsar's time is now dry land, and has become so partly by the natural
increase of shingle thrown up by the waves. The coarsest shingle
usually accumulates towards the upper part of the beach, and the rest
arranges itself generally according to size and weight, that which is
finest being nearest to low-water mark.
It is often long before the stuff brought down by the rivers settles
on the floor of the ocean. The finer particles may be carried out to
sea for three hundred miles or more before they settle. Within this
three-hundred-mile zone the land-derived materials are distributed
over the floor in orderly succession. Nearer the land we shall find
coarse gravel and sand. Beyond there will be tracts of finer sand
and silt, with patches of gravel here and there. Still farther off
will come fine blue and green muds, which are made of the tiny
particles of such materials as form the ordinary rocks of the land.
But when we are once past this zone of land material we come upon
deposits which are the ocean's own freehold--materials which it does
not derive from the continents, but which may be called oceanic in
origin. First there are vast sheets of exceedingly fine red and brown
clay. Whence comes it? It is by far the most common deposit in all
the deeper parts of the ocean. It may either be the dust of volcanic
fragments washed away from volcanic islands, or (which is much more
likely) it may be supplied by eruptions under the sea. For it must
be remembered that the sea floor is two to five miles nearer the hot
rocks that are in the interior of the earth than the land surface is,
and that consequently the water coming into contact with them may
cause explosions arising from the action of steam. This is a question
we shall have to consider later, and for the present we must ask the
reader to accept the fact--and read on.
There is one very curious thing about this red clay, and it is that
the accumulations of it appear to be built up very slowly. Where it
occurs farthest from land great numbers of sharks' teeth with ear
bones and other bones of whales have been dredged up from it. Some
of these relics are quite fresh; others are coated with a crust of
brown peroxide of manganese. Some are covered with this material and
hidden in it. One haul of an ocean dredge will bring up the bones in
all these states, so that they must be lying side by side. The bones
are probably those of many generations of animals, and it must take a
long time to cover them with the manganese deposit. But the clay is
deposited even more slowly than the manganese, so that it must fall
very slowly indeed.
But besides these things the bottom of the sea receives deposits of
the remains of all kinds of shells, corals, and all sorts of marine
creatures, great and small. As the countless myriads of the animals
of the sea die, the shells with which they are covered, or the bones
which form their framework, fall continually to the bottom of the
oceanic gulfs in which they dwell. Then the ocean floor is covered
with the remains of tiny animals incomparably more numerous than the
stars of the sky; and this grey slimy ooze of organic matter hardens
by pressure into sedimentary rock. In the course of ages, when the
slow decline of the water lays it bare, it may become part of the
land on which men dwell. But it is always forming, has always been
forming, since life first appeared on the earth. It is on this ocean
floor that man to-day lays his telegraph cables. Mr. Rudyard Kipling,
in his verses "The Deep Sea Cables," has drawn a vivid picture of the
bed of the deep ocean:--
The wrecks dissolve above us: their dust drops down from afar--
Down to the dark, to the utter dark where the blind white
sea-snakes are.
There is no sound, no echo of sound, in the deserts of the deep,
On the great grey level plains of ooze, where the shell-burred
cables creep.
It is in these silent depths that for uncounted and innumerable
years the crust of the earth has been forming and has been growing
outwards, while it has been slowly hardening inwards above the fires
of its unplumbed interior.
It has been calculated that in a square mile of the ocean down to
a depth of one hundred fathoms there exist more than sixteen tons
of carbonate of lime in the form of the bones or shells of living
animals. A continual fine "snow-storm" of dead chalky animals is
therefore falling on to the bottom. Here and there, especially among
volcanic islands, portions of the sea-bed have been raised up into
land and masses of modern limestone. Though these rocks are full of
the same kinds of shells as are still living in the neighbouring sea,
they have been cemented into hard rock. This cementing is due to
the water which has penetrated and permeated the stone, dissolving
chalky matter from the outside shells, and depositing it once more
lower down and farther in, like a fine mortar, so as to bind the mass
together.
Every one has heard of coral reefs. They are one of the best and most
familiar examples of the way in which great masses of solid rock
can be built up by the dead bodies of animals. In the warmer seas
of the earth, and notably in the track of the great ocean currents,
various kinds of coral polyps, as they are called, take root on the
edges and summits of submerged rocks and peaks, as well as on the
shelving shores of islands. The coral polyp is a jelly-like creature,
but it has a hard chalky skeleton inside its transparent body. It is
a great colonist, with no liking for a solitary life, but with, on
the contrary, a great fancy for its neighbours; in fact, the polyps
grow and thrive in clumps, and the clumps unite to form communities,
and the communities increase to colonies and nations, till they unite
to form what is called a reef. The coral polyps are rather exigent
in the choice of their residential neighbourhoods. They cannot live
at a greater depth than fifteen or twenty fathoms, and in defiance
of the inclinations which rule human beings, they have the strongest
distaste for sun and air; in fact, they die when exposed to it.
Now when the polyp dies its skeleton remains behind it, and millions
upon millions of these coral skeletons make a layer of coral. These
layers of coral gradually lift the generations of polyps upwards to
the surface of the water. But as we have seen, the living polyps
die when they get so far, and consequently the reef then spreads
outwards. On the outer edges of the reef the coral polyps flourish in
the most vigorous way. There they are as completely provided for as
in a County Council Utopia. The breakers bring them the food on which
they live; the water and the climate suit them exactly. The only blot
on their lives are the occasional storms which break off fragments of
the coral foundation on which they live. But even this, while it is
disastrous to the individual polyp, is for the good of the community,
because these blocks as they roll down form a new foundation on
which new generations of polyps can grow and feed. Moreover, it is
better for the polyp to take the risks of these evictions than to
vegetate _inside_ the reef, for there in the calmer water he will
not have enough to eat, and will dwindle and die. Thus the tendency
of all reefs must be to grow seawards, and to increase in breadth.
Perhaps their breadth may tell us roughly how old they are. But there
is another possibility to be taken into consideration, which is that
while the polyps are building the sea bottom or island foundation may
be slowly sinking. In that case it is quite likely that the coral
builders might just keep pace with the subsiding foundations of
their home, and build up a great thickness of coral rock during the
countless years of change.
Sir Archibald Geikie has called attention to the swiftness with
which the structure of the coral polyp's skeleton is effaced from
the foundation and a compact mass of rock put in its place. The
sea-water's chemical and dissolving action, and the vast amount of
mud and sand produced by the breakers are chiefly responsible for
this. As the rock is being formed it is always being cemented. On
the portion of a reef laid dry at low water, the coral rock looks in
many places as solid and old as some of the ancient white limestones
and marbles of the land. In pools where a current of water keeps the
grains of coral sand in motion, each grain may be seen to be rounded.
This is because on each particle of coral the dissolved carbonate of
lime in the water is always being deposited (like the sediment in the
bottom of a kettle). A mass of these rounded or egg-like grains all
gathered together in a lump is called _oolite_, from the Greek word
"oon" (Latin "ovum"), an egg. In many limestones, now forming parts
of agricultural land, this _oolitic_ structure is strikingly shown,
and there can be no doubt that in such cases it was produced just as
now coral reefs are being formed before our eyes. In the coral tracts
of the Pacific Ocean there are nearly three hundred coral islands,
besides extensive reefs round volcanic islands. Others occur in the
Indian Ocean. Coral reefs abound in the West Indian seas, where in
many of the islands they have been upraised into dry land--in Cuba
to a height of 1100 feet above the sea-level. The Great Barrier Reef
that fronts the north-eastern coast of Australia is 1250 miles long
and from ten to ninety miles broad.
It will thus be seen that, apart from any other consideration, the
animals of past ages leave permanent records of their existence
merely by the accumulation of their dead bodies. Nevertheless, alike
on land and on sea, the proportions of organic remains thus sealed
and preserved is only a small part of the total population of plants
and animals living at any given time.