Part 4

Now after studying these various rocks all over our country, we find that there is a certain regular order in which they are found; some have been made a long time before others, and while most kinds contain some fossils, those found in the oldest rocks are much less like the living plants and animals than the fossils we find in the newer rocks.

But you will want to know how it is that we can tell that one rock is older than another, when both appear at the surface of the earth. It would take a long time to make sure of this for ourselves, but it will be enough to say that the various cliffs, quarries, and railway-cuttings often show one kind of rock resting upon another, and these always occur in a certain order. Thus we never find the Chalk resting on the London Clay, but we constantly find the London Clay resting on the Chalk. And this is proved in another way, by deep well-borings. Underneath London many wells have been carried down right through the London Clay, and if only continued deep enough they always reach the Chalk. In the same way, the order of the other rocks has been ascertained in different parts of the country, by examining all the pits and quarries, and cliffs and cuttings, with the help of what knowledge can be obtained from deep mines and wells.

You will now begin to wonder why the older rocks should appear at the surface. I have told you about earthquakes, and you will find that many dreadful earthquakes must in former times have ravaged our country. The reason why the old rocks come to the surface is because they have been lifted up sometimes violently, but more often very slowly. And the newer rocks which formerly rested on them have very often been quite washed away, either by the sea or by rivers and little streams which formerly acted upon them.

Suppose then we take six books, some thick and some thin, and pile them up together on the table, the lowest being a good thick one. The lowest we will call granite, the next slate, the third sandstone, the fourth coal, the fifth chalk, and the sixth the London clay. These will represent some of the principal kinds of earths, and you can fancy many more with other names coming between them; but the London clay can never be below the granite nor the chalk below the coal, for the great coal beds were formed long before the chalk and clay. They generally come in much the same order as we have named them, hard rocks like granite at the bottom, and softer earths, like sandstone, chalk and clay, a long way above them. But we do not always find all these earths in one place even if we dig ever so deeply, though the granite would always be found at the bottom if we went deep enough.

Sometimes the granite and other old rocks have been pushed through the upper layers by some great force, and have broken them and risen above them in magnificent mountain chains, like those of the Andes in South, and the Rocky Mountains in North America, the Wicklow Mountains in Ireland, the Grampians in Scotland, and the Cornish mountains in England. We can easily suppose that the lowest of our books (the granite book) has been pushed upwards by some great force from below, and parts of it broken through the others, and raised high above them; and this is what has actually been done with real rocks. And as this kind of upheaval has taken place at different periods of the earths history, we find that granites have come to the surface at different times.

When the layers are thus broken through they are often tilted up on end and tumbled about in confusion. But where there has been no disturbance like this, they generally rest evenly upon one another in their proper order.

Granite, and rocks of the same kind, are not in the least like chalk, or clay, or even sandstone, and when once you have seen any of these you will not be likely to mistake it for the others. Granite is excessively hard, and has a beautiful appearance when polished, with a number of brilliant white and some dark specks in it. It is used for paving the streets of towns, for which purpose it is cut into oblong blocks, and for the pillars of fine buildings. Sometimes it is dark brown, sometimes reddish, but generally a bluish grey. This rock is composed of a great quantity of crystals, and for this reason it is thought it must have been melted at one time by intense heat in the earth, and afterwards slowly cooled. Chalk is very different, and sandstone, though it is also hard, not in the least like granite.

HOW THE ROCKS WERE FORMED.

What I have just said is about all that we know of the formation of the oldest and hardest granite rocks: but there is something going on now which confirms the belief that the materials of which they are made were melted together by a greater heat than we can make in our furnaces for melting iron; for I should tell you that it is easier to melt iron and copper than granite rocks. Volcanoes often throw out melted earths which when cooled appear to be made of much the same materials as these granites.

SANDSTONE.

But we know more of the manner of the formation of sandstone. This rock is composed of rounded grains of sand just like that we find upon the sea shore. If you take a handful of this sand and squeeze it tightly, it will keep together a little while. Now suppose a quantity of this sand was pressed by a very great weight—the weight of a large hill for instance—after many years the grains would stick firmly together, and become a sort of stone. It is in this way the sandstones must have been formed, and perhaps heat helped the work, though not so great a heat as melted the granite. The sand, after it had been washed upon the sea shore, became gradually covered with other earths hundreds of feet thick, and the immense weight above it pressed it into stone: but you may imagine how very long a time it took to do this. Sandstones are used for building, but they do not last very long; the frost makes little cracks in them and they soon crumble away to the grains of sand of which they were made. Several fossils are found in some of these sandstones, which have been formed at many different periods of the earth’s history.

CHALK.

You have seen those high cliffs of chalk along the south coast of England, perhaps, and you have wondered what that beautiful white earth was, and how it came there. It is found in many parts of the world, and the south and south-east of England are to a great extent composed of it. The material is called by chemists carbonate of lime. It is almost entirely made up of minute shells called foraminifera, from two Latin words which mean that there are many openings or chambers in their shells, and there are many beautiful fossils called ammonites imbedded in the chalk. These are shell-fish, two or three inches, and sometimes a foot across, and their shape is very like that of the young leaves of the common fern before it has opened in the spring.

Millions of these tiny foraminifera are living now in parts of the Atlantic and Pacific oceans, and when they die their shells sink to the bottom and form a greyish mud, something like chalk.

When H.M.S. “Challenger” was sent out in the year 1873, to find out what was at the bottom of the deepest seas of the World, great interest was felt in the expedition, because we were sure that we should learn something about the manner in which some of the rocks were formed.

We knew that the whole of the beds of the present seas must be receiving the washings of the rivers and the bodies of many fishes and animals, and that the rocks of the future must be forming down there by these accumulations. Long lines were let down from the ship with a dredge at the end, and thus parts of the bottom of the sea were brought up and carefully examined. It was found that the washings, stones, clay, and mud of the land were carried hundreds of miles out to sea, and laid upon the bottom. But in the deeper parts, where the Alps would be almost covered—there was a fine grey mud composed almost entirely of the shells of the little foraminifera, and this, no doubt, is the chalk of future times, or perhaps limestone of a harder kind. Deeper, too, than where this grey mud is found, there is a reddish mud, exceedingly fine and soft. We cannot exactly say yet whether this is formed from the remains of shell-fish; but it is, at all events, very like the clay of the land, and in some future time will most likely become like that stiff mud we know so well. So that even the materials for bricks are being made now, and perhaps when all those hundreds of islands scattered about the Pacific Ocean are joined into one great Continent, this red mud will be raised and made use of for building the houses of new peoples and nations.

When we see this going on now, of course it is very easy to conclude that the chalk, a great deal of which is above the sea now, must have been formed in the same way at the bottom of an ancient ocean, and afterwards raised by the same kind of upward force which made the granite break through other earths.

If we did not know that the same cause was at work now, and that the same kinds of shell-fish were living and laying down new beds of chalk under the sea, we should not know how to account for the quantities of chalk in the world. For innumerable ages these little creatures have thus been paving the floor of the ocean with their dead bodies, and you may suppose that countless millions of them must have lived and died! In some of the chalks the shells of the foraminifera can be quite distinctly seen with a microscope, and when these are compared with the shells of living ones, they are seen to be almost exactly alike. Next time you pass through one of the railway cuttings through the chalk in going to Brighton, or Ramsgate, or Dover, remember that those high cliffs were built up by these Liliputian giants under the sea, and you may think of the chalk as “foraminifera earth”.

COAL.

You see this black shining substance almost every day, and you know it is dug up from very deep pits where the poor miners are often killed by explosions of gas escaping from it. But it is as well to know what it is and how it comes to be so useful to us. In the language of chemistry it is called “carbon”, and a great writer has given it the poetical name of “compressed sunlight”. But you will ask how sunlight could possibly get into a deep mine, and how it could be compressed there. You will see that the explanation is really quite simple by-and-by. This coal was once above ground, and was a splendid forest of waving palm-trees, and ferns, and gigantic mosses, as you will see by the pictures of the fossils of them.

Many of the animals and plants of past times were giants compared to those living now, of the same species or kind, and many of the plants of the present time are dwarfs to those of the same kind which formed the coal beds. Many generations of trees must have grown and died, and others must have sprung up, and so on, until beds of them, some ten, others twenty, or even thirty feet thick, were formed. Here, buried in the coal, are the stems, leaves, bark, roots, fruit, and seeds of these trees, and we can have no doubt that almost the whole of the coal is composed of them. You must not expect to find the shapes of these in every piece of coal you may happen to look at, because most of it has been greatly changed by the great weight and pressure upon it, and the length of time: but it is certainly all the same substance—wood turned into coal. The fossil plants of the coal are of course entirely black, but there is no mistake about their having once been living plants.

You will ask perhaps how the coal came to be buried so deep. It is not so always, being sometimes at the surface. But just as the granite has been pushed up through the other rocks, so has the coal in some places been uplifted and in others has sunk down. It was often covered up by other earths to a great depth, after the trees which composed it had died; but where it is now at the surface these newer earths have been afterwards worn away. When the sun shone upon these coal trees they took its warmth and light into their stems and leaves, for they could not live without, and this made them grow so fast and become so large that it is not untrue to call coal “compressed sunlight.” Charcoal is in some respects so like coal that it would seem to you at once that they were probably the same material. Charcoal is simply burnt wood, and when the coal forests had died down, and when these beds sank down beneath other layers the pressure and heat together turned the wood and leaves into a hard mass like charcoal in colour, but heavier and more solid, and just enough of the stems and leaves have been left to enable us to know with certainty that coal was once wood.

We light our fires now and drive our steam-engines with the heat of the sun which shone upon the coal forests, and has been stored up for many thousands of years in the Earth, to be brought out once more to give us light and warmth.

CLAY AND MUD.

While the ancient forests were growing up to form the coal beds, and the foraminifera were slowly building up the chalk, as I have explained, the Earth was covered with water in some places which are now dry land, and the sea now flows over parts of the World which were once the habitations of plants and animals. These great changes have left their marks upon many a mountain side, and many an old river or sea bed has become filled up. A map of Europe during the chalk period would show that the places where Paris, London, Copenhagen, and Berlin now are were then under the ocean; but since then these places have been lifted up, and mud, clay, and gravel swept over the chalk in many places by the action of new rivers and seas. Water, you perceive, has had a great deal to do with these changes, and indeed it is one of Nature’s most powerful tools, for it can wash down rocks and cliffs and cut its way in rivers for thousands of miles over the Earth’s surface. It carries down mud, and clay, and gravel, and this soil, which has been named alluvium, is one of the most interesting of all to us, because it contains the bones of the immense animals we shall talk about presently, as well as those of the oldest races of men with their weapons and ornaments.

The mud age, and we are in the mud and gravel age now, belongs to what is called the Tertiary period, and we shall see that this age has lasted a very long time already, so long that though it is still going on, the most extraordinary animals have lived and died, and not one of them is now left alive. Still the same washing and cutting of water is going on which buried their bones in swamps, and bogs, and river caves, and may perhaps carry away some of the bones of us who are living now, to be found ages afterwards by future generations who will read our history in these silent witnesses, as we read the history of the tree ferns and foraminifera in the coal and the chalk.

The present age of the World’s history is the Mud age, or, as we shall call it in future, the Tertiary period, and I think you will agree with me when I come to describe it, that it contains the most interesting of all the pieces of “the puzzle of life.”

The earth of the Tertiary period is very different from a great many of the older earths. Clay, mud, and gravel are the washings only of the older rocks, the fine particles which have been worn off from them by frost and water and carried down by rivers and left in large beds, and sometimes they have a good deal of decayed wood and weeds mixed with them. Here are found the bones of the great animals which were so much larger and stronger than those of the same kind living now, or any that lived before them.