Chapter I. The Rocks and Their Story

THE ROCKS AND THEIR STORY

Walking along the sea shore, with all its varied interest, many must from time to time have had their attention attracted by the shells to be seen, not lying on the sands, or in the pools, but firmly embedded in the solid rock of the cliffs and of the rock ledges which run out on to the shore, and have, it may be, wondered sometimes how they got there. At almost any point of the coast of the Isle of Wight, in bands of limestone and beds of clay, in cliffs of sandstone or of chalk, we shall have no difficulty in finding numerous shells. But it is not only in the rocks of the sea coast that shells are to be found. In quarries for building stone and in the chalk pits of the downs we see shells in the rock, and may often notice them in the stones of walls and buildings. How did they get there? The sea, we say, must once have been here. It must have flowed over the land at some time. Now let us think. We are going to read a wonderful story, written not in books, but in the rocks. And it will be much more valuable if we learn to read it ourselves, than if we are just told what other people have made out. We know a thing much better if we see the answers to questions for ourselves than if we are told the answers, and take some one else's word for it. And if we learn to ask questions of Nature, and get answers to them, it will be useful in all sorts of ways all through life. Now, look at the shells in the rock of cliff and quarry. How are they there? The sea cannot have just flowed over and left them. The rock could not have been hard, as it is now, when they got in. Some of the rocks are sandstone, much like the sand on the sea shore, but they are harder, and their particles are stuck together. Does sand on a sea shore ever become hard like rock, so that shells buried in it are found afterwards in hard rock? Now we are getting the key to a secret. We are learning the way to read the story of the rocks. How? In this way. Look around you. See if anything like this is happening to-day. Then you will be able to read the story of what happened long, long ago, of how this world came to be as it is to-day. We have asked a question about the sandstone. What about the clays and the limestone? As before, what is happening to-day? Is limestone being made anywhere to-day, and are shells being shut up in it? Are shells in the sea being covered up with clay,--with mud,--and more shellfish living on the top of that; and then, are they, too, being covered up? So that in years to come they will be found in layers of clay and stone like those we have been looking at in quarry and sea cliff?

We have asked our questions. Now we must look around, and see if we can find the answers. After it has been raining heavily for two or three days go down to the marshes of the Yar, and stand on one of the bridges over the stream. We have seen it flowing quite clear on some days. Now it is yellow or brown with mud. Where did the mud come from? Go into a ploughed field with a ditch by the side. Down the ditch the rain water is pouring from the field away to the stream. It is thick with mud. Off the ploughed field little trickles of water are running into the ditch. Each brings earth from the field with it. Off all the country round the rain is trickling away, carrying earth into the ditches and on into the stream, and the stream is carrying it down into the sea. Now think. After every shower of rain earth is carried off the land into the sea. And this goes on all the year round, and year after year. If it goes on long enough--? Look a long way ahead, a hundred years,--a thousand,--thousands of years. We shall be talking soon of what takes many thousands of years to do. Why, you say, if it goes on long enough, all the land will be carried into the sea. So it will be. So it must be. You see how the world is changing. You will soon see how it has changed already, what wonderful changes there have been. You will see that things have happened in the world which you never guessed till you began to study Geology.

Now, let us go a bit further. What becomes of all the mud the streams and rivers are carrying down into the sea? Look at a stream coming steeply down from the hills. How it rushes along, rolling pebbles against one another, sweeping everything before it, clearing out its channel, polishing the rocks, and carrying all it rubs off down towards the sea. Now look at a river near its mouth in flat lowland country. It flows now much slower; and so it has not power to bear along all the material it swept down from the hills. And so it drops a great deal; it is always silting up its own channel, and in flood time depositing fresh layers of mud on the flat meadow land,--the alluvial flat,--through which it generally flows in the last part of its course. But a good deal of sediment is carried by the river out to sea. The water of the river, moving slower as it enters the sea, has less and less power to sweep along its burden of sand and mud, and it drops it on the sea bottom,--first the bigger coarser particles like the sand, then the mud; farther out, the finer particles of mud drop to the bottom.

During the exploring cruise of the Challenger, under the direction of Sir Wyville Thomson, in 1872-6, the most extensive exploration of the depths of the sea that has been made up to the present time, it was found that everything in the nature of gravel or sand was laid down within a very few miles, only the finer muddy sediments being carried as far as 20 to 50 miles from the land, the very finest of all, under most favourable conditions, rarely extending beyond 150, and never exceeding 300 miles from land into the deep ocean. So gradually layer after layer of sand and mud cover the sea bed round our coasts; and shells of cockles and periwinkles, of crabs and sea urchins, and other sea creatures that have lived on the bottom of the sea are buried in the growing layers of sand and mud. As layer forms on layer, the lower layers are pressed together, and become more and more solid. And so we have got a good way towards seeing the making of clay and sandstone with shells in them, such as we saw in the sea cliffs and the quarries.

But it is not only rain and rivers that are wearing the land away. All round the coasts the sea is doing the same work. We see the waves beating against the shores, washing out the softer material, hollowing caves into the cliffs, eating away by degrees even the hardest rock, leaving for a while at times isolated rocks like the Needles to mark the former extension of the land. Most people see for themselves the work of the sea, but do not notice so much what the rain and the frost, the streams and the rivers are doing. But these are wearing away the ground over the whole country, while the sea is only eating away at the coast line. So the whole of the land is being worn away, and the sand and mud carried out into the sea, and deposited there, the material of new land beneath the waters.

How do these beds rise up again, so that we find them with their sea shells in the quarry? Well, we look at the sea heaving up and down with the tides, and we think of the land as firm and fixed. And yet the land also is continually heaving up and down--very slowly,--far too slowly for it to be noticed, but none the less surely. The exact causes of this are not yet well understood, because we know but little about the inside of the earth. The deepest mine goes a very little way. We know that parts of the interior are intensely hot. The temperature in a mine becomes hotter, about 1 deg.F. for every 60 ft. we go down on the average. We know that there are great quantities of molten rock in places, which, in a volcanic eruption is poured out in sheets of lava over the land. There are great quantities of water turned into steam by the heat, and in an eruption the steam pours out of the crater of the volcano like the clouds of steam out of the funnel of a locomotive. The people who live about a volcano are living, as it were, on the top of the boiler of a steam engine; and their country is sometimes shaken up and down like the lid of a kettle by the escaping steam. In such a country the land is often changing its level. A few miles from Naples at Pozzuoli, the ancient Puteoli, may be seen columns of what appears to be an ancient market hall, though it goes by the name of the Temple of Serapis. About half way up the columns are holes bored by boring shellfish, such as we may find on the shore here at low tide. We see from this that since the building was constructed in Roman times the land has sunk, and carried the columns into the sea, and shellfish have bored into them. Then the land has risen, and lifted the columns out of the sea again.

But it is not only in the neighbourhood of volcanoes that the land is moving. Not suddenly and violently, but slowly and gradually great tracts of land rise and sink. Sometimes the land may remain for a long time nearly stationary. The Southern coasts of England seem to stand at much the same level as in the time of the Romans 1,500 or 2,000 years ago. On the other hand there is evidence which seems to show that the coast of Norway has for some time been gradually rising.

It was thought at one time that the interior of the earth was liquid like molten lava, and that the land we see was a comparatively thin crust over this like the crust of a pie. But it is now believed for various mathematical reasons, that the main mass of the earth is rigid as steel. Still underneath the surface rocks there must be a quantity of semi-fluid matter, like molten rock, and on this the solid land sways about, as we see the ice on a pond sway with the pressure of the skaters on it. So the solid land, pressed by internal forces, rises and falls like the elastic ice, sometimes sinking and letting the sea flow over, then rising again, and bringing up the land from beneath the sea.

Again, as the heated interior of the earth gradually cools by the radiation of the earth's heat into space, it will tend to shrink away from the cooler rocks of the crust. This then, sinking in upon the shrinking interior, will be thrown into folds, like the skin on a shrivelled apple. Seeing, as we often do, layers of rock thrown into numerous folds, so as to occupy a horizontal space far less than that in which they were originally laid down, we can hardly resist the conclusion that shrinkage of the cooling interior of the earth has been a chief cause of the greatest movements of the surface, and of the lateral pressure we so often find the strata to have undergone.

As we study geology we shall find plenty to show that the land does rise and fall, that where now is land the sea has been, that land once stretched where now is sea, though there is still much which is not well understood about the causes of its movements. We have seen how many of the rocks are made in the sea,--the sandstones and the clays,--but there are two other kinds of rocks, about which we must say a little. The first are the Igneous rocks, which means rocks made by fire. These rocks have solidified, most frequently in crystalline forms, from a molten mass. Lava, which flows hot and fluid, from a volcano, and cooling becomes a sheet of solid rock, is an igneous rock. Some igneous rocks solidify under ground under great pressure, and become crystalline rocks such as granite. We shall not find these rocks in the Isle of Wight. We should find them in Cornwall, Wales, and Scotland; and, if we could go deep enough, we should find some such rock as granite underneath the other rocks all the world over. The other rocks, such as the sandstones and clays, are called Sedimentary rocks, because they are formed of sediment, material carried by the sea and rivers, and dropped to the bottom. They are also called Stratified rocks, because they are formed of Strata, i.e., beds or layers, as we see in cliff and quarry.

But we have seen another kind of rock,--the limestones. In Sandown Bay towards the Culvers, bands of limestone run through the dark clay cliffs, and broken fragments lie on the shore, looking like pieces of paving stone. Examining these we find that they are made up of shells, one band of small oysters, the others of shells of other kinds. You see how they have been made. There has been an oyster bed, and the shells have been pressed together, and somehow stuck together, so that they have formed a layer of rock. They are stuck together in this way. The atmosphere contains a small quantity of carbonic dioxide, and the soil a larger quantity, the result of vegetable decomposition. Rain water absorbs some of it, and carries it into the rocks, as it soaks into the ground. This gas has the property of combining with carbonate of lime,--the material of which shells and limestone are made. The bicarbonate of lime so formed is soluble in water, which is not the case with the simple carbonate. Water containing carbonic dioxide soaking into a limestone rock or a mass of shells dissolves some of the carbonate of lime, and carries it on with it. When it comes to an open space containing air, some of the carbonic dioxide is given off, leaving the insoluble carbonate of lime again. So by degrees the hollows are filled up, and a solid layer of rock is formed. Even while gathering in the sea the shell-fragments may be cemented by the deposit of carbonate of lime from sea-water containing more of the soluble bicarbonate than it can hold.

These limestones are examples of rocks which are said to be of organic origin, that is to say, they are formed by living things. Organic rocks may be formed by animal or vegetable growth. Rocks of vegetable origin are seen in the coals. A peat bog is composed of a mass of vegetable matter, chiefly bog moss, which for centuries has been growing and accumulating on the spot. At the bottom of the bog will frequently be found trunks of oak, or other trees, the remains of a forest of former days. The wood has undergone chemical changes, has lost much of its moisture, and often become very hard, as in bog oak. Beds of coal have been formed by a similar process, on a much vaster scale, and continued much longer. The remains of ancient forests have been buried under sand stones and other rocks, have undergone chemical change, and been compressed into the hard solid mass we call coal. Fossil wood, which has not reached the stage of hard coal, but forms a soft brown substance, is called lignite. This is of frequent occurrence in various strata in the Isle of Wight.

Of organic rocks of animal origin the most remarkable are the chalk, of which we shall speak later, and the coral-reefs, which are found in the warm waters of tropical seas. Sailing over the South Pacific you will see a line of trees--coconut trees chiefly--looking as if they rose up from the sea. Coming nearer you see that they grow on a low island, which rises only a few feet above the water. These islands are often in the form of a ring, and look "like garlands thrown upon the waters." Inside the ring is a lagoon of calm water. Outside the heavy swell of the Southern Ocean thunders on the coral shore. If a sounding line be let down from the outer edge of the reef, it will be found that the wall of coral goes down hundreds of feet like a precipice. On an island in the Southern Sea, Funafuti, a deep boring has been made 1,114 ft. deep. As far as the boring went all was coral. All this mass of coral is formed by living things,--polyps they are called. They are like tiny sea anemones, only they grow attached to one another, forming a compound animal, like a tree with stem and branches, and little sea anemones for flowers. The whole organism has a sort of shell or skeleton, which is the coral. Blocks are broken off by the waves, and ground to a coral mud, which fills up the interstices of the coral; and as more coral grows above, the lower part of the reef becomes, by pressure and cementing, a solid coral limestone. Once upon a time there were coral islands forming in a sea, where now is England. These old coral reefs form beds of limestone in Devon, Derbyshire, and other parts of England. In the Isle of Wight we have no old coral reefs, but we shall easily find fossil corals in the rocks. They helped to make up the rocks, but there were not enough here to make reefs or islands all of coral.

The great branching corals that form the reefs can only live in warm waters. So we see that when corals were forming reefs where now is England the climate must have been warm like the tropics. That is a story we shall often read as we come to hear more about the rocks. We shall find that the climate has often been quite warm as the tropics are now: and we shall also read another wonderful story of a time when the climate was cold like the Arctic regions.