In fact, the sand-bed may become a kind of sponge, quite saturated with the water which has filtered down from the surface.
A bed of clay, on the other hand, is impervious; it is made up of very small particles fitting closely to each other, and therefore offering resistance to the passage of water. Wherever such a bed occurs, it hinders the free passage of the water, which, unable to sink through it from above on the way down, or from below on the way up to the surface again, is kept in by the clay, and forced to find another line of escape.
Sandy soils are dry because the rain at once sinks through them; clay soils are wet because they retain the water, and prevent it from freely descending into the earth. Now the rocks beneath us, besides being in many cases porous in their texture, such as sandstone, are all more or less traversed with cracks—sometimes mere lines, like those of a cracked window-pane, but sometimes wide and open clefts and tunnels.
These numerous channels serve as passages for the underground water. Hence, although a rock may be so hard and close-grained that water does not soak through it at all, yet if that rock is plentifully supplied with these cracks, it may allow a large quantity of water to pass through. In hilly districts, where the surface of the ground has not been brought under the plow, you will notice that many places are marshy and wet, even when the weather has long been dry.
The soil everywhere around has perhaps been baked quite hard by the sun; but these places remain still wet, in spite of the heat. Whence do they get their water?
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Plainly not directly from the air, for in that case the rest of the ground would also be damp. They get it not from above, but from below. It is oozing out of the ground; and it is this constant outcome of water from below which keeps the ground wet and marshy. In other places you will observe that the water does not merely soak through the ground, but gives rise to a little run of clear water. If you follow such a run up to its source, you will see that it comes gushing out of the ground as a spring.
Springs are the natural outlets for the underground water. But, you ask, why should this water have any outlets, and what makes it rise to the surface?
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Let us suppose that a flat layer of some impervious rock, like clay, underlies another layer of a porous material, like sand. The rain which falls on the surface of the ground, and sinks through the upper bed, will be arrested by the lower one, and made either to gather there, or find its escape along the surface of that lower bed.
If a hollow or valley should have its bottom below the level of the line along which the water flows, springs will gush out along the sides of the valley. The line of escape may be either the junction between two different kinds of rock, or some of the numerous joints already referred to. Whatever it be, the water can not help flowing onward and downward, as long as there is any passage along which it can find its way; and the rocks underneath are so full of cracks, that it has no difficulty in doing so.
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But it must happen that a great deal of the underground water descends far below the level of the valleys, and even below the level of the sea. And yet, though it should descend for several miles, it comes at last to the surface again. To realize clearly how this takes place, let us follow a particular drop of water from the time when it sinks into the earth as rain, to the time when, after a long journey up and down in the bowels of the earth, it once more reaches the surface. It soaks through the soil together with other drops, and joins some feeble trickle, or some more ample flow of water, which works its way through crevices and tunnels of the rocks.
It sinks in this way to perhaps a depth of several thousand feet, until it reaches some rock through which it can not readily make further way. Unable to work its way downward, the pent-up water must try to find escape in some other direction. By the pressure from above it is driven through other cracks and passages, winding up and down until at last it comes to the surface again. It breaks out there as a gushing spring. Rain is water nearly in a state of purity.
After journeying up and down underground it comes out again in springs, always more or less mingled with other materials, which it gets from the rocks through which it travels. They are not visible to the eye, for they are held in what is called chemical solution. When you put a few grains of salt or sugar upon a plate, and pour water over them, they are dissolved in the water and disappear.
They enter into union with the water. You can not see them, but you can still recognize their presence by the taste which they give to the water which holds them in solution. So water, sinking from the soil downward, dissolves a little of the substance of the subterranean rocks, and carries this dissolved material up to the surface of the ground. One of the important ingredients in the air is carbonic acid gas, and this substance is both abstracted from and supplied to the air by plants and animals. In descending through the atmosphere rain absorbs a little air.
As ingredients of the air, a little carbonic acid gas, particles of dust and soot, noxious vapors, minute organisms, and other substances floating in the air, are caught up by the descending rain, which in this way washes the air, and tends to keep it much more wholesome than it would otherwise be. But rain not merely picks up impurities from the air, it gets a large addition when it reaches the soil. Armed with the carbonic acid which it gets from the air, and with the larger quantity which it abstracts from the soil, rainwater is prepared to attack rocks, and to eat into them in a way which pure water could not do.
Water containing carbonic acid has a remarkable effect on many rocks, even on some of the very hardest.
It dissolves more or less of their substance, and removes it. When it falls, for instance, on chalk or limestone, it almost entirely dissolves and carries away the rock in solution, though still remaining clear and limpid. In countries where chalk or limestone is an abundant rock, this action of water is sometimes singularly shown in the way in which the surface of the ground is worn into hollows.
In such districts, too, the springs are always hard; that is, they contain much mineral matter in solution, whereas rainwater and springs which contain little impurity are termed soft. When a stone building has stood for a few hundred years, the smoothly-dressed face which its walls received from the mason is usually gone.
Again, in the burying-ground surrounding a venerable church you see the tombstones more and more mouldered the older they are. This crumbling away of hard stone with the lapse of time is a common familiar fact to you. But have you ever wondered why it should be so? What makes the stone decay, and what purpose is served by the process?
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If it seem strange to you to be told that the surface of the earth is crumbling away, you should take every opportunity of verifying the statement. Examine your own district. You will find proofs that, in spite of their apparent steadfastness, even the hardest stones are really crumbling down. In short, wherever rocks are exposed to the air they are liable to decay.
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Now let us see how this change is brought about. First of all we must return for a moment to the action of carbonic acid, which has been already described.
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You remember that rainwater abstracts a little carbonic acid from the air, and that, when it sinks under the earth, it is enabled by means of the acid to eat away some parts of the rocks beneath. The same action takes place with the rain, which rests upon or flows over the surface of the ground. The rainwater dissolves out little by little such portions of the rocks as it can remove. In the case of some rocks, such as limestone, the whole, or almost the whole, of the substance of the rock is carried away in solution.
In  other kinds, the portion dissolved is the cementing material whereby the mass of the rock was bound together; so that when it is taken away, the rock crumbles into mere earth or sand, which is readily washed away by the rain. Hence one of the causes of the mouldering of stone is the action of the carbonic acid taken up by the rain.
In the second place, the oxygen of the portion of air contained in rainwater helps to decompose rocks. When a piece of iron has been exposed for a time to the weather, in a damp climate, it rusts. This rust is a compound substance, formed by the union of oxygen with iron.
What happens to an iron railing or a steel knife, happens also, though not so quickly nor so strongly, to many rocks. They, too, rust by absorbing oxygen. A crust of corroded rock forms on their surface, and, when it is knocked off by the rain, a fresh layer of rock is reached by the ever-present and active oxygen. In the third place, the surface of many parts of the world is made to crumble down by means of frost.
Sometimes during winter, when the cold gets very keen, pipes full of water burst, and jugs filled with water crack from top to bottom. The reason of this lies in the fact that water expands in freezing. Ice requires more space than the water would if it remained fluid. When ice forms within a confined space, it exerts a great pressure on the sides of the vessel, or cavity, which contains it.
If these sides are not strong enough to bear the strain to which they are put, they must yield, and therefore they crack.