Water freezes not only between the component grains, but in the numerous crevices or joints, as they are called, by which rocks are traversed. You have, perhaps, noticed that on the face of a cliff, or in a quarry, the rock is cut through by lines running more or less in an upright direction, and that by means of these lines the rock is split up by nature, and can be divided by the quarrymen into large four-sided blocks or pillars. These lines, or joints, have been already referred to as passages for water in descending from the surface. You can understand that only a very little water may be admitted at a time into a joint. But by degrees the joint widens a little, and allows more water to enter. Every time the water freezes it tries hard to push asunder the two sides of the joint. After many winters, it is at last able to separate them a little; then more water enters, and more force is exerted in freezing, until at last the block of rock traversed by the joint is completely split up. When this takes place along the face of a cliff, one of the loosened parts may fall and actually roll down to the bottom of the precipice.
In addition to carbonic acid, oxygen, and frost, there are still other influences at work by which the surface of the earth is made to crumble. For example, when, during the day, rocks are highly heated by strong sunshine, and then during night are rapidly cooled by radiation, the alternate expansion and contraction caused by the extremes of temperature loosen the particles of the stone, causing them to crumble away, or even making successive crusts of the stone fall off.
Again, rocks which are at one time well soaked with rain, and at another time are liable to be dried by the sun’s rays and by wind, are apt to crumble away. If then it be true, as it is, that a general wasting of the surface of the land goes on, you may naturally ask why this should be. Out of the crumbled stones all soil is made, and on the formation and renewal of the soil we depend for our daily food.
Take up a handful of soil from any field or garden, and look at it attentively. What is it made of? You see little pieces of crumbling stone, particles of sand and clay, perhaps a few vegetable fibers; and the whole soil has a dark color from the decayed remains of plants and animals diffused through it. Now let us try to learn how these different materials have been brought together.
Every drop of rain which falls upon the land helps to alter the surface. You have followed the chemical action of rain when it dissolves parts of rocks. It is by the constant repetition of the process, drop after drop, and shower after shower, for years together, that the rocks become so wasted and worn. But the rain has also a mechanical action.
Watch what happens when the first pattering drops of a shower begin to fall upon a smooth surface of sand, such as that of a beach. Each drop makes a little dint or impression. It thus forces aside the grains of sand. On sloping ground, where the drops can run together and flow downward, they are able to push or carry the particles of sand or clay along. This is called a mechanical action; while the actual solution of the particles, as you would dissolve sugar or salt, is a chemical action. Each drop of rain may act in either or both of these ways.
Now you will readily see how it is that rain does so much in the destruction of rocks. It not only dissolves out some parts of them, and leaves a crumbling crust on the surface, but it washes away this crust, and thereby exposes a fresh surface to decay. There is in this way a continual pushing along of powdered stone over the earth’s surface. Part of this material accumulates in hollows, and on sloping or level ground; part is swept into the rivers, and carried away into the sea. As the mouldering of the surface of the land is always going on, there is a constant formation of soil. Indeed, if this were not the case, if after a layer of soil had been formed upon the ground, it were to remain there unmoved and unrenewed, the plants would by degrees take out of it all the earthy materials they could, and leave it in a barren or exhausted state. But some of it is being slowly carried away by rain, fresh particles from mouldering rocks are being washed over it by the same agent, while the rock or sub-soil underneath is all the while decaying into soil. The loose stones, too, are continually crumbling down and making new earth. And thus, day by day, the soil is slowly renewed.
Plants, also, help to form and renew the soil. They send their roots among the grains and joints of the stones, and loosen them. Their decaying fibers supply most of the carbonic acid by which these stones are attacked, and furnish also most of the organic matter in the soil. Even the common worms, which you see when you dig up a spadeful of earth, are of great service in mixing the soil and bringing what lies underneath up to the surface.
One part of the rain sinks under the ground, and you have traced its progress there until it comes to the surface again. You have now to trace, in a similar way, the other portion of the rainfall which flows along the surface in brooks and rivers.
You can not readily meet with a better illustration of this subject than that which is furnished by a gently sloping road during a heavy shower of rain. Let us suppose that you know such a road, and that just as the rain is beginning you take up your station at some part where the road has a well-marked descent. At first you notice that each of the large heavy drops of rain makes in the dust, or sand, one of the little dints or rain-prints already described. As the shower gets heavier these rain-prints are effaced, and the road soon streams with water. Now mark in what manner the water moves.