The folding and warping of the rock layers has brought some of the stratified beds which were originally horizontal into an almost vertical position so that we now find at the surface the worn-off edges of these beds. The different kinds of rocks and the different positions in which the rock layersare presented to the forces which
FOLDED ROCKS.
Stratified rocks which have been folded since they were formed.
are active in wearing them away cause great variety in the forms of the surface features.
It is not necessary to consider all the causes which may have disturbed the position of the rock layers, but the most important of them deserves attention. It has already been found that, although the exterior of the earth is cool, the interior is hot. Now it is known that almost all substances contract when cooled. If the interior of the earth is cooling, and there is every reason to believe that it is, then it must be contracting. As the crust is already cool it has ceased to contract and thus the interior shrinks away from it and it must fold up in order still to rest upon the shrinking interior. The cooling of the earth is so slow that the folding under ordinary conditions disturbs the surface but little.
42.Rock Weathering.
Experiment 42. -Weigh carefully a piece of dry coarse sandstone or coquina. Allow this to remain in water for several days. Wipe dry and weigh again. Why has there been a change in weight?
Experiment 43. -Fill a test tube or small glass dish about half full of limewater, made by putting about 2 ounces of quicklime into a pint of water. Blow from the mouth through a glass tube into the limewater. There is formed in the limewater a white substance which chemists tell us is of the same composition as limestone.
ROCKS WEATHERING AND FORMING DEEP SLOPES.
Experiment 44. -Continue to blow from the mouth for a considerable time through a tube into a dish of limewater. The white substance disappears. A gas in our breath called carbon dioxidedissolves in the water, forming a weak acid and causes the change. Now if we heat the water, thus decomposing the acid and driving out the gas, the white substance again appears. This gas is found everywhere in the air and is given out in the decay and burning of substances.
Rocks which are exposed to the atmosphere, e sp e cially in moist climate s, u nd e rg o decomposition. If the climate is warm and dry, rocks may stand for hundreds of years without apparent change, whereas the same rock in another locality, where the weather conditions are different, will crumble rapidly. A striking example of this is found in the great stone obelisk, called Cleopatra"s Needle, which was brought from Egypt to Central Park, New York, some time ago. Although it had stood for 3000 years in Egypt without losing the distinctness of the carving upon it, yet inCLEOPATRA"S NEEDLE, CENTRAL PARK, NEW YORK.
the moist and changeable climate of New York it was found necessary within a year to cover its surface with a preservative substance.
Not only do different climates affect differently the wearing away of rocks, but different kinds of rocks themselves vary much in the rate at which they crumble. It has been found that while marble inions, in a large town where there is much coal smoke and considerable rain, will become illegible in fifty years, that after a hundred years inions cut in slate are sharp and distinct.
Experiment 45. -Allow a test tube filled with water and tightly corked to freeze. What happens? If the temperature of the air is not cold enough, place the test tube in a mixture of chopped ice and salt, or better, chopped ice and ammonium chloride (sal ammoniac), and allow it to remain for some time.
Water getting into the cracks of rocks and expanding when it freezes splits them apart and aids much in their destruction. Plant roots penetrate into the crevices of rocks and by their growth split off pieces of the rock. Water, especially when it has passed through decaying vegetable matter, has the power of dissolving some rock minerals. Certain minerals of which rocks are composed change when exposed to the air somewhat as iron does when it rusts.
ROCKS SPLIT BY ROOTS OF A TREE.
WIND-CUT ROCKS.
These rocks have been fantastically cut by wind-blown sand.
W h e r e t h e t e m p e r a t u r e varies greatly during the day the expansion and contraction due to the heating and cooling sometimes cause a chipping off of the rock surfaces. In some localities, the winds, by blowing sand particles against the rocks, cut them away quite rapidly. All these agenciesand others tend to break up and decompose the rocks, thus forming soil. The actions of some of these agencies were seen in the previous experiments.
43.Soil.
Experiment 46. -Into a 16 oz. bottle nearly full of water put a small handful of sand, and into another bottle about the same amount of pulverized clay. Shake each bottle thoroughly and allow the water to settle. Which settles the more rapidly? Which would settle first if washed by a stream whose current was gradually checked?
Wherever the inclination is not too steep, we find the surface of the bed rocks covered for varying depths with a loose material which we call soil. It is upon this that plants grow and in it lies the wealth of our agricultural communities. On examining this soil, it will be found that in some places it grows coarser and coarser the farther down we dig. The coarser the pieces become, the more they resemble the bed rock, until finally they pass by imperceptible stages into it. This kind of soil is called local or sedentary soil.
LOCAL SOIL.
This soil is being weathered from the underlying rock.
In other localities the coarseness of the soil does not materially change as we dig into it, but suddenly we come upon the surface of the bed rock, which may contain few if any of the constituents which were in the soil. This soil, which in no way resembles the underlying rock, is called transported soil. We shall find out later how most of it reached its present position.