Читать книгу The Romance of Modern Geology - Edwin Sharpe Grew - Страница 11

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The same causes that produced the layers of peat or sand, or limestone, or clay, which we find by examination of the earth's surface, are acting to-day. Coal is forming now; and so is limestone; and so is sandstone; so even is granite. But these layers or strata form very slowly, so that since man has kept historical records the thickness of new strata laid down could be measured in inches. Consequently we are only able to see the beginnings of the processes. After the materials were laid down by water or the shifting winds, or by the decay of other materials already in position, they underwent various changes. For example, many layers, instead of consisting of loose materials such as gravel, sand, or mud, are now hard stone. Sometimes this consolidation has been the result of pressure. As bed was piled over bed those at the bottom would be more and more compressed by the increasing weight of those laid down upon them; the water would be squeezed out; the particles would stick closer together. Mud, for example, might thus turn into clay; and clay, pressed harder and harder, might be converted into mudstone or shale. But there is another agency at work. We have all seen mortar hardening and binding bricks together; or cement hardening into concrete. Similarly sedimentary deposits are bound together by cements, of which there are many which exist naturally. For example, silica is a natural cement; and so is carbonate of lime; and so is peroxide of iron. All these will bind other particles together. But how do they arrive at the layers of particles? By the same action which lays down the particles themselves. They are rubbed off the places where they exist by the wind or by water. Perhaps they were laid down among the deposited particles of mud or sand. Perhaps they were brought to them by streams or rivers or lakes, and sank with the water into them. In a red sandstone, for example, the quartz grains of the rock may be often observed to be coated with earthy iron peroxide, which serves to bind them together into a rather hard stone. On the other hand, the process is often being reversed. The weather frequently conspires by frost and wind and rain to remove the binding cement, and thereby to allow the stone to return to its original condition of loose sediment.

One of the Colossal Natural Bridges of Utah

This is an instance in which water has hollowed out the lower strata, leaving a harder upper stratum partially intact.

For millions of years the winds have blown over the surface of the earth, the rain has fallen on it, the sun heated it by day, the frost cracked it. Consider the winds that have circled the earth. All movements of the air are due in the first place to the sun which heats the atmosphere and causes it to expand. The sun's rays passing through the air do not heat it at once, or directly, but heat the land and the sea, which absorb some of the rays and reflect others and so warm the air in contact with them. But, as will readily be understood, the land and the sea do not absorb and reflect the heat rays in the same way or to the same extent; nor do the sun's rays fall equally or constantly on all portions of the earth's surface. So that from various causes one part of the earth is always being warmed in a different way from other parts, and the air above the earth is being warmed in an immeasurable number of different ways. Even if the earth's surface were all water or all land, we should expect therefore that there would be movements of the air due to unequal heating. If, however, the earth's surface were quite even and uniform, we should expect that there would be a certain evenness and uniformity about the movements of the air. These movements would be due partly to the regular heating and regular cooling of the surface, and partly due to the fact that the earth is spinning round taking the air with it—but not taking it quite evenly. The air does not fit tightly on to the earth. It is rather like a loose, baggy envelope with a tendency to slip as the earth moves round. Furthermore, a point situated on the Equator has much farther to travel in twenty-four hours as the earth spins round than a point situated in the Arctic Circle, where a tape measure placed along one of the parallels of latitude (let us say the eighty-sixth parallel, where Nansen turned back in his search for the Pole) would show the earth's girth there to be, not twenty-four thousand miles, but only so many hundreds. This also would make a difference in the way the air would be whirled round the earth; but we could take this point into consideration, and should be able, if, as we have said, our earth were quite uniform, to say always and at all times of the year in what direction the prevailing wind should blow.

Even with all the earth's irregularities we do know a good deal with certainty about the earth's prevailing winds: the trades; the anti-trades; the south-west monsoon, which sets in so regularly in India that year by year its advent hardly varies by more than a day; and, in the descending scale of regularity, the east winds that usually sweep England in March, and the prevailing south-westerly to westerly winds which bend most of the young trees of the country a little to the north-east. Besides these regularly or irregularly defined winds, there are certain paths along the earth's surface where the winds always move like a trout stream with eddies in it. These eddies of the air we call cyclones, and they are continually travelling in one direction. No doubt they arise from the air in one place becoming hotter or moister than in the surrounding regions. As the air grows hotter it becomes lighter and ascends, while the heavier air round it pours in. These eddies always travel eastwards and incline in the northern hemisphere towards the north. They usually originate somewhere on the North American continent, and move across the Atlantic about the pace of a slow railway train, winds whirling round them all the time at a much greater pace. Usually the centres of these eddies bear northward past the north coast of Scotland to the north-west of Norway. Sometimes, however, they take a more southerly course, keeping to the south of the British Isles and passing over Central Europe on to Siberia, where they appear to die away.

Such are the cyclones which are in the main part responsible for British weather; and the winds that accompany them vary a great deal in strength. They depend on the size of the eddy. If the eddy is a very big one (and sometimes the eddies are thousands of miles across) the winds will not be so strong as in the smaller ones. It is, therefore, the smaller ones which cause the violent storms. In the tropical regions whirling eddies of a rather different character occur. To quote Mr. J. H. N. Stephenson: "Instead of being measured by some hundreds or even thousands of miles, they are usually only some hundreds of yards across; and as we found that the smaller the cyclone the more violent the wind, we shall not be surprised that the wind in these is more violent than anything we ever experience in this part of the world. They are called by many different names; in the West Indies they are known as hurricanes, in the south-east of Asia as typhoons, and in North America as tornadoes. These hurricanes or tornadoes travel much faster than the larger cyclones, and the winds blowing into them are so violent that everything—trees, houses, bridges—are swept before them, and so strong is the in-draught of air in the centre that strong walls are sucked in just as a piece of paper is in front of a grate when the fire begins to blaze up; and even heavy metal objects are carried upwards. Fortunately these tornadoes do not travel continuously along the ground but bump along it, so to speak, sometimes passing harmlessly overhead, then striking the earth again and causing more havoc. Where they pass over the surface of the sea the water is sometimes sucked in just in the same way, causing what is known as a waterspout. These may do even more damage than a tornado on land, for the water is sometimes carried bodily on to the land, sweeping everything away in a deluge. This happened many years ago in the delta of the Ganges, when thousands of people perished."

Now let us see how these winds might leave traces in the geological record. When soil is exposed to the sun its surface becomes dust, and the wind carries it off. Even where turf protects the surface, bare places may always be found whence this covering has been removed. Rabbits and moles bring up the earth to the surface; the earthworms sometimes bring as much as ten tons of earth to the surface of a single acre of turf in the course of a year. The earthworms bring up only the finest particles of mould; and these, of course, are the very particles readily converted into dust and borne away by the wind if they are not washed away by rain. In tropical countries the white ant conveys a prodigious amount of fine earth up into the open air, building walls sometimes sixty feet high. Although, therefore, the layer of vegetable soil which covers the land appears to be a permanent protection, it does not really prevent a large amount of material from being removed even from grassy ground. The wind carries this fine dust far and wide over the land, and over the sea as well. After the eruption of the island of Krakatoa in 1883, the dust which was the product of that mighty explosion was carried round the world, and even in England we saw the dust particles furnishing extraordinary colours in sunset skies.

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The Garden of the Gods, Colorado

These peaks exhibit the gradual wearing away of hard rocks by the action of rain and wind.

In dry countries, especially in the large tracts of Central Asia and of Africa, the air is often so thick with a fine yellow dust that the sun's light struggles through it as through a London fog. The dust settles on everything, and after many centuries a deposit, which may be hundreds of feet deep, is thus accumulated on the surface of the land. Some of the ancient cities of the old world, Nineveh and Babylon for example, after being long abandoned by man, have gradually been buried under the fine soil which the wind blew over them. Even in England the Roman town of Silchester, not far from Reading, after falling into decay when its inhabitants left it, has been buried under the accumulations of two thousand years, and its walls and floors now lie underground and have to be carefully unearthed in order to lay them bare. But we need not seek these exceptional cases in order to perceive what the wind is doing with sand and the fine dust of the earth's uppermost layers. At many places round the coast are sand-dunes. On sandy shores, exposed to the winds that blow off the sea, the sand is dried and carried away from the beach, gathering into long mounds or ridges which run parallel to the coast-line. These ridges are often fifty or sixty feet, sometimes even more than 250 feet high, with deep troughs and irregular hollows between them, and they sometimes form a strip several miles broad bordering the sea. These sand-hills creep farther inland, till their progress is stopped by the fields or woods they encounter, or till, by seeds finding a root, vegetation springs up on them and they harden and consolidate under the influence of their own vegetation and move inland no farther. But in many parts of Western Europe and Eastern America the dunes are marching inland at the rate of twenty feet a year. Off the coast of Friesland and North Germany the danger has grown so threatening that scientific attention has been given to the problem; and the German scientific men have employed ingenious devices of planting wind-stakes—something like the wooden breakwaters that are to be found along every seaside beach, but arranged at different angles,—of forcing the sand-dune to heap itself up so as to form an obstruction to further arrivals; or of sowing those plants in the sand that will bind its particles together, in order to preserve the land from further invasion.

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A Curious Rock greatly revered by the Natives

This is the Dance Rock of the Walpi Indians of Arizona. Its curious shape is the result of weathering.

What goes on along the coast finds a parallel in the interior of continents where, as in Arizona, in America, or by the desert of Gobi, in Asia, or in the Karroo of South Africa, or in Central Australia and Africa, there is great dryness of climate and a continual disintegration of the surface rocks. Sometimes the dust or sand remains and gradually consolidates or hardens. More often it is only a temporary visitor. Wind and rain are continually removing it, sometimes in vast quantities, into the sea; and in the course of time the most astounding changes are wrought in the surface and appearance of the land. The softer rocks are worn down; the harder ones are left sticking out. Gradually the surface is carved out into heights and hollows. The harder rocks become the hills and ridges; the softer rocks are worn into valleys and plains. If there were no water left on the earth's surface a great deal of this process would still go on. In some respects it might become more violent, for owing to the absence of moisture the winds of the earth would always be laden with fine particles; and every one who has seen a "sand-blast" at work, or even the modified sand-blast which is sometimes used for cleaning the stonework of some of our cities, will appreciate what a tornado laden with sand grains might do in the way of destroying the surface of any rock on which it was playing. But, as a matter of fact, the action of water in carving the surface of the earth is the most important of all the factors we have at present to consider.

As rain falls from the clouds it absorbs the gases of the air, including oxygen and carbonic acid. Now both these are what we call corroding agents. If water is allowed to fall on a steel knife the knife rusts; but it has been shown by Dr. Gerald Moody, during the last few years, that if there were no acid gas present, the rusting would not take place. Oxygen and carbonic acid will rust other things beside metal; they will rust stone. Moreover, when the rain reaches the earth it absorbs any other acids of the soil which rotting vegetation may afford, and reinforced by these it goes on to attack the stones over which it flows. When it rolls along as a brook or a river it is no doubt attacking in this way the rocks and stones of its channel, though this action is not very strikingly shown. But sometimes the rusting or dissolving action of water is very evident. When it issues from a peat bog, for example, and is consequently highly charged with acid, it will make a very great impression on any limestones it may encounter; for as any schoolboy knows who has ever put a piece of chalk in vinegar, or in any of the stronger acids of the school laboratory, all the limestones are peculiarly susceptible to this form of chemical attack. Peat-water eats into limestone rapidly, while the limestone above the stream escapes, though it is a little (and much more slowly) dissolved by rain. Hence arise some curious features in the scenery of limestone districts. The walls of limestone above the water are not eaten away so fast as their base over which the water flows. Consequently they are undermined and are sometimes cut into tunnels and caverns and caves.

The rivers carry away the dissolved material. The carbonate of lime is taken to the sea; and this substance, of which sea shells, for example, are principally formed, is constantly supplied to the sea by the rivers that transport it from the land. The rivers of Western Europe have been known to convey one part of dissolved mineral matter in every 5000 parts of water, and of this mineral matter one half is carbonate of lime. The Rhine alone bears enough carbonate of lime to the sea every year to make 332,000,000,000 oyster shells of the usual size. The Thames conveys 180,000 tons of sulphate of lime past London every year. It has been computed that more than 8,000,000 tons of dissolved mineral matter are removed from the rocks of England and Wales in one year. That is equivalent to a general lowering of the surface of the country, by chemical solution alone, at the rate of one foot in 13,000 years. That is not much, it may seem; but in a million years, which is not a long period in geological time, half the present towns of England would be sunk under water by this cause alone.