Читать книгу The Romance of Modern Geology - Edwin Sharpe Grew - Страница 13

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When we come to examine more closely the work which rivers do in removing mineral substances from the land by washing particles of them from the surface, we find that the records they leave in geological history must be plainly marked. Every stream, large or small, is always busy carrying mud, sand, or gravel. Rivers are the "navvies" of geology. When they are swollen by rain they sweep large stones away with them. If we look at the bed of a mountain torrent we shall often see huge blocks of stone that have fallen from the cliffs on either side blocking the pathway of the stream. To all appearance the stream is quite powerless to remove these blocks, and has to circumnavigate them. But visit such a torrent when the snows are melting, or heavy rain has fallen, and you will hear the stones knocking against each other or on the rocky bottom as they are driven downwards by the flood. It is not easy to estimate the driving power of water. M. Gustave le Bon has furnished an illustration of its power which is very curious. In the south of France a stream is led downwards from the mountains to drive the turbine of some machinery at a manufactory. It comes down several thousand feet. In the manufactory there is a vent-hole, out of which the water can be allowed to shoot. The vent-hole is about an inch in diameter; and the water rushes out with such swiftness and force that the water-jet becomes as rigid as steel. It is impossible to cut through this water-jet; and if any one were to try to do so with a sword, the sword might break but it could never pierce or pass through those swiftly moving particles of water. A more commonplace illustration is the use that is sometimes made of water-jets to break up the surfaces of rock in quarries; nor must it be forgotten that horse-power of great value and extent for electric lighting and other purposes is always being drawn from waterfalls. Thus as a mechanical force merely the river can be immensely powerful; and must leave marks of its power on the rocks.

The aspect of its force with which we are, however, most concerned is that which is directed to lowering gradually the surface of the land. In the last chapter we showed how much mineral might be dissolved in the waters of rivers. If we are to include also the amount of mud, sand, and other things classed altogether as silt which a river carries down, the figures become much more imposing. Sir Archibald Geikie says that, taking the Mississippi as a typical river (it is as good an example as would be found, because in its great length it passes through many different kinds of land, soil, and climate), we may assume that the average amount of sediment carried down by a river is one part of sediment to every 1500 parts of water.

If now, says he, we assume that all over the world this is the amount carried down, we can see how seriously the level of the land is lowered by rivers. The Mississippi carries from the land it drains every year the ¹/₆₀₀₀th part of a foot of rock. If we take the general height of the land of the whole globe to be 2100 feet, and suppose it to be continuously wasted at this rate, then the whole dry land would be carried into the sea in 12,600,000 years. Or if we assume the average height of the continent of Europe to be 940 feet, and to be lowered by its rivers at the same rate, then the last vestige of Europe would have disappeared in 8,640,000 years. Such figures are of course not exact; and it must always be remembered that the rivers are merely robbing Peter to pay Paul, and whatever they take away are always putting somewhere else, but we may learn from the foregoing considerations that the lowering of the land is much more rapid than is sometimes supposed. Another thing about the excavating work of rivers has to be remembered. The torrents carry sand, shingle, and rock with them, and these very materials act as agents of destruction on the beds of the water-courses. If we want to polish brass or steel we mix emery powder (or something finer or coarser) with the polishing liquid. The torrent or river uses sand or shingle as its polishing powder. It then wears out the rock over which it travels, and sometimes carves it into holes or caverns, gorges or ravines. Sometimes the process is varied, as when a stream finds its way over a hard rock which overlies a softer rock. If the arrangement is like that of a series of steps (there may be only one or two steps) it is possible for the river as it foams in a waterfall over the hard step at the top to eat its way into the lower softer step. The lower softer step will gradually disappear, and then the waterfall, still eating its way in, will begin to undermine the hard top step, and when that has gone on long enough the hard top step will fall down and the waterfall will have to begin a little farther up the stream. In this way a waterfall, gorge, or ravine can be constructed by a river.

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The Grand Cañon of Arizona

The Colorado River at this point is nearly 200 feet wide. The man is seated about 1200 feet above the river's level. This whole cañon up to the top of the mountains in the distance has been worn away by prehistoric current; and the river has gradually cut its bed deeper.

The Falls of Niagara are an illustration of this method. The river flows from Lake Erie through a level country for a few miles, then begins to go faster as the path becomes steeper, and finally plunges over a hard limestone precipice. Beneath the hard limestone (the top step) are softer beds of shale and sandstone. As the water eats into them and removes them, large portions of the face of the limestone precipice on the top fall into the stream below. Thus gradually the Falls of Niagara are eating their way back to Lake Erie, and have been doing so for hundreds of thousands of years. In the process of doing so the Niagara River has cut out below the Falls a gorge which is not less than seven miles long, from two hundred to four hundred yards wide, and from two hundred to three hundred feet deep. There is no reason to doubt that the Niagara gorge has been entirely cut out in this way, and that at first the river fell over cliffs seven miles farther down its course at Queenstown. The amount of rock thus tunnelled would make a rampart about twelve feet high and six feet thick going round the world at the Equator. Still more gigantic are the gorges or caverns of the Colorado and its tributaries in Western America. The Grand Cañon of the Colorado is three hundred miles long, and in some places more than six thousand feet deep. The country traversed by it is a network of deep ravines, at the bottom of which flow the streams that have dug themselves down from the top of the Colorado tableland.

Now suppose that the river has dug itself in as far as it can go. There must be a limit, and the limit is reached when the slope of the bed has been made so slight that the current can only go on languidly. In that case it cannot sweep along stones, or shingle, or even coarse gravel; and then the river so far from deepening its channel begins to raise it by allowing more of the transported sediment to settle down. If a fast stream meets a slower one deposition of material will take place; and the same thing will occur when the rivers meet a lake or a sea. Whatever checks the swiftness of a current weakens its carrying power and causes it to drop some of its sediment to the bottom. Therefore accumulations of sediment occur at the foot of torrent slopes along the lower and more level ground. These deposits we call alluvium, and sometimes when the mountain torrent ends abruptly in the plain they may stand up in cones of silt. They are sometimes called alluvium cones or fans. Quitting the steep descents, and reinforced by tributaries on either side, the stream ceases to be a torrent and becomes a river. It goes fast enough at first to carry still coarse gravel; but the big angular blocks of rock have been dropped, and the stones it now leaves in its bed are smaller, and become rounded and smoothed as it goes farther and farther across the plain. At many places it deposits gravel or sand, more especially at the inner side of the curves which the stream makes as it winds down the valley. When the stream runs low in summer, strips of bare sand and shingle are seen at each of these bends; and the stones are always well smoothed and lie on the whole regularly. Those that are oblong are so placed that the greater length of the stone points across the stream; those that are flat usually slope upstream. These facts, though apparently insignificant, are really of importance, because they point to us a method by which geology can determine, after a river has disappeared, the slope of the bed and the direction of the curves which once it had. If we examine the steep banks or cliffs by the side of a river the layers of gravel or shingle in the strata may be found to lie not flat on one another but in sloping planes. That at once will furnish a clue to the direction of the river. Another thing of great importance are the terraces which a river forms by the side of itself. When it overflows in floods it deposits mud on either side, and when after the flood it subsides the mud is left. If the reader will imagine the river in the course of ages sinking lower into its bed he will see that successive eras of flood-levels will leave their mark in a series of steps, or river terraces as they are called, on either side of the channel.

But besides the stones and gravel and mud carried down by a river, we must also consider the fate of the remains of plants and animals that are swept along by it, especially in flood-time. In any ordinary flood trees and shrubs, and the smaller animals like mice and moles and rabbits, are drowned by the flood. In greater floods birds and even large animals are drowned, and their remains are buried in the sediment. If they are quite covered over they may perhaps be preserved, and their bones may last for an indefinite period. If, further, the mud deposit hardens, these remains may be preserved so well and so long that they become the fossil records of creatures which lived before man emerged to dwell in the world and to become the arbiter of many of its destinies.

What we have said of rivers is true also of lakes. Rivers pour into lakes, bringing with them, especially in flood-time, enormous freights of gravel, sand, and mud, and mingled with them the remains of vegetation and of animal life. Hundreds of thousands of tons may be swept down by one storm. To the Lake of Lucerne, for example, the River Reuss, which comes down from the St. Gothard, brings seven million cubic feet of sediment every year with it. Since the time of the Romans the Rhone has so filled up part of the Lake of Geneva that the Roman harbour, Port Valais, is now nearly two miles from the edge of the lake. The ground between it and the Lake first became marsh. It is now farm land. And though these accumulations are most marked where the rivers drain into the lake, there are deposits always taking place from the hills all round the lake. Thus lake bottoms become most interesting and valuable receptacles of the life that has for ages lived by or near their shores. These deposits are in many ways peculiar. The snails that live in lake waters are distinct from the land snails of the adjoining shores. Their dead shells gather at the bottom of some lakes in such numbers as to form there a deposit of white crumbling marl, sometimes many yards in thickness. On the sites of lakes that have been gradually filled up, or artificially drained, this marl shows at once where the lake borders were, and, roughly, the period of the lake. In some lakes also are found concretions of iron-oxide, which are formed by the chemical action of the water on some of the rocks by the lake-side; and in several Swedish lakes this ironstone forms so fast that the lakes are regularly dredged for it.

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Cleopatra Terrace, with its Mirror-like Pools, Yellowstone Park, U.S.A.

These beautiful basins are formed of incrustations of volcanic limestone. They are of all colours: pink, orange-yellow, green, and blue. The water in them is of a brilliant blue, caused by the growth of water plants (algæ), which live in water with as high a temperature as 150 F.

Thus among the rocks which form the dry lands of the globe there occur masses of limestone, sand, marl, and other materials which we know were deposited in lakes, because they contain types of plants and animals like those found in the lakes of our own time. From this kind of evidence we can mark out the places of great lakes that have long ago vanished from the face of Europe and North America.

There are also the so-called Salt Lakes to consider. These are generally the lakes that have no outlet and into which a small amount of water now flows, but never enough to cause the lake now to overflow, whatever it may have done in past times. The water that now runs in escapes merely by evaporation. But just as the bottom of a kettle in which hard water is constantly boiled gradually becomes furred, so a lake bottom into which water is continually pouring, bringing dissolved in it all sorts of mineral salts, becomes coated with sediment. The mineral salts are not evaporated, consequently the lakes become gradually more mineral—or, for convenience, let us say, become salter. Among the mineral salts common salt and gypsum are most important; but some bitter lakes contain sodium carbonate or magnesium chloride. The Dead Sea and the Great Salt Lake of Utah show by the deposits round them how they have changed their shape and depth. In the upper terraces of the Great Salt Lake, 1000 feet above the present level of the water, fresh-water shells occur, showing that the basin was at first fresh. The valley bottoms around salt lakes are now crusted with gypsum, salt, or other deposits, and their waters are without sign of life. Such conditions help us to understand how great deposits of salt or gypsum were once laid down in England, Poland, and Germany, and in many other places where now the climate would not permit of the necessary evaporation and condensation of the water.

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A Petrified Tree

This magnificent fossil is in the Petrified Forest of Arizona; and it affords one of the most striking examples known of the solidification and petrifaction of material by the infiltration of mineral salts. The trunk is now not merely encrusted with stone: it is permeated by silica, and is, in fact, itself a stone as hard as flint.