These accumulations are gradually carried away by the larger mountain streams, which in hurrying them along cause a vast amount of wear and tear; so that their corners are worn off, and they get further and further reduced in size, becoming mere round pebbles lining the bed of the stream, and finally by the time they reach the large slow-moving rivers of the plains are mainly reduced to tiny specks of mud or grains of sand. So then the rivers and streams not only transport sediment, but they manufacture it as they go along. And thus they may be considered as great grinding-mills, where large pieces of stone go in at one end, and only fine sand and mud come out at the other.
The amount of land débris thus transported depends partly on the carrying power of rivers, which varies with the seasons and the annual rainfall; partly on the size of the area drained by a river; and again, partly on the nature of the rocks of which that area is composed.
A stream, moving along at the rate of about half a mile (880 yards) an hour, which is a slow, rate, can carry along ordinary sandy soil suspended in a cloud-like fashion in the water; when moving at the rate of two thirds of a mile (about 1,173 yards) an hour, it can roll fine gravel along its bed; but when the rate increases to a yard in a second, or a little more than two miles an hour, it can sweep along angular stones as large as an egg. But streams often flow much faster than this, and so do rivers when swollen by heavy rain.
A rapid torrent often flows at the rate of eighteen or twenty miles an hour, and then we may hear the stones rattling against each other as they are irresistibly rolled onward; and during very heavy floods, huge masses of rock as large as a house have been known to be moved.
These are the two principal ways in which streams and rivers act as transporting agents: they carry the finer materials in a suspended state (though partly drifting it along their beds); and they push the coarser materials, such as gravel, bodily along. But there is one other way in which they carry on the important work of transportation, which, being unseen, might easily escape our notice. Every spring is busily employed in bringing up to the surface mineral substances which the water has dissolved out of the underground rocks. This invisible material finds its way, as the springs do, to the rivers, and so finally is brought into that great reservoir, the sea. Rain and river water also dissolve a certain amount of mineral matter from rocks lying on the surface of the earth. Now, the material which is most easily dissolved is carbonate of lime. Hence if you take a small quantity of spring or river water and boil it until the whole is evaporated, you will find that it leaves behind a certain amount of deposit. This, when analysed by the chemist, proves to be chiefly carbonate of lime; but it also contains minute quantities of other minerals, such as common salt, potash, soda, oxide of iron, and silica, or flint. All these and other minerals are found to be present in sea water.
The waters of some of the great rivers of the world have been carefully examined at different times, in order to form some idea of the amount of solid matter which they contain, both dissolved and suspended; and the results are extremely important and interesting, for they enable us to form definite conclusions with regard to their capacity for transport. This subject has been investigated with great skill by eminent men of science. The problem is a very complicated one; but it is easy to see that if we know roughly the number of gallons of water annually discharged into the sea by a big river, and the average amount of solid matter contained in such a gallon of water, we have the means of calculating, by a simple process of multiplication, the amount of solid matter annually brought down to the sea by that river. But we must also add the amount of sand, gravel, and stones pushed along its bed. This may be roughly estimated and allowed for. These are some of the results:
The amount of solid matter discharged every year by that great river, the Mississippi, if piled up on a single square mile of the bed of the sea,—say, in the Gulf of Mexico, where that river discharges itself,—would make a great square-shaped pile 268 feet high. But the Gulf Stream, sweeping through this gulf, carries the materials for many and many a mile away; so that in course of time it gradually sinks and spreads itself as a fine film or layer over part of the great Atlantic Ocean. The mud brought down by the great river Amazon spreads so far into the Atlantic Ocean as to discolour the water even at a distance of three hundred miles. The Ganges and the Brahmapootra, flowing into the Bay of Bengal, discharge every year into that part of the Indian Ocean 6,368,000,000 cubic feet of solid matter. This material would in one year raise a space of fifteen square miles one foot in height. The weight of mud, etc., that these rivers bring down is sixty times that of the Great Pyramid of Egypt, or about six million tons.
Or, to put the matter in another way, if a fleet of more than eighty "Indiamen," each with a cargo of fourteen hundred tons of solid matter, sailed down every hour, night and day, for four months, and discharged their burdens into the waters of the Indian Ocean, they would only do what the mighty Ganges does quietly and easily in the four months of the flood season.
It is probable that even the Thames, a small river compared to those just mentioned, manages to bring down, in one way or another, fourteen million cubic feet of solid matter. These few figures may suffice to give the reader some idea of the enormous amount of rock-forming materials brought down to the seas at the present day.
Of course they are spread out far and wide by the numerous ocean currents, some of which flow for hundreds of miles; and so the bed of the sea can only be very slowly raised by their accumulation. Still the geologist can allow plenty of time, for there is no doubt that the world is immensely old; and if we allow thousands of years, we may easily comprehend that deposits of very considerable thickness may in this way accumulate on the floors of the oceans. Also the coasts of continents and islands suffer continual wear and tear at the hands of sea waves; and thus the supply of sediment is increased.
When the geologist comes to study the great rock-masses—hundreds, and even thousands, of feet in thickness—of which mountain-ranges are composed, he finds all those kinds of rock which we have just been considering,—sandstones, shales (or hardened clays), pebble-beds, and limestones,—and endeavours to picture to himself their gradual growth in the ways we have described. In so doing, he is driven to the conclusion that many thousands of years must have been occupied in their construction.
We must now say a few words about those other aqueous rocks which have an organic origin, of which limestone is the chief. It is indeed a startling conclusion that deposits of great thickness, and ranging for very many miles over the earth's surface, have been slowly built up through the agency of marine animals extracting carbonate of lime from the sea. Yet such is undoubtedly the case. Of this important process of rock-building coral reefs are the most familiar example. The great barrier reef along the northeast coast of Australia is about 1,250 miles long, from ten to ninety miles in width, and rises at its seaward edge from depths which in some places certainly exceed eighteen hundred feet. It may be likened to a great submarine wall. Now, all this solid masonry is the work of humble coral polypes (not "insects"), building up their own internal framework or skeleton by extracting carbonate of lime from sea water. Then the breakers dashing against coral reefs produce, by their grinding action, a great deal of fine "coral-sand" and calcareous mud, which covers the surrounding bed of the sea for many miles.
Now, geologists find that some limestone formations met with in the stratified rocks have certainly been formed in this way; for example, certain parts of the great "mountain limestone." This is proved by the fossil corals it contains, and by tracing the old coral reefs; but it is also largely formed by the remains of other graceful calcareous creatures known as encrinites, or "sea-lilies," with long branching arms that waved in the clear water. Such creatures still exist in some deeper parts of the sea, and look more like plants than animals. In former ages they existed in great abundance, and so played an important part as rock-formers,—for their stems, branches, and all are made of little plates of carbonate of lime, beautifully fitting together like the separate bones, or vertebræ, composing the backbone of a fish; and when the creatures died, these little plates no longer held together, but were scattered on the floor of the sea-bed. Shell-fish abounded too, and their shelly remains accumulated into regular shell-beds in some places. But at times mud and sand would come and cover over all these organic deposits.
But of all rocks that have an organic origin, chalk is the most interesting. Geologists were for a long time puzzled to know how this rock could have been formed; but some soundings made in the Atlantic Ocean previous to the laying of the first Atlantic cable led to a very important discovery, which at once threw a flood of light on the question. Samples of the mud lying on the bed of this ocean at considerable distances from the European and American coasts, and at depths varying from one thousand to three thousand fathoms, were brought up by sounding apparatus.
Little was it thought that the dull grey ooze covering a large part of the Atlantic bed would bring a message from the depths of the sea, and furnish the answer to a great geological problem. Yet such was the case; for under the microscope this mud was seen to be chiefly composed of very minute and very beautiful shells, now known as foraminifera, and much prized by microscopists. These tiny shells are found at or near the surface of the sea; and after the death of the creatures that inhabit them (which are only lumps of protoplasm with no organs of any kind), the shells slowly sink down to the bed of the ocean. Now, these creatures multiply at so inconceivable a rate that a continuous shower of dead shells seems to be taking place, and the result is the slow accumulation over vast areas of the Atlantic and Pacific oceans of a great deposit of calcareous ooze, which if raised above the sea-level would harden into a rock very similar to chalk.
Microphotographs illustrating Rock Formation.
I. Foraminifera. II. Section of Granite. III. Nummulitic Limestone.
But this process only takes place in the deeper parts of our seas, far removed from land, where the supply of land-derived materials fails,—for even the finest mud supplied by rivers probably all settles down before travelling two or three hundred miles from its native shores.
Thus we learn that when one agency fails, Nature makes use of another to take up the important work of rock-building. How the other rocks which we mentioned in our list were formed,—such as granite, basalt, and the metamorphic rocks,—we must explain in a future chapter dealing with volcanoes and their work.
The notion that the ground is naturally steadfast is an error,—an error which arises from the incapacity of our senses to appreciate any but the most palpable, and at the same time most exceptional, of its movements. The idea of terra firma belongs with the ancient belief that the earth was the centre of the universe. It is, indeed, by their mobility that the continents survive the increasing assaults of the ocean waves, and the continuous down-wearing which the rivers and glaciers bring about.—Professor Shaler.
We have found out the quarries which supplied the rocky framework of mountains, and have learned how the work of transporting these vast quantities of stone was accomplished by the agency of ever-flowing glaciers, rivers, and streams.
We must now consider the second stage of the work, and inquire how the mountains were raised up. Referring back to our illustration of the cathedral (see pages 143-147), it will be remembered that this work was included under the head of Elevation. But perhaps some one might ask: "How do you know that the mountains have been elevated or upheaved? Is it not enough to suppose that they owe their height entirely to the fact that they are composed of harder rock, and so have been more successful in resisting the universal decay and destruction?" Now, such an objection contains a good deal of truth, for mountains are formed of hard rocks; but at the same time we know that the agents of denudation are more active among them than on the plains below, so that, in the higher mountain regions at least, the work of demolition may actually proceed faster than it does on low ground.
Mountains are higher than the rest of the world, not merely because they are built of more lasting material, but also because they have been uplifted for thousands of feet above the level of the sea; and the evidence of their upheaval is so plain as to be entirely beyond doubt.
Let us inquire into the nature of this evidence. We have seen that the rocks of which mountains are composed were for the most part formed at the bottom of the sea. When the geologist finds, as he frequently does, buried in mountain rocks the fossil remains of creatures that must have lived in the sea (and often very similar to those living there now), he is compelled to think of the gigantic upheavals that must have taken place before those remains could arrive at their present elevated position.
Numerous examples might be given; but we will only mention three. In the Alps marine fossils have been detected at a height of 10,000 feet above sea-level, in the Himalayas at a height of 16,500 feet, and in the Rocky Mountains at a height of 11,000 feet.
Again we must take it for granted that all the stratified or sedimentary rocks (see pages 148-149) with some trivial exceptions, such as beds of shingle and conglomerates, have been formed in horizontal layers. This is one of the simple axioms of geology to which every one must assent.
Now, if we find in various parts of the continents, and especially among the mountains, such strata sloping or "dipping" in various directions, sometimes only slightly, but sometimes very steeply,—nay, even standing up on end,—the conclusion that they have been upheaved and pushed or squeezed into these various positions by some subsequent process is irresistible. But this is not all; for in every mountain region we find that the rocks have been crumpled, twisted, and folded in a most marvellous manner. Solid sheets of limestone may be seen, as it were, to writhe from the base to the summit of a mountain; yet they present everywhere their truncated ends to the air, and from their incompleteness it is easy to see what a vast amount of material has been worn away, leaving, as it were, mere fragments behind. The whole geological aspect of the Alps (for example) is suggestive of intense commotion; and they remain a marvellous monument of stupendous earth-throes, followed by prolonged and gigantic denudation (see diagrams, chap. ix., p. 307).
There are certain features found in all mountain-chains which must be carefully borne in mind, especially when we are considering the explanations that have been suggested with regard to their upheaval. These may be briefly stated as follows:—
Having arrived at the conclusion that the mountains show evident signs of upheaval, let us proceed to inquire whether any movements, either upward or downward, are taking place now on the earth, or can be proved to have done so within comparatively recent times. On this question there is ample evidence at our disposal.
More than one hundred and thirty years ago, Celsius, the Swedish astronomer, was aware, from the unanimous testimony of the inhabitants of the sea-coasts, that the Gulf of Bothnia was constantly diminishing both in depth and extent. He resorted to measurements in order to prove (as he thought) that the waters of the Baltic were changing their level. This was a mistaken idea; and we now understand that the level of the sea does not change, except under the influence of the daily rise and fall of the tide, which is easily allowed for. However, that was the idea then; and it survived for some time. But if the sea-level were continually sinking, the water, which, owing to the influence of gravitation, must always remain horizontal, would equally retreat all round the Scandinavian peninsula and on all our seashores. But this is not the case. Again, it would be impossible on this theory to explain the curious fact that in some parts of the world the sea is gaining on the land, while in other places it is as surely retreating; for we cannot believe that in one part the sea-level is rising, while in another (not far off in some cases) it is sinking. No body of water could behave in this irregular fashion; and the sea could not possibly be rising and falling at the same time.
Hence we may take it for granted that any change that we may notice in the relative level of land and sea must be due to upward or downward movements in the land.
But to return to Celsius. Old men pointed out to him various points on the coast, over which during their childhood the sea was wont to flow, and besides, showed him the water-lines which the waves had once traced out farther inland. And besides this, the names of places which implied a position on the shore, former harbours or ports now abandoned and situated inland, the remains of boats found far from the sea, and lastly, the written records and popular songs, left no doubt that the sea had retreated; and it seemed both to themselves and to the astronomer that the waters were sinking. In the year 1730 Celsius, after comparing all the evidence he had collected, announced that the Baltic had sunk three feet, four inches, every hundred years. In the course of the following year, in company with Linnæus, the naturalist, he made a mark at the base of a rock in the island of Leoffgrund, not far from Jelfe, and thirteen years afterwards was able to prove, as he thought, that the waters were still subsiding at the same rate, or a little faster. In reality, he had proved, not that the sea was sinking, but that the land was rising.
Similar observations show that nearly the whole of Scandinavia is slowly rising out of the sea. At the northern end of the Gulf of Bothnia the land is emerging at the rate of five feet, three inches, in a century; but by the side of the Aland Isles it only rises three and one quarter feet in the same time. South of this archipelago it rises still more slowly; and farther down, the line of shore does not alter as compared with the level of the sea.
But it is a curious fact that the extreme southern end of this peninsula is subsiding, as proved by the forests that have been submerged. Several streets of some towns there have already disappeared, and the coast has lost on the average a belt of land thirty-two yards in breadth.
The upward movement of the Scandinavian peninsula must have been going on for a long time, if we assume that it was always at the same rate as at present; for we find beds of seashells of living species at heights of six or seven hundred feet above the level of the sea. Great dead branches of a certain pink coral, found in the sea at a depth of over one hundred and fifty to three hundred fathoms, are now seen in water only ten or fifteen fathoms deep. It must have been killed as it was brought up into the upper and warmer layers of water. This is striking testimony.
The pine woods too, which clothe the hills, are continually being upheaved towards the lower limit of snow, and are gradually withering away in the cooler atmosphere; and wide belts of forest are composed of nothing but dead trees, although some of them have stood for centuries.
Geologists have proved that the Baltic Sea formerly communicated by a wide channel with the North Sea, the deepest depressions of which are now occupied by lakes in the southern part of Sweden; for considerable heaps of oyster-shells are now found in several places on the heights commanding these great lakes. Then we have in Denmark the celebrated "kitchen-middens," heaps of rubbish also largely composed of oyster-shells which the inhabitants, in the "Stone Age," collected from the bottoms of the neighbouring bays. At the present day the waters of the Baltic, into which rivers bring large quantities of fresh water, do not contain enough salt for oysters to grow there; but the oyster-shells prove that the Baltic Sea and these inland lakes were once as salt as the North Sea is now. This can only be explained by supposing that the Baltic was not so shut in then as it is in these days. The bed of the old wide channel has risen, and what once was sea is now land.
Again, it is very probable that the great lakes and innumerable sheets of water which fill all the granite basins of Finland have taken the place of an arm of the sea which once united the waters of the Baltic to those of the great Polar Ocean. And so there must have been upheaval here as well.
The old sea-beaches, now above the level of the highest tides, that are found in many parts of the Scandinavian, Scottish, and other coasts, furnish plain evidence of upheaval.
At the present day, between the lines of high tide and low tide, the sea is constantly engaged in producing sand and shingle, spreading them out upon the beach, mingling them with the remains of shells and other marine animals, and sometimes piling them up, sometimes sweeping them away. In this way a beach often resembles a terrace. When the land is upheaved rapidly enough to carry up this line of beach-deposits before they are washed away by the waves, they form a flat terrace, or what is known as a "raised beach." The old high-water mark is then inland; its sea-worn caves become in time coated with ferns and mosses; the old beach forms an admirable platform on which meadows, fields, villages, and towns spring up; and the sea goes on forming a new beach below and beyond the margin of the old one.
The Scottish coast-line, on both sides, is fringed with raised beaches, sometimes four or five occurring above each other, at heights of from twenty-five to seventy-five feet above the present high-water mark. Each of these lines of terrace marks a former lower level at which the land stood with regard to the sea; and the spaces between them represent the amount of each successive rise of the land. Each terrace was formed during a pause, or interval, in the upward movement, during which the waves had time to make a terrace, whereas, while the land kept on rising, they had no time to do so. Thus we learn that the upheaval of the country was interrupted by considerable pauses.
Sometimes old ports and harbours furnish evidence of upheaval. Thus, the former Roman port of Alaterva (Cramond) in Scotland, the quays of which are still visible, is now situated at some distance from the sea, and the ground on which it stands has risen at least twenty-four feet. In other places the scattered débris shows that the coast has risen twenty-six feet. And by a remarkable coincidence, the ancient wall of Antoninus, which in the time of the Romans stretched from sea to sea, and served as a barrier against the Picts, comes to an end at a point twenty-six feet above the level of high tides. In the estuary of the Clyde there are deposits of mud, containing rude canoes and other relics of human workmanship, several feet above the present high-water mark.
Raised beaches are found on many parts of the coast of Great Britain. Excellent examples occur on the coasts of Devon and Cornwall. On the sides of the mountainous fiords of Norway similar terraces are found up to more than six hundred feet above the sea; and as some of these rise to a greater height at a distance of fifty miles inland, it seems that there was a greater upward movement towards the interior of Norway than on the coasts.
There is a celebrated raised beach on the side of a mountain in North Wales, known as Moel Tryfaen, where the writer gathered a number of marine shells at a height of 1,357 feet.
But Scandinavia and Great Britain are not the only parts of Europe where an upward movement has taken place, for the islands of Nova Zembla and Spitzbergen show evidence of the same kind; and the coast of Siberia, for six hundred miles to the east of the river Lena, has also been upraised. On the banks of the Dwina and the Vega, 250 miles to the south of the White Sea, Murchison found beds of sand and mud with shells similar to those which inhabit the neighbouring seas, so well preserved that they had not lost their colours.
Again, the ground of the Siberian toundras is to a large extent covered with a thin coating of sand and fine clay, exactly similar to that which is now deposited on the shores of the Frozen Ocean. In this clay, the remains of the mammoth, or woolly elephant, now extinct, are preserved in great numbers.
Parts of Northern Greenland have also risen; while at the southern end of this frozen land a downward movement is still taking place.
The best-known example of these slow movements within historic times is the so-called Temple of Serapis in the Bay of Baie, near Naples. The ruins of this building, which was probably a Roman bath, consist of a square floor paved with marble, showing that it possessed a magnificent central court. This court, when perfect, was covered with a roof supported by forty-six fine columns, some of marble, others of granite. There is still a hot spring behind, from which water was conducted through a marble channel. All the columns but three were nearly buried in the soil which covered the whole court, when the ruins were first discovered. Now, each of the three marble columns that are still standing shows clear evidence of having been depressed below the sea-level, for they all exhibit a circular row of little holes bored by a certain marine shell-fish, known as Lithodomus dactylus, at a height of twelve feet from the floor; each row is about eight feet broad. The shells may still be seen inside the little pear-shaped holes which the shell-fish bored for themselves; and the same shell-fish still live in the waters of the Mediterranean and bore holes in the limestone rocks.
It is therefore quite clear that these columns must have been under water to a depth of twenty feet or so, and also that they must have remained under water for some considerable time, during which the shell-fish made these borings. Then an upheaval took place whereby the whole building was elevated to its present level. But underneath the present floor, at a depth of five feet, were discovered the remains of an older floor. This probably belonged to an earlier building which had in like manner been depressed below sea-level. We thus learn that the land in this spot had been sinking for a long time, and that at some subsequent time it rose. The fallen columns suggest the idea that they were thrown down by earthquakes. At the present time the land here is again sinking at the rate of one inch in three or four years.
But the first example of upheaval within comparatively recent times, and one which is instructive as throwing some light on the subject of the present chapter,—namely, the upheaval of mountain-chains,—is to be found along the western mountainous coast of South America. Here we have the magnificent ranges of the Andes running along the whole length of this continent. The illustrious Charles Darwin, during his famous trip in the "Beagle," discovered numerous raised beaches along this coast, and at once perceived their importance to the geologist. The terraces are not quite horizontal, but rise towards the south. On the frontier of Bolivia, they are seen at heights of from sixty-five to eighty feet above sea-level; but nearer the higher mass of the Chilian Andes they are found at one thousand feet, and near Valparaiso, in Chili, at thirteen hundred feet above the sea. Darwin also discovered that some of the upheavals thus indicated took place during the human period; for he found in one of the terraces opposite Callao, in Peru, at a height of eighty feet, shells with bones of birds, ears of wheat, plaited reeds, and cotton thread, showing that men had lived on the terrace. These relics of human industry are exactly similar to those that are found in the huacas, or burial-places, of the ancient Peruvians. There can be no doubt that the island of San Lorenzo, and probably the whole of the coast in its neighbourhood, have risen eighty feet or more since the Red Man inhabited the country.
Callao probably forms the northern limit of the long strip of coast that has been upheaved, and the island of Chiloe the southern limit; but even thus the region of elevation has a length from north to south of about 2,480 miles.
We noticed in the case of Scandinavia that the upward movement is greater in the interior of the mountain-range than at or near the coast; and it is interesting to find that the same difference has been observed in the case of the Andes. The upheaving force, whatever its nature, acts with more energy under the Chilian Andes than under the rocks of the adjacent coast.
In New Zealand we have also evidences of upheaval; and if we trace out on the map a long line from the Friendly Isles and Fiji, through the Eastern Archipelago, and then on through the Philippine Islands, and finally to Japan and the Kurile Islands, we shall find scattered regions of elevation all along this great line, which is probably a mountain-chain, partly submerged, and along which numerous active volcanoes are situated.
Putting together all the evidence that has been gathered on this subject, of which only a very small part is here given, we are warranted in concluding that taking the world generally, regions where active volcanoes exist are generally regions where upheaval is taking place. There is also a very interesting connection between mountain-chains and lines of volcanic action. From this it seems to follow, if lines of volcanic action are also lines of upheaval, that mountain-chains are undergoing upheaval at the present time. This is a conclusion in favour of which a good deal may be said. It is certainly true in the cases of the Scandinavian range, and also of a very large part of the Andes, to which we have already referred. The Highlands of Scotland and Scandinavia form the northern end of an old line of volcanic action running down the Atlantic Ocean through the Azores, Madeira, Cape Verde Islands, Ascension, St. Helena, right down to Tristan d'Acunha.
In many other parts of the world we have evidences from submerged forests, the positions of certain landmarks with regard to the sea, and in some cases submerged towns, that movements of a downward nature are taking place.
It is important to distinguish from these evidences the changes that take place where the waves of the sea are rapidly washing away the coast-line. Putting aside these cases, however, it has been clearly proved that in many regions a slow sinking of the land is going on.
The eastern side of South America has not been so thoroughly observed as its western side; but there is still good reason to believe that a large part of this coast is sinking. So it appears that a see-saw movement is affecting South America, and that while one side is going up, the other is going down; and it is interesting to observe other examples of the same thing,—such as are afforded by Greenland and Norway.
THE SKAEGGDALFORS, NORWAY.
From a Photograph by J. Valentine.
Again, while part of Labrador is rising, parts of the eastern coast of North America, as far down as Florida, are slowly sinking. Thus along the New England coast between New York and Maine, and again along the Gulf of St. Lawrence, we find numerous submerged forests with quantities of trees standing upright with their roots in old forest-beds, but with the tops of their stumps some feet below the level of high tide. In the case of New Jersey the subsidence is probably taking place at the rate of two feet in a hundred years.
Before passing on to consider upward movements of a more rapid nature, such as are frequently caused by earthquakes, we may pause for a few moments to consider certain very slight, but nevertheless very interesting little movements, such as slight pulsations and tremors, which have been observed to take place in the earth's crust (as it is called), and which of late years have been carefully studied.
Professor Milne, a great authority on earthquakes, has noticed slight swayings of the earth, which though occupying a short time—from a few seconds to a few hours—are still too slow to produce a shock of any kind. These he calls "earth pulsations." They have been observed by means of delicate spirit-levels, the bubbles of which move with very slight changes of level at either end of the instrument. At present only a few experiments of this kind have been made; but they tell us that the surface of the earth (which is apparently so firm and immovable) is subject to slight but frequent oscillations. Some think that they depend upon changes in the weight of the atmosphere. If this is so, the balance between the forces at work below the earth's surface and those that operate on its surface must be very easily disturbed. Still we cannot see that this is a serious objection; on the contrary, there is much reason to think that any slight extra weight on the surface, such as might be caused by an increase of the pressure of the atmosphere, and still more by the accumulation of vast sedimentary deposits on the floor of the ocean, may be quite sufficient to cause a movement to take place. Moreover, Mr. G. H. Darwin has shown that the earth's crust daily heaves up and down under the attraction of the moon in the same kind of way that the ocean does; so that we must give up all idea of the solid earth being fixed and immovable, and must look upon it as a flexible body, like a ball of india-rubber (see chap. ix., pp. 314-315).
Slight movements of rather a different kind have been noticed, to which the name of "earth-tremors" has been given. These are very slight jarrings or quiverings of the earth, too slight to be observed by our unaided senses, but rendered visible by means of very delicate pendulums and other contrivances. Now wherever such observations have been made it has been discovered that the earth is constantly quivering as if it were a lump of jelly. In Italy, where this subject has been very carefully studied, the tremors that are continually going on are found to vary considerably in strength; for instance, when the weather is very disturbed and unsettled, the movements of the pendulum are often much greater. Again, before an earthquake the instrument shows that the tremors are more frequent and violent.
Another way of observing these curious little movements is by burying microphones in the ground. The microphone is a little instrument invented of late years which is capable of enormously magnifying the very slightest sounds, such as our ears will not detect. By its means one can hear, as some one said, "the tramp of a fly's foot," if he will be so obliging as to walk over it. It has thus been proved in Italy that the earth sends forth a confused medley of sounds caused by little crackings and snappings in the rocks below our feet.
In this way it will be possible to predict a serious earthquake, because it will give warning some days before, by the increase of the little tremors and sounds; and it is to be hoped that by this simple means human lives may be saved.
Now, these disturbances are of precisely the same nature as earthquakes,—in fact, we may call them microscopic earthquakes. To the geologist they are of great interest, as they seem to afford some little insight into the difficult question of the upheaval of mountains, and to show us something of the constant working of those wonderful forces below the surface of the earth by means of which continents are raised up out of the sea, and mountain-chains are elevated thousands of feet. It is probable that both are due to the working of the same forces, and are accomplished by the same machinery.
We now pass on to consider those more violent movements of the solid land known as earthquakes. This kind of disturbance is such as might be produced by a sudden shock or blow given below the ground, from which waves travel in all directions. First comes a rumbling noise like the roar of distant artillery; then come the earthquake waves one after another, causing the ground to rise and fall as a ship does on the waves of the sea; the ground is frequently rent asunder, so that chasms are formed, into which in some cases men and animals have been hurled alive. In the case of a very violent earthquake the waves travel long distances. Thus the great earthquake by which Lisbon was destroyed in the year 1755 disturbed the waters of Loch Lomond in Scotland. In this fearful catastrophe sixty thousand human beings perished. If the disturbance takes place near the sea, great sea waves are formed, which cause fearful destruction to life and property. This happened in the case of the Lisbon earthquake; and in the year 1868, when Ecuador and Peru were visited by a fearful earthquake, a great sea wave swept over the port of Arica, and in a few minutes every vessel in the harbour was either driven ashore or wrecked, and a man-of-war was swept inland for a quarter of a mile.
Earthquakes bring about many changes on the surface of the earth. For example, on mountain-slopes forests are shattered, and large masses of soil and débris are shaken loose from the rock on which they rested, and hurled into the valleys; streams are thus choked up, and sometimes lakes formed, either by the damming up of a river or by the subsidence of the ground.
It is frequently found after an earthquake that the level of the ground has been permanently altered; and this effect of earthquakes is important in connection with the subject we are now considering,—namely, how mountains are upheaved. Sometimes, it is true, the movement is a downward one; but more generally it takes place in an upward direction. As an example of this, we may mention the Chilian earthquake of 1835, which was very violent, and destroyed several towns on that coast, from Copiapo to Chile. It was afterwards found that the land in the Bay of Conception had been raised four or five feet. At the island of Santa Maria, to the southwest of this bay, the land was raised eight feet, and in one part ten feet; for beds of dead mussels were seen at that height above high water, and a considerable rocky flat that formerly was covered by the sea now became dry land. It was also proved by means of soundings that the sea round the island was shallower by about nine feet.
Now the question arises, "How are earthquakes caused?" Various suggestions have been made; but it is pretty clear that all earthquakes are not produced in the same way. For instance, volcanic eruptions are frequently attended by earthquakes. Violent shocks of this nature generally precede and accompany a great eruption, as is frequently the case before an eruption of Mount Vesuvius.
Steam plays a very important part in all volcanic eruptions; and these earthquakes are probably caused by great quantities of pent-up steam at a high pressure struggling to escape. It is also possible that when molten rock is forcibly injected into the crevices and joints of overlying rocks earthquake shocks may be produced by the concussion. The old Roman poet and philosopher, Lucretius, endeavoured to solve this problem, and concluded that "the shakings of the surface of the globe are occasioned by the falling in of enormous caverns which time has succeeded in destroying." But though the explanation might possibly apply to a few cases of small earthquakes, it is not a satisfactory one, for it is not at all likely that many large cavities exist below the earth's surface, because the great weight of the overlying rock would inevitably crush them in.
We have already pointed out that earthquakes frequently happen in mountainous regions; and this fact alone suggests that perhaps the same causes which upheave mountains may have something to do with earthquakes. But there are other reasons for believing that the same force which causes earthquakes also upheaves mountain-chains. The reader will remember the case of the Chilian earthquake that raised part of the Andes a few feet in height.
Now, it is quite clear that the rocks of which mountains are composed have suffered a great deal of disturbance. We have only to look at the crumbled and contorted strata to see that they have been forced into all kinds of positions, sometimes standing bolt upright (see diagrams, chap. ix., p. 307). And as we cannot believe, for many reasons, that these movements were of a very sudden or violent kind, we must consider that they took place slowly on the whole; but besides being folded and twisted, the rocks of mountains frequently exhibit clear signs of having been split and cracked. The fractures are of all sizes, from an inch or more up to hundreds or even thousands of feet. They tell us plainly that the rocks were once slowly bent, and that after a certain amount of bending had taken place, the strain put upon them became greater than they could bear, and consequently they snapped and split along certain lines. This is just what might be expected. For instance, ice on a pond will bend a good deal, but only up to a certain amount; after that, it cracks in long lines with a remarkably sharp and smooth fracture. But suppose the pressure came from below instead of from above, as when a number of people are skating on a pond. Should we not see the ice forced up in some places, so that some sheets stood up above the others after sliding past their broken edges? This is just what the rocks in different places have frequently done. After a fracture has taken place the rock on one side has slid up over the other, and the two surfaces made by the fracture—like two long walls—are no longer seen at the same level. One has been pushed up, while the other has gone down (see diagram of the ranges of the Great Basin, chap. viii., p. 273).
Now, it is almost impossible to conceive of these tremendous fractures taking place in the rocks below our feet without causing sudden jars or shocks. Here, then, we seem to have a clue to the problem. Even if the movements took place only a few inches or a few feet at a time, that does not spoil our theory, but rather favours it; for in that case the upheaval of a mountain-chain will have taken a very long time (which is almost certain), and may have been accomplished bit by bit. Hundreds and thousands of earthquake shocks, some slight, and others severe, may have attended the upheaval of a mountain-range.
This explanation is accepted by many authorities. It does not exactly imply that mountains were upheaved by earthquakes; but it means that the same forces that elevate continents, heaving them up out of the sea into ridges and very low arches, have been at work to crumple and fold their rocks in some places into stupendous folds, such as we now find form part of the general structure of mountains; and that in so doing they caused fearful strains, too great for the rocks to bear, so that they split over and over again, and in so doing produced jars and shocks that must have been very similar to, if not identical with, earthquake shocks as we know them at the present day.
Such an explanation is in striking harmony with what we have already learned about the operations of Nature. It was from the long-continued operation of rain and rivers that the materials now forming mountains were transported to the seas in which they were slowly formed. It was also by the ordinary operations of frost, heat and cold, snow and ice, streams, rain, and rivers that the mountains received their present shapes (see chapters v. and vii.). And now we learn that the gigantic work of upheaval took place in a tolerably quiet and uniform manner,—with perhaps only an occasional catastrophe of a more violent kind, but still according to the same law of uniformity which is the very basis of modern geology, and by means of which so much can be explained.
We could give other proofs of the gradual elevation of mountains if they were wanted. But at least enough has been said to give the reader a glimpse into the methods employed by geologists in endeavouring to explain how mountains were upheaved; and to show that it is only by a careful study of all that is taking place now on the earth that we can ever hope to solve the difficult questions that present themselves to all who study those stony records on which the earth has written for our enlightenment the chapters of her ancient history.
In conclusion, it may be asked what is the nature of the force that accomplishes all this titanic work of upheaval. Although the question has been much discussed, and some very ingenious suggestions brought forward, we cannot say that any of them are entirely satisfactory. But we know that the earth is a cooling body which loses so much heat every year; and it may be that the shrinking that takes place as it cools, by leaving the crust of the earth in some places unsupported, causes it to settle down, to adapt itself to a smaller surface below, and in so doing it would inevitably throw itself into a series of folds, or wrinkles, like those on the skin of a dried apple. Many think that mountain-ranges may be explained in this way.