Outlines of the Earth's History Part 16

We should here note something of the conditions which determine the supply of food which the marine animals obtain. First of all, we may recur to the point that the ocean waters appear to contain something of all the earth materials which do not readily decompose when they are taken into the state of solution. These mineral substances, including the metals, are obtained in part from the lands, through the action of the rain water and the waves, but perhaps in larger share from the volcanic matter which, in the form of floating lava, pumice, or dust, is plentifully delivered to the sea. Except doubtfully, and at most in a very small way, this chemical store of the sea water can not be directly taken into the structures of animals; it can only be immediately appropriated by the marine plants. These forms can only develop in that superficial realm of the seas which is penetrated by the sunlight, or say within the depth of five hundred feet, mostly within one hundred feet of the surface, about one thirtieth of the average, and about one fiftieth of the maximum ocean depth. On this marine plant life, and in a small measure on the vegetable matter derived from the land, the marine animals primarily depend for their provender. Through the conditions which bring about the formation of _Sarga.s.sum_ seas, those areas of the ocean where seaweeds grow afloat, as well as by the water-logging and weighting down of other vegetable matter, some part of the plant remains is carried to the sea floor, even to great depths; but the main dependence of the deep-sea forms of animals is upon other animal forms, which themselves may have obtained their store from yet others. In fact, in any deep-sea form we might find it necessary to trace back the food by thousands of steps before we found the creature which had access to the vegetable matter. It is easy to see how such conditions profoundly limit the development of organic being in the abysm of the ocean.

The sedentary animals, or those which are fixed to the sea bottom--a group which includes the larger part of the marine species--have to depend for their sustenance on the movement of the water which pa.s.ses their station. If the seas were perfectly still, none of these creatures except the most minute could be fed; therefore the currents of the ocean go far by their speed to determine the rate at which life may flourish. At great depths, as we have seen, these movements are practically limited to that which is caused by the slow movement which the tide brings about. The amount of this motion is proportional to the depth of the sea; in the deeper parts, it carries the water to and fro twice each day for the distance of about two hundred and fifty feet. In the shallower water this motion increases in proportion to the shoaling, and in the regions near the sh.o.r.es the currents of the sea which, except the ma.s.sive drift from the poles, do not usually touch the bottom, begin to have their influence. Where the water is less than a hundred feet in depth, each wave contributes to the movement, which attains its maximum near the sh.o.r.e, where every surge sweeps the water rapidly to and fro. It is in this surge belt, where the waves are broken, that marine animals are best provided with food, and it is here that their growth is most rapid. If the student will obtain a pint of water from the surf, he will find that it is clouded by fragments of organic matter, the quant.i.ty in a pound of the fluid often amounting to the fiftieth part of its weight. He will thus perceive that along the sh.o.r.e line, though the provision of victuals is most abundant, the store is made from the animals and plants which are ground up in the mill. In a word, while the coast is a place of rapid growth, it is also a region of rapid destruction; only in the case of the coral animals, which a.s.sociate their bodies with a number of myriads in large and elaborately organized communities, do we find animals which can make such head against the action of the waves that they can build great deposits in their realm.

It should be noted that a part of the advantage which is afforded to organic life by the sh.o.r.e belt is due to the fact that the waters are there subjected to a constant process of aeration by the whipping into foam and spray which occurs where the waves overturn.

It will be interesting to the student to note the great number of mechanical contrivances which have been devised to give security to animals and plants which face these difficult conditions arising from successive violent blows of falling water. Among these may be briefly noted those of the limpets--mollusks which dwell in a conical sh.e.l.l, which faces the water with a domelike outside, and which at the moment of the stroke is drawn down upon the rock by the strong muscle which fastens the creature to its foundation. The barnacles, which with their wedge-shaped prows cut the water at the moment of the stroke, but open in the pauses between the waves, so that the creature may with its branching arms grasp at the food which floats about it; the nullipores, forms of seaweed which are framed of limestone and cling firmly to the rock--afford yet other instances of protective adaptations contrived to insure the safety of creatures which dwell in the field of abundant food supply.

The facts above presented will show the reader that the marine sediments are formed under conditions which permit a great variety in the nature of the materials of which they are composed. As soon as the deposits are built into rocks and covered by later acc.u.mulations, their materials enter the laboratory of the under earth, where they are subjected to progressive changes. Even before they have attained a great depth, through the laying down of later deposits upon them, changes begin which serve to alter their structure. The fragments of a soluble kind begin to be dissolved, and are redeposited, so that the ma.s.s commonly becomes much more solid, pa.s.sing from the state of detritus to that of more or less solid rock. When yet more deeply buried, and thereby brought into a realm of greater warmth, or perhaps when penetrated by dikes and thereby heated, these changes go yet further. More of the material is commonly rearranged by solution and redeposition, so that limestone may be converted into crystalline marble, granular sandstones into firm ma.s.ses, known as quartzites, and clays into the harder form of slate. Where the changes go to the extreme point, rocks originally distinctly bedded probably may be so taken to pieces and made over that all traces of their stratification may be destroyed, all fossils obliterated, and the stone transformed into mica schist, or granite or other crystalline rock. It may be injected into the overlying strata in the form of dikes, or it may be blown forth into the air through volcanoes. Involved in mountain-folding, after being more or less changed in the manner described, the beds may become tangled together like the rumpled leaves of a book, or even with the complexity of snarled thread. All these changes of condition makes it difficult for the geologist to unravel the succession of strata so that he may know the true order of the rocks, and read from them the story of the successive geological periods. This task, though incomplete, has by the labours of many thousand men been so far advanced that we are now able to divide the record into chapters, the divisions of the geologic ages, and to give some account of the succession of events, organic and geographic, which have occurred since life began to write its records.

EARTHQUAKES.

In ordinary experience we seem to behold the greater part of the earth which meets our eyes as fixed in its position. A better understanding shows us that nothing in this world is immovable. In the realm of the inorganic world the atoms and molecules even in solid bodies have to be conceived as endowed with ceaseless though ordered motions. Even when matter is built into the solid rock, it is doubtful whether any grain of it ever comes really to rest. Under the strains which arise from the contraction of the earth's interior and the chemical changes which the rocks undergo, each bit is subject to ever-changing thrusts, which somewhat affect its position. If we in any way could bring a grain of sand from any stratum under a microscope, so that we could perceive its changes of place, we should probably find that it was endlessly swaying this way and that, with reference to an ideally fixed point, such as the centre of the earth. But even that centre, whether of gravity or of figure, is probably never at rest.

Earth movements may be divided into two groups--those which arise from the bodily s.h.i.+fting of matter, which conveys the particles this way or that, or, as we say, change their place, and those which merely produce vibration, in which the particles, after their vibratory movement, return to their original place. For purposes of ill.u.s.tration the first, or translatory motion, may be compared to that which takes place when a bell is carried along upon a locomotive or a s.h.i.+p; and the second, or vibratory movement, to what takes place when the bell is by a blow made to ring. It is with these ringing movements, as we may term them, that we find ourselves concerned when we undertake the study of earthquakes.

It is desirable that the reader should preface his study of earthquakes by noting the great and, at the same time, variable elasticity of rocks. In the extreme form this elasticity is very well shown when a toy marble, which is made of a close-textured rock, such as that from which it derives its name, is thrown upon a pavement composed of like dense material. Experiment will show that the little sphere can often be made to bounce to the height of twenty feet without breaking. If, then, with the same energy the marble is thrown upon a brick floor, the rebound will be very much diminished. It is well to consider what happens to produce the rebound. When the sphere strikes the floor it changes its shape, becoming shorter in the axis at right angles to the point which was struck, and at the same instant expanded along the equator of that axis. The flattening remains for only a small fraction of a second; the sphere vibrates so that it stretches along the line on which it previously shortened, and, as this movement takes place with great swiftness, it may be said to propel itself away from the floor. At the same time a similar movement goes on in the rock of the floor, and, where the rate of vibration is the same, the two kicks are coincident, and so the sphere is impelled violently away from the point of contact. Where the marble comes in contact with brick, in part because of the lesser elasticity of that material, due to its rather porous structure, and partly because it does not vibrate at the same rate as the marble, the expelling blow is much less strong.

All rocks whatever, even those which appear as incoherent sands, are more or less set into vibratory motion whenever they are struck by a blow. In the crust of the earth various accidents occur which may produce that sudden motion which we term a blow. When we have examined into the origin of these impulses, and the way in which they are transmitted through the rocks, we obtain a basis for understanding earthquake shocks. The commonest cause of the jarrings in the earth is found in the formation of fractures, known as faults. If the reader has ever been upon a frozen lake at a time when the weather was growing colder, and the ice, therefore, was shrinking, he may have noted the rending sound and the slight vibration which comes with the formation of a crack traversing the sheet of ice. At such a time he feels a movement which is an earthquake, and which represents the simpler form of those tremors arising from the sudden rupture of fault planes. If he has a mind to make the experiment, he may hang a bullet by a thread from a small frame which rests upon the ice, and note that as the vibration occurs the little pendulum sways to and fro, thus indicating the oscillations of the ice. The same instrument will move in an identical manner when affected by a quaking in the rocks.

Where the rocks are set in vibration by a rent which is formed in them, the phenomena are more complicated, and often on a vastly larger scale than in the simple conditions afforded by a sheet of ice. The rocks on either side of the rupture generally slide over each other, and the opposing ma.s.ses are rent in their friction upon one another; the result is, not only the first jar formed by the initial fracture, but a great many successive movements from the other breakages which occur. Again, in the deeper parts of the crust, the fault fissures are often at the moment of their formation filled by a violent inrush of liquid rock. This, as it swiftly moves along, tears away ma.s.ses from the walls, and when it strikes the end of the opening delivers a blow which may be of great violence. The nature of this stroke may be judged by the familiar instance where the relatively slow-flowing stream from a hydrant pipe is suddenly choked by closing the stopc.o.c.k.

Unless the plumber provides a cus.h.i.+on of air to diminish the energy of the blow, it is often strong enough to shake the house. Again, when steam or other gases are by a sudden diminution of pressure enabled to expand, they may deliver a blow which is exactly like that caused by the explosion of gunpowder, which, even when it rushes against the soft cus.h.i.+on of the air, may cause a jarring that may be felt as well as heard to a great distance. Such movements very frequently occur in the eruptions of volcanoes; they cause a quivering of the earth, which may be felt for a great distance from the immediate seat of the disturbance.

When by any of the sudden movements which have been above described a jar is applied to the rocks, the wave flies through the more or less elastic ma.s.s until the energy involved in it is exhausted. This may not be brought about until the motion has travelled for the distance of hundreds of miles. In the great earthquake of 1755, known as the Lisbon shock, the records make it seem probable that the movement was felt over one eighth part of the earth's surface. Such great disturbances probably bring about a motion of the rocks near the point of origin, which may be expressed in oscillations having an amplitude of one to two feet; but in the greater number of earthquakes the maximum swing probably does not exceed the tenth of that amount. Very sensible shaking, even such as may produce considerable damage to buildings, are caused by shocks in which the earth vibrates with less than an inch of swing.

When a shock originates, the wave in the rocks due to the compression which the blow inflicts runs at a speed varying with the elasticity of the substance, but at the rate of about fifteen hundred feet a second.

The movements of this wave are at right angles to the seat of the originating disturbance, so that the shock may come to the surface in a line forming any angle between the vertical and the nearly horizontal. Where, as in a volcanic eruption, the shock originates with an explosion, these waves go off in circles. Where, however, as is generally the case, the shock originates in a fault plane, which may have a length and depth of many miles, the movement has an elliptical form.

If the earthquake wave ran through a uniform and highly elastic substance, such as gla.s.s, it would move everywhere with equal speed, and, in the case of the greater disturbances, the motion might be felt over the whole surface of the earth. But as the motion takes place through rocks of varying elasticity, the rate at which it journeys is very irregular. Moving through materials of one density, and with a rate of vibration determined by those conditions, the impulse is with difficulty communicated to strata which naturally vibrate at another speed. In many cases, as where a shock pa.s.sing through dense crystalline strata encounters a ma.s.s of soft sandstone, the wave, in place of going on, is reflected back toward its point of origin. These earthquake echoes sometimes give rise to very destructive movements.

It often happens that before the original tremors of a shock have pa.s.sed away from a point on the surface the reflex movements rush in, making a very irregular motion, which may be compared to that of the waves in a cross-sea.

The foregoing account of earthquake action will serve to prepare the reader for an understanding of those very curious and important effects which these accidents produce in and on the earth. Below the surface the sensible action of earthquake shocks is limited. It has often been observed that people in mines hardly note a swaying which may be very conspicuous to those on the surface, the reason for this being that underground, where the rocks are firmly bound together, all those swingings which are due to the unsupported position of such objects as buildings, columnar rocks, trees, and the waters of the earth, are absent. The effect of the movements which earthquakes impress on the under earth is mainly due to the fact that in almost every part of the crust tensions or strains of other kinds are continually forming. These may for ages prove without effect until the earth is jarred, when motions will suddenly take place which in a moment may alter the conditions of the rocks throughout a wide field.

In a word, a great earthquake caused by the formation of an extensive fault is likely to produce any number of slight dislocations, each of which is in turn shock-making, sending its little wave to complicate the great oscillation. Nor does the perturbing effect of these jarring movements cease with the fractures which they set up and the new strains which are in turn developed by the motions which they induce.

The alterations of the rocks which are involved in chemical changes are favoured by such motions. It is a familiar experience that a vessel of water, if kept in the state of repose, may have its temperature lowered three or four degrees below the freezing point without becoming frozen. If the side of the vessel is then tapped with the finger, so as to send a slight quake through the ma.s.s, it will instantly congeal. Molecular rearrangements are thus favoured by shocks, and the consequences of those which run through the earth are, from a chemical point of view, probably important.

The reader may help himself to understand something of the complicated problem of earth tensions, and the corresponding movements of the rocks, by considering certain homely ill.u.s.trations. He may observe how the soil cracks as it shrinks in times of drought, the openings closing when it rains. In a similar way the frozen earth breaks open, sometimes with a shock which is often counted as an earthquake. Again, the ashes in a sifter or the gravel on a sieve show how each shaking may relieve certain tensions established by gravity, while they create others which are in turn to be released by the next shock. An ordinary dwelling house sways and strains with the alternations of temperature and moisture to which it is subjected in the round of climatal alterations. Now and then we note the movements in a cracking sound, but by far the greater part of them escape observation.

With this sketch of the mechanism of earthquake shocks we now turn to consider their effects upon the surface of the earth. From a geological point of view, the most important effect of earthquake shocks is found in the movement of rock ma.s.ses down steep slopes, which is induced by the shaking. Everywhere on the land the agents of decay and erosion tend to bring heavy ma.s.ses into position where gravitation naturally leads to their downfall, but where they may remain long suspended, provided they are not disturbed. Thus, wherever there are high and steep cliffs, great falls of rock are likely to occur when the earthquake movements traverse the under earth. In more than one instance observers, so placed that they commanded a view of distant mountains, have noticed the downfall of precipices in the path of the shock before the trembling affected the ground on which they stood. In the famous earthquake of 1783, which devastated southern Italy, the Prince of Scylla persuaded his people to take refuge in their boats, hoping that they might thereby escape the destruction which threatened them on the land. No sooner were the unhappy folk on the water than the fall of neighbouring cliffs near the sea produced a great wave, which overwhelmed the vessels.

Where the soil lies upon steep slopes, in positions in which it has acc.u.mulated during ages of tranquillity, a great shock is likely to send it down into the valleys in vast landslides. Thus, in the earthquake of 1692, the Blue Mountains of Jamaica were so violently shaken that the soil and the forests which stood on it were precipitated into the river beds, so that many tree-clad summits became fields of bare rock. The effect of this action is immensely to increase the amount of detritus which the streams convey to the sea.

After the great Jamaica shock, above noted, the rivers for a while ceased to flow, their waters being stored in the ma.s.ses of loose material. Then for weeks they poured forth torrents of mud and the _debris_ of vegetation--materials which had to be swept away as the streams formed new channels.

In all regions where earthquake movements are frequent, and the shock of considerable violence, the trained observer notes that the surfaces of bare rock are singularly extensive, the fact being that many of these areas, where the slope lies at angles of from ten to thirty degrees, which in an unshaken region would be thickly soil-covered, are deprived of the coating by the downward movement of the waste which the disturbances bring about. A familiar example of this action may be had by watching the workmen engaged in sifting sand, by casting the material on a sloping grating. The work could not be done but for an occasional blow applied to the sifter. An arrangement for such a jarring motion is commonly found in various ore-dressing machines, where the object is to move fragments of matter over a sloping surface.

Even where the earth is so level that an earthquake shock does not cause a sliding motion of the materials, such as above described, other consequences of the shaking may readily be noted. As the motion runs through the ma.s.s, provided the movement be one of considerable violence, crevices several feet in width, and sometimes having the length of miles, are often formed. In most cases these fissures, opened by one pulsation of the shock, are likely to be closed by the return movement, which occurs the instant thereafter. The consequences of this action are often singular, and in cases const.i.tute the most frightful elements of a shock which the sufferer beholds. In the great earthquake of 1811, which ravaged the section of the Mississippi Valley between the mouth of the Ohio and Vicksburg, these crevices were so numerously formed that the pioneers protected themselves from the danger of being caught in their jaws by felling trees so that they lay at right angles to the direction in which the rents extended, building on these timbers platforms to support their temporary dwelling places. The records of earthquakes supply many instances in which people have been caught in these earth fissures, and in a single case it is recorded that a man who disappeared into the cavity was in a moment cast forth in the rush of waters which in this, as in many other cases, spouts forth as the walls of the opening come together.

Sometimes these rents are attended by a dislocation, which brings the earth on one side much higher than on the other. The step thus produced may be many miles in length, and may have a height of twenty feet or more. It needs no argument to show that we have here the top of a fault such as produced the shock, or it may be one of a secondary nature, such as any earthquake is likely to bring about in the strata which it traverses. In certain cases two faults conjoin their action, so that a portion of the surface disappears beneath the earth, entombing whatever may have stood on the vanished site. Thus in the great shock known as that of Lisbon, which occurred in 1755, the stone quay along the harbour, where many thousand people had sought refuge from the falling buildings of the city, suddenly sank down with the mult.i.tude, and the waters closed over it; no trace of the people or of the structure was to be found after the shock was over. There is a story to the effect that during the same earthquake an Arab village in northern Africa sank down, the earth on either side closing over it, so that no trace of the habitations remained. In both these instances the catastrophes are best explained by the diagram.

[Ill.u.s.tration: Fig. 21.--Diagram showing how a portion of the earth's surface may be sunk by faulting. Fig. A shows the original position; B, the position after faulting; b b' and c c' the planes of the faults; the arrows the direction of the movement.]

In the earthquake of 1811 the alluvial plains on either side of the Mississippi at many points sank down so that arable land was converted into lakes; the area of these depressions probably amounted to some hundred square miles. The writer, on examining these sunken lands, found that the subsidences had occurred where the old moats or abandoned channels of the great river had been filled in with a mixture of decaying timber and river silt. When violently shaken, this loose-textured _debris_ naturally settled down, so that it formed a basin occupied by a crescent-shaped lake. The same process of settling plentifully goes on wherever the rocks are still in an uncemented state. The result is often the production of changes which lead to the expulsion of gases. Thus, in the Charleston earthquake of 1883, the surface over an area of many hundred square miles was pitted with small craters, formed by the uprush of water impelled by its contained gases. These little water volcanoes--for such we may call them--sometimes occur to the number of a dozen or more on each acre of ground in the violently shaken district. They indicate one result of the physical and chemical alterations which earthquake shocks bring about. As earthquakes increase in violence their effect upon the soil becomes continually greater, until in the most violent shocks all the loose materials on the surface of the earth may be so shaken about as to destroy even the boundaries of fields. After the famous earthquake of Riobamba, which occurred on the west coast of South America in 1797, the people of the district in which the town of that name was situated were forced to redivide their land, the original boundaries having disappeared. Fortunately, shocks of this description are exceedingly rare. They occur in only a few parts of the world.

Certain effects of earthquakes where the shock emerges beneath the sea have been stated in the account of volcanic eruptions (see page 299).

We may therefore note here only certain of the more general facts.

While pa.s.sing through the deep seas, this wave may have a height of not more than two or three feet and a width of some score miles. As it rolls in upon the sh.o.r.e the front of the undulation is r.e.t.a.r.ded by the friction of the bottom in such a measure that its speed is diminished, while the following part of the waves, being less checked, crowds up toward this forward part. The result is, that the surge mounts ever higher and higher as it draws near the sh.o.r.e, upon which it may roll as a vast wave having the height of fifty feet or more and a width quite unparalleled by any wave produced from wind action. Waves of this description are most common in the Pacific Ocean. Although but occasional, the damage which they may inflict is very great. As the movement approaches the sh.o.r.e, vessels, however well anch.o.r.ed, are dragged away to seaward by the great back lash of the wave, a phenomenon which may be perceived even in the case of the ordinary surf. Thus forced to seaward, the crews of the s.h.i.+ps may find their vessels drawn out for the distance of some miles, until they come near the face of the advancing billow. This, as it approaches the sh.o.r.e, straightens up to the wall-fronted form, and then topples upon the land. Those vessels which are not at once crushed down by the blow are generally hurled far inland by the rush of waters. In the great Jamaica earthquake of 1692 a British man-of-war was borne over the tops of certain warehouses and deposited at a distance from the sh.o.r.e.

Owing to the fact that water is a highly elastic material, the shocks transmitted to it from the bottom are sent onward with their energy but little diminished. While the impulse is very violent, these oscillations may prove damaging to s.h.i.+pping. The log-books of mariners abound in stories of how vessels were dismasted or otherwise badly shaken by a sudden blow received in the midst of a quiet sea. The impression commonly conveyed to the sailors is that the craft has struck upon a rock. The explanation is that an earthquake jar, in traversing the water, has delivered its blow to the s.h.i.+p. As the speed of this jarring movement is very much greater than that of any ordinary wave, the blow which it may strike may be most destructive.

There seems, indeed, little reason to doubt that a portion of the vessels which are ever disappearing in the wilderness of the ocean are lost by the crus.h.i.+ng effect of these quakings which pa.s.s through the waters of the deep.

We have already spoken of the earthquake shock as an oscillation. It is a quality of all bodies which oscillate under the influence of a blow, such as originates in earthquake shocks, to swing to and fro, after the manner of the metal in a bell or a tuning fork, in a succession of movements, each less than the preceding, until the impulse is worn out, or rather, we should in strict sense say, changed to other forms of energy. The result is, that even in the slightest earthquake shock the earth moves not once to and fro, but very many times. In a considerable shock the successive diminis.h.i.+ng swingings amount to dozens before they become so slight as to elude perception. Although the first swaying is the strongest, and generally the most destructive, the quick to-and-fro motions are apt to continue and to complete the devastation which the first brings about. The vibrations due to any one shock take place with great rapidity. They may, indeed, be compared to those movements which we perceive in the margin of a large bell when it has received a heavy blow from the clapper. The reader has perhaps seen that for a moment the rim of the bell vibrates with such rapidity that it has a misty look--that is, the motions elude the sight. It is easy to see that a shaking of this kind is particularly calculated to disrupt any bodies which stand free in the air and are supported only at their base.

In what we may call the natural architecture of the earth, the pinnacles and obelisks, such as are formed in many high countries, the effect of these shakings is destructive, and, as we have seen, even the firmer-placed objects, such as the strong-walled cliffs and steep slopes of earth, break down under the a.s.saults. It is therefore no matter of surprise that the buildings which man erects, where they are composed of masonry, suffer greatly from these tremblings. In almost all cases human edifices are constructed without regard to other problems of strength than those which may be measured by their weight and the resistance to fracture from gravitation alone. They are not built with expectation of a quaking, but of a firm-set earth.

The damage which earthquakes do to buildings is in most cases due to the fact that they sway their walls out of plumb, so that they are no longer in position to support the weight which they have to bear. The amount of this swaying is naturally very much greater than that which the earth itself experiences in the movement. A building of any height with its walls unsupported by neighbouring structures may find its roof rocked to and fro through an arc which has a length of feet, while its base moves only through a length of inches. The reader may see an example of this nature if he will poise a thin book or a bit of plank a foot long on top of a small table; then jarring the table so that it swings through a distance of say a quarter of an inch, he will see that the columnar object swings at its top through a much greater distance, and is pretty sure to be overturned.

Where a building carries a load in its upper parts, such as may be afforded by its heavy roof or the stores which it contains, the effect of an earthquake shock such as carries the earth to and fro becomes much more destructive than it might otherwise be. This weight lags behind when the earth slips forward in the first movement of the oscillation, with the effect that the walls of the building are pretty sure to be thrust so far beyond the perpendicular that they give way and are carried down by the weight which they bore. It has often been remarked in earthquake shocks that tall columns, even where composed of many blocks, survive a shock which overturns lower buildings where thin walls support several floors, on each of which is acc.u.mulated a considerable amount of weight. In the case of the column, the strains are even, and the whole structure may rock to and fro without toppling over. As the energy of the undulations diminish, it gradually regains the quiet state without damage. In the ordinary edifice the irregular disposition of the weight does not permit the uniform movement which may insure safety. Thus, if the city of Was.h.i.+ngton should ever be violently shaken, the great obelisk, notwithstanding that it is five hundred feet high, may survive a disturbance which would wreck the lower and more ma.s.sive edifices which lie about it.

Where, as is fortunately rarely the case, the great shock comes to the earth in a vertical direction, the effect upon all movable objects is in the highest measure disastrous. In such a case buildings are crushed as if by the stroke of a giant's hand. The roofs and floors are at one stroke thrown to the foundations, and all the parts of the walls which are not supported by strong masonry continuous from top to bottom are broken to pieces. In such cases it has been remarked that the bodies of men are often thrown considerable distances. It is a.s.serted, indeed, that in the Riobamba shock they were cast upward to the height of more than ninety feet. It is related that the solo survivor of a congregation which had hastened at the outset of the disturbance into a church was thrown by the greatest and most destructive shock upward and through a window the base of which was at the height of more than twenty feet from the ground.

It is readily understood that an earthquake shock may enter a building in any direction between the vertical and the horizontal. As the movement exhausts itself in pa.s.sing from the place of its origin, the horizontal shocks are usually of least energy. Those which are accurately vertical are only experienced where the edifices are placed immediately over the point where the motion originates. It follows, therefore, that the destructive work of earthquakes is mainly performed in that part of the field where the motion is, as regards its direction, between the vertical and the horizontal--a position in which the edifice is likely to receive at once the destructive effect arising from the sharp upward thrust of the vertical movement and the oscillating action of that which is in a horizontal direction. Against strains of this description, where the movements have an amplitude of more than a few inches, no ordinary masonry edifice can be made perfectly safe; the only tolerable security is attained where the building is of well-framed timber, which by its elasticity permits a good deal of motion without destructive consequences. Even such buildings, however, those of the strongest type, may be ruined by the greater earthquakes. Thus, in the Mississippi Valley earthquake of 1811, the log huts of the frontiersmen, which are about as strong as any buildings can be made, were shaken to pieces by the sharp and reiterated shocks.

It is by no means surprising to find that the style of architecture adopted in earthquake countries differs from that which is developed in regions where the earth is firm-set. The people generally learn that where their buildings must meet the trials of earthquakes they have to be low and strong, framed in the manner of fortifications, to withstand the a.s.sault of this enemy. We observe that Gothic architecture, where a great weight of masonry is carried upon slender columns and walls divided by tall windows, though it became the dominant style in the relatively stable lands of northern Europe, never gained a firm foothold in those regions about the Mediterranean which are frequently visited by severe convulsions of the earth. There the Grecian or the Romanesque styles, which are of a much more ma.s.sive type, retain their places and are the fas.h.i.+ons to the present day.

Even this manner of building, though affording a certain security against slight tremblings, is not safe in the greater shocks. Again and again large areas in southern Italy have been almost swept of their buildings by the destructive movements which occur in that realm. The only people who have systematically adapted their architectural methods to earthquake strains are the j.a.panese, who in certain districts where such risks are to be encountered construct their dwellings of wood, and place them upon rollers, so that they may readily move to and fro as the shock pa.s.ses beneath them. In a measure the people of San Francisco have also provided against this danger by avoiding dangerous weights in the upper parts of their buildings, as well as the excessive height to which these structures are lifted in some of our American towns.

Earthquakes of sensible energy appear to be limited to particular parts of the earth's crust. The regions, indeed, where within the period of human history shocks of devastating energy have occurred do not include more than one fifteenth part of the earth's surface. There is a common notion that these movements are most apt to happen in volcanic regions. It is, indeed, true that sensible shocks commonly attend the explosions from great craters, but the records clearly show that these movements are very rarely of destructive energy. Thus in the regions about the base of Vesuvius and of aetna, the two volcanoes of which most is known, the shocks have never been productive of extensive disaster. In fact, the reiterated slight jarrings which attend volcanic action appear to prevent the formation of those great and slowly acc.u.mulated strains which in their discharge produce the most violent tremblings of the earth. The greatest and most continuous earthquake disturbances of history--that before noted in the early days of this century, in the Mississippi Valley, where shocks of considerable violence continued for two years--came about in a field very far removed from active volcanoes. So, too, the disturbances beneath the Atlantic floor which originated the shocks that led to the destruction of Lisbon, and many other similar though less violent movements, are developed in a field apparently remote from living volcanoes. Eastern New England, which has been the seat of several considerable earthquakes, is about as far away from active vents as any place on the habitable globe. We may therefore conclude that, while volcanoes necessarily produce shocks resulting from the discharge of their gases and the intrusion of lava into the dikes which are formed about them, the greater part of the important shocks are in no wise connected with volcanic explosions.

With the exception of the earthquake in the Mississippi Valley, all the great shocks of which we have a record have occurred in or near regions where the rocks have been extensively disturbed by mountain-building forces, and where the indications lead us to believe that dislocations of strata, such as are competent to rive the beds asunder, may still be in progress. This, taken in connection with the fact that many of these shocks are attended by the formation of fault planes, which appear on the surface, lead us to the conclusion that earthquakes of the stronger kind are generally formed by the riving of fissures, which may or may not be developed upward to the surface. This view is supported by many careful observations on the effect which certain great earthquakes have exercised on the buildings which they have ravaged. The distinguished observer, Mr. Charles Mallet, who visited the seat of the earthquake which, in 1854, occurred in the province of Calabria in Italy, with great labour and skill determined the direction in which the shock moved through some hundreds of edifices on which it left the marks of its pa.s.sage.

Platting these lines of motion, he found that they were all referred to a vertical plane lying at the depth of some miles beneath the surface, and extending for a great distance in a north and south direction. This method of inquiry has been applied to other fields, with the result that in the case of all the instances which have been subjected to this inquiry the seat of the shock has been traced to such a plane, which can best be accounted for by the supposition of a fault.

The method pursued by Mr. Mallet in his studies of the origin of earthquakes, and by those who have continued his inquiry, may be briefly indicated as follows: Examining disrupted buildings, it is easy to determine those which have been wrecked by a shock that emerged from the earth in a vertical direction. In these cases, though tall walls may remain standing, the roofs and floors are thrown into the cellars. With a dozen such instances the plane of what is called the seismic vertical is established (_seismos_ is the Greek for earthquake). Then on either side of this plane, which indicates the line but not the depth of the disturbance, other observations may be made which give the clew to the depth. Thus a building may be found where the northwest corner at its upper part has been thrown off. Such a rupture was clearly caused by an upward but oblique movement, which in the first half of the oscillation heaved the structure upwardly into the northwest, and then in the second half, or rebound, drew the ma.s.s of the building away from the unsupported corner, allowing that part of the masonry to fly off and fall to the ground. Constructing a line at right angles to the plane of the fracture, it will be found to intersect the plane, the position of which has been in part determined by finding the line where it intersects the earth, or the seismic vertical before noted. Multiplying such observations on either side of the last-mentioned line, the att.i.tude of the underground parts of the plane, as well as the depth to which it attained, can be approximately determined.

It is worth while to consider the extent to which earthquake shocks may affect the general quality of the people who dwell in countries where these disturbances occur with such frequency and violence as to influence their lives. There can be no question that wherever earthquakes occur in such a measure as to produce widespread terror, where, recurring from time to time, they develop in men a sense of abiding insecurity, they become potent agents of degradation. All the best which men do in creating a civilization rests upon a sense of confidence that their efforts may be acc.u.mulated from year to year, and that even after death the work of each man may remain as a heritage to his kind. It is likely, indeed, that in certain realms, as in southern Italy, a part of the failure of the people to advance in culture is due to their long experience of such calamities, and the natural expectation that they will from time to time recur. In a similar way the Spanish settlements in Central and South America, which lie mostly in lands that are subject to disastrous shocks, may have been r.e.t.a.r.ded by the despair, as well as the loss of property and life, which these accidents have so frequently inflicted upon them. It will not do, however, to attribute too much to such terrestrial influences. By far the most important element in determining the destiny of a people is to be found in their native quality, that which they owe to their ancestors of distant generations. In this connection it is well to consider the history of the Icelandic people, where a small folk has for a thousand years been exposed to a range and severity of trials, such as earthquakes, volcanic explosions, and dearth of harvests may produce, and all these in a measure that few if any other countries experience.

Notwithstanding these misfortunes, the Icelanders have developed and maintained a civilization which in all else, except its material results, on the average transcends that which has been won by any other folk in modern times. If a people have the determining spirit which leads to high living, they can successfully face calamities far greater than those which earthquakes inflict.

It was long supposed that the regions where earthquakes are not noticeable by the unaided senses were exempt from all such disturbances. The observations which seismologists have made in recent years point to the conclusion that no part of the earth's surface is quite exempt from movements which, though not readily perceived, can be made visible by the use of appropriate instruments. With an apparatus known as the horizontal pendulum it is possible to observe vibrations which do not exceed in amplitude the hundredth part of an inch. This mechanism consists essentially of a slender bar supported near one end by two wires, one from above, the other from below. It may readily be conceived that any measurable movement will cause the longer end of the rod to sway through a considerable arc. Wherever such a pendulum has been carefully observed in any district, it has been found that it indicates the occurrence of slight tremors. Even certain changes of the barometer, which alter the weight of the atmosphere that rests upon the earth to the amount indicated by an inch in the height of the mercury column, appears in all cases to create such tremors. Many of these slight shocks may be due to the effect of more violent quakings, which have run perhaps for thousands of miles from their point of origin, and have thus been reduced in the amplitude of their movement. Others are probably due to the slight motion brought about through the chemical changes of the rocks, which are continuously going on. The ease with which even small motions are carried to a great distance may be judged by the fact that when the ground is frozen the horizontal pendulum will indicate the jarring due to a railway train at the distance of a mile or more from the track.

In connection with the earth jarring, it would be well to note the occurrence of another, though physically different, kind of movement, which we may term earth swayings, or ma.s.sive movements, which slowly dislocate the vertical, and doubtless also the horizontal, position of points upon its surface. It has more than once been remarked that in mountain countries, where accurate sights have been taken, the heights of points between the extremities of a long line appear somewhat to vary in the course of a term of years. Thus at a place in the Apennines, where two buildings separated by some miles of distance are commonly intervisible over the crest of a neighbouring peak, it has happened that a change of level of some one of the points has made it impossible to see the one edifice from the other. Knowing as we do that the line of the seacoast is ever-changing, uprising taking place at some points and down-sinking at others, it seems not unlikely that these irregular swayings are of very common occurrence. Moreover, astronomers are beginning to remark the fact that their observatories appear not to remain permanently in the same position--that is, they do not have exactly the same lat.i.tude and longitude. Certain of these changes have recently been explained by the discovery of a new and hitherto unnoted movement of the polar axis. It is not improbable, however, that the irregular swaying of the earth's crust, due to the folding of strata and to the alterations in the volume of rocks which are continually going on, may have some share in bringing about these dislocations.

Measured by the destruction which was wrought to the interests of man, earthquakes deserve to be reckoned among the direst calamities of Nature. Since the dawn of history the records show us that the destruction of life which is to be attributed to them is to be counted by the millions. A catalogue of the loss of life in the accidents of this description which have occurred during the Christian era has led the writer to suppose that probably over two million persons have perished from these shocks in the last nineteen centuries.

Nevertheless, as compared with other agents of destruction, such as preventable disease, war, or famine, the loss which has been inflicted by earth movements is really trifling, and almost all of it is due to an obstinate carelessness in the construction of buildings without reference to the risks which are known to exist in earthquake-ridden countries.

Although all our exact knowledge concerning the distribution of earthquakes is limited to the imperfect records of two or three thousand years, it is commonly possible to measure in a general way the liability to such accidents which may exist in any country by a careful study of the details of its topography. In almost every large area the process of erosion naturally leaves quant.i.ties of rock, either in the form of detached columns or as detrital acc.u.mulations deposited on steep slopes. These features are of relatively slow formation, and it is often possible to determine that they have been in their positions for a time which is to be measured by thousands of years. Thus, on inspecting a country such as North America, where the historic records cover but a brief time, we may on inquiry determine which portions of its area have long been exempt from powerful shocks.

Where natural obelisks and steep taluses abound--features which would have disappeared if the region had been moved by great shocks--we may be sure that the field under inspection has for a great period been exempt from powerful shaking. Judged by this standard, we may safely say that the region occupied by the Appalachian Mountains has been exempt from serious trouble. So, too, the section of the Cordilleras lying to the east of what is commonly called the Great Basin, between the Rocky Mountains and the Sierra Nevada, has also enjoyed a long reign of peace. In glaciated countries the record is naturally less clear than in those parts of the world which have been subjected to long-continued, slow decay of the rocks. Nevertheless, in those fields boulders are often found poised in position which they could not have maintained if subjected to violent shaking. Judged by this evidence, we may say that a large part of the northern section of this continent, particularly the area about the Great Lakes, has been exempt from considerable shocks since the glacier pa.s.sed away.

The sh.o.r.es which are subject to the visitations of the great marine waves, caused by earthquake shocks occurring beneath the bottom of the neighbouring ocean, are so swept by those violent inundations that they lose many features which are often found along coasts that have been exempted from such visitations. Thus wherever we find extensive and delicately moulded dunes, poised stones, or slender pinnacled rocks along a coast, we may be sure that since these features were formed the district has not been swept by these great waves.