Chapter IX: The Rocks and Their Order (1)

In the preceding chapters of this book the attention of the student has been directed mainly to the operations of those natural forces which act upon the surface of the earth. Incidentally the consequences arising from the applications of energy to the outer part of the planet have been attended to, but the main aim has been to set forth the work which solar energy, operating in the form of heat, accomplishes upon the lands. We have now to consider one of the great results of these actions, which is exhibited in the successive strata that make up the earth's crust.

The most noteworthy effect arising from the action of the solar forces on the earth and their co-operation with those which originate in our sphere is found in the destruction of beds or other deposits of rock, and the removal of the materials to the floors of water basins, where they are again aggregated in strata, and gradually brought once more into a stable condition within the earth. This work is accomplished by water in its various states, the action being directly affected by gravitation. In the form of steam, water which has been built into rocks and volcanically expelled by tensions, due to the heat which it has acquired at great depths below the surface, blows forth great quantities of lava, which is contributed to the formation of strata, either directly in the solid form or indirectly, after having been dissolved in the sea. Acting as waves, water impelled by solar energy transmitted to it by the winds beats against the shores, wearing away great quantities of rock, which is dragged off to the neighbouring sea bottoms, there to resume the bedded form. Moving ice in glaciers, water again applying solar energy given to it by its elevation above the sea, most effectively grinds away the elevated parts of the crust, the _débris_ being delivered to the ocean. In the rain the same work is done, and even in the wind the power of the sun serves to abrade the high-lying rocks, making new strata of their fragments.

As gravity enters as an element in all the movements of divided rock, the tendency of the waste worn from the land is to gather on to the bottoms of basins which contain water. Rarely, and only in a small way, this process results in the accumulation of lake deposits; the greater part of the work is done upon the sea floor. When the beds are formed in lake basins, they may be accumulated in either of two very diverse conditions. They may be formed in what are called dead seas, in which case the detrital materials are commonly small in amount, for the reason that the inflowing streams are inconsiderable; in such basins there is normally a large share of saline materials, which are laid down by the evaporation of the water. In ordinary lakes the deposits which are formed are mostly due to the sediment that the rivers import. These materials are usually fine-grained, and the sand or pebbles which they contain are plentifully mingled with clay. Hence lake deposits are usually of an argillaceous nature. As organic life, such as secretes limestone, is rarely developed to any extent in lake basins, limy beds are very rarely formed beneath those areas of water. Where they occur, they are generally due to the fact that rivers charged with limy matter import such quantities of the substance that it is precipitated on the bottom.

As lake deposits are normally formed in basins above the level of the sea, and as the drainage channels of the basins are always cutting down, the effect is to leave such strata at a considerable height above the sea level, where the erosive agents may readily attack them. In consequence of this condition, lacustrine beds are rarely found of great antiquity; they generally disappear soon after they are formed. Where preserved, their endurance is generally to be attributed to the fact that the region they occupy has been lowered beneath the sea and covered by marine strata.

The great laboratory in which the sedimentary deposits are accumulated, the realm in which at least ninety-nine of the hundred parts of these materials are laid down, is the oceanic part of the earth. On the floors of the seas and oceans we have not only the region where the greater part of the sedimentation is effected, but that in which the work assumes the greatest variety. The sea bottoms, as regards the deposits formed upon them, are naturally divided into two regions--the one in which the _débris_ from the land forms an important part of the sediment, and the other, where the remoteness of the shores deprives the sediment of land waste, or at least of enough of that material in any such share as can affect the character of the deposits.

What we may term the littoral or shore zone of the sea occupies a belt of prevailingly shallow water, varying in width from a few score to a few hundred miles. Where the bottom descends steeply from the coast, where there are no strong off-shore setting currents, and where the region is not near the mouth of a large river which bears a great tide of sediment to the sea, the land waste may not affect the bottom for more than a mile or two from the shore. Where these conditions are reversed, the _débris_ from the air-covered region may be found three or four hundred miles from the coast line. It should also be noted that the incessant up-and-down goings of the land result in a constant change in the position of the coast line, and consequently in the extension of the land sediment, in the course of a few geological periods over a far wider field of sea bottom than that to which they would attain if the shores remained steadfast.

It is characteristic of the sediments deposited within the influence of the continental detritus that they vary very much in their action, and that this variation takes place not only horizontally along the shores in the same stratum, but vertically, in the succession of the beds. It also may be traced down the slope from the coast line to deep water. Thus where all the _débris_ comes from the action of the waves, the deposits formed from the shore outwardly will consist of coarse materials, such as pebbles near the coast, of sand in the deeper and remoter section, and of finer silt in the part of the deposit which is farthest out. With each change in the level of the coast line the position of these belts will necessarily be altered. Where a great river enters the sea, the changes in the volume of sediment which it from time to time sends forth, together with the alternations in the position of its point of discharge, led to great local complexities in the strata. Moreover, the turbid water sent forth by the stream may, as in the case of the tide from the Amazon, be drifted for hundreds of miles along the coast line or into the open sea.

The most important variations which occur in the deposits of the littoral zone are brought about by the formations of rocks more or less composed of limestone. Everywhere the sea is, as compared with lake waters, remarkably rich in organic life. Next the shore, partly because the water is there shallow, but also because of its relative warmth and the extent to which it is in motion, organic life, both that of animals and plants, commonly develops in a very luxuriant way. Only where the bottom is composed of drifting sands, which do not afford a foothold for those species which need to rest upon the shore, do we fail to find that surface thickly tenanted with varied forms. These are arranged according to the depth of the bottom. The species of marine plants which are attached to fixed objects are limited to the depth within which the sunlight effectively penetrates the water; in general, it may be said that they do not extend below a depth of one hundred feet. The animal forms are distributed, according to their kinds, over the floor, but few species having the capacity to endure any great range in the pressure of the sea water. Only a few forms, indeed, extend from low tide to the depth of a thousand feet.

The greatest development of organic life, the realm in which the largest number of species occur, and where their growth is most rapid, lies within about a hundred feet of the low-tide level. Here sunlight, warmth, and motion in the water combine to favour organic development. It is in this region that coral reefs and other great accumulations of limestone, formed from the skeletons of polyps and mollusks, most abundantly occur. These deposits of a limy nature depend upon a very delicate adjustment of the conditions which favour the growth of certain creatures; very slight geographic changes, by inducing movements of sand or mud, are apt to interrupt their formation, bringing about a great and immediate alteration in the character of the deposits. Thus it is that where geologists find considerable fields of rock, where limestones are intercalated with sandstones and deposits of clay, they are justified in assuming that the strata were laid down near some ancient shore. In general, these coast deposits become more and more limy as we go toward the tropical realms, and this for the reason that the species which secrete large amounts of lime are in those regions most abundant and attain the most rapid growth. The stony polyps, the most vigorous of the limestone makers, grow in large quantities only in the tropical realm, or near to it, where ocean streams of great warmth may provide the creatures with the conditions of temperature and food which they need.

As we pass from the shore to the deeper sea, the share of land detritus rapidly diminishes until, as before remarked, at the distance of five hundred miles from the coast line, very little of that waste, except that from volcanoes, attains the bottom of the sea. By far the larger part of the contributions which go to the formation of these deep-sea strata come from organic remains, which are continually falling upon the sea floor. In part, this waste is derived from creatures which dwell upon the bottom; in considerable measure, however, it is from the dead bodies of those forms which live near the surface of the sea, and which when dying sink slowly through the intermediate realm to the bottom.

Owing to the absence of sunlight, the prevailingly cold water of the deeper seas, and the lack of vegetation in those realms, the growth of organic forms on the deep-sea floor is relatively slow. Thus it happens that each shell or other contribution to the sediment lies for some time on the bottom before it is buried. While in this condition it is apt to be devoured by some of the many species which dwell on the bottom and subsist from the remains of animals and plants which they find there. In all cases the fossilization of any form depends upon the accumulation of sediment before the processes of destruction have overtaken them, and among these processes we must give the first place to the creatures which subsist on shells, bones, or other substances of like nature which find their way to the ocean floor. In the absolute darkness, the still water, and the exceeding cold of the deeper seas, animals find difficult conditions for development. Moreover, in this deep realm there is no native vegetation, and, in general, but little material of this nature descends to the bottom from the surface of the sea. The result is, the animals have to subsist on the remains of other animals which at some step in the succession have obtained their provender from the plants which belong on the surface or in the shallow waters of the sea. This limitation of the food supply causes the depths of the sea to be a realm of continual hunger, a region where every particle of organic matter is apt to be seized upon by some needy creature.

In consequence of the fact that little organic matter on the deeper sea floors escapes being devoured, the most of the material of this nature which goes into strata enters that state in a finely divided condition. In the group of worms alone--forms which in a great diversity of species inhabit the sea floor--we find creatures which are specially adapted to digesting the _débris_ which gathers on the sea bottom. Wandering over this surface, much in the manner of our ordinary earthworms, these creatures devour the mud, voiding the matter from their bodies in a yet more perfectly divided form. Hence it comes about that the limestone beds, so commonly formed beneath the open seas, are generally composed of materials which show but few and very imperfect fossils. Studying any series of limestone beds, we commonly find that each layer, in greater or less degree, is made up of rather massive materials, which evidently came to their place in the form of a limy mud. Very often this lime has crystallized, and thus has lost all trace of its original organic structure.

One of the conspicuous features which may be observed in any succession of limestone beds is the partings or divisions into layers which occur with varied frequency. Sometimes at vertical intervals of not more than one or two inches, again with spacings of a score of feet, we find divisional planes, which indicate a sudden change in the process of rock formation. The lime disappears, and in place of it we have a thin layer of very fine detritus, which takes on the form of a clay. Examining these partings with care, we observe that on the upper surface on the limestone the remains of the animal which dwelt on the ancient sea floor are remarkably well preserved, they having evidently escaped the effect of the process which reduced their ancestors, whose remains constitute the layer, to mud. Furthermore, we note that the shaly layer is not only lacking in lime, but commonly contains no trace of animals such as might have dwelt on the bottom. The fossils it bears are usually of species which swam in the overlying water and came to the bottom after death. Following up through the layer of shale, we note that the ordinary bottom life gradually reappears, and shortly becomes so plentiful that the deposit resumes the character which it had before the interruption began. Often, however, we note that the assemblage of species which dwelt on the given area of sea floor has undergone a considerable change. Forms in existence in the lower layer may be lacking in the upper, their place being taken by new varieties.

So far the origin of these divisional planes in marine deposits has received little attention from geologists; they have, indeed, assumed that each of these alterations indicates some sudden disturbance of the life of the sea floors. They have, however, generally assumed that the change was due to alterations in the depth of the sea or in the run of ocean currents. It seems to the writer, however, that while these divisions may in certain cases be due to the above-mentioned and, indeed, to a great variety of causes, they are in general best to be explained by the action of earthquakes. Water being an exceedingly elastic substance, an earthquake passes through it with much greater speed than it traverses the rocks which support the ocean floor. The result is that, when the fluid and solid oscillate in the repeated swingings which a shock causes, they do not move together, but rub over each other, the independent movements having the swing of from a few inches to a foot or two in shocks of considerable energy.

When the sea bottom and the overlying water, vibrating under the impulse of an earthquake shock, move past each other, the inevitable result is the formation of muddy water; the very fine silt of the bottom is shaken up into the fluid, which afterward descends as a sheet to its original position. It is a well-known fact that such muddying of water, in which species accustomed to other conditions dwell, inevitably leads to their death by covering their breathing organs and otherwise disturbing the delicately balanced conditions which enable them to exist. We find, in fact, that most of the tenants of the water, particularly the forms which dwell upon the bottom, are provided with an array of contrivances which enable them to clear away from their bodies such small quantities of silt as may inconvenience them. Thus, in the case of our common clam, the breathing organs are covered with vibratory cilia, which, acting like brooms, sweep off any foreign matter which may come upon their surfaces. Moreover, the creature has a long, double, spoutlike organ, which it can elevate some distance above the bottom, through which it draws and discharges the water from which it obtains food and air. Other forms, such as the crinoids, or sea lilies, elevate the breathing parts on top of tall stems of marvellous construction, which brings those vital organs at the level, it may be, of three or four feet above the zone of mud. In consequence of the peculiar method of growth, the crinoids often escape the damage done by the disturbance of the bottom, and thus form limestone beds of remarkable thickness; sometimes, indeed, we find these layers composed mainly of crinoidal remains, which exhibit only slight traces of partings such as we have described, being essentially united for the depth of ten or twenty feet. Where the layers have been mainly accumulated by shellfish, their average thickness is less than half a foot.

When we examine the partitions between the layers of limestone, we commonly find that, however thin, they generally extend for an indefinite distance in every direction. The writer has traced some of these for miles; never, indeed, has he been able to find where they disappeared. This fact makes it clear that the destruction which took place at the stage where these partings were formed was widespread; so far as it was due to earthquake shocks, we may fairly believe that in many cases it occurred over areas which were to be measured by tens of thousands of square miles. Indeed, from what we know of earthquake shocks, it seems likely that the devastation may at times have affected millions of square miles.

Another class of accidents connected with earthquakes may also suddenly disturb the mud on the sea bottom. When, as elsewhere noted, a shock originates beneath the sea, the effect is suddenly to elevate the water over the seat of the jarring and the regions thereabouts to the height of some feet. This elevation quickly takes the shape of a ringlike wave, which rolls off in every direction from its point of origin. Where the sea is deep, the effect of this wave on the bottom may be but slight; but as the undulation attains shallower water, and in proportion to the shoaling, the front of the surge is retarded in its advance by the friction of the bottom, while the rear part, being in deeper water, crowds upon the advancing line. The action is precisely that which has been described as occurring in wind-made waves as they approach the beach; but in this last-named group of undulations, because of the great width of the swell, the effect of the shallowing is evident in much deeper water. It is likely that at the depth of a thousand feet the passing of one of these vast surges born of earthquakes may so stir the mud of the sea floor as to bring about a widespread destruction of life, and thus give rise to many of the partitions between strata.

If we examine with the microscope the fine-grained silts which make up the shaly layers between limestones, we find the materials to be mostly of inorganic origin. It is hard to trace the origin of the mineral matter which it contains; some of the fragments are likely to prove of Volcanic origin; others, bits of dust from meteorites; yet others, dust blown from the land, which may, as we know, be conveyed for any distance across the seas. Mingled with this sediment of an inorganic origin we almost invariably find a share of organic waste, derived not from creatures which dwelt upon the bottom, but from those which inhabited the higher-lying waters. If, now, we take a portion of the limestone layer which lies above or below the shale parting, and carefully dissolve out with acids the limy matter which it contains, we obtain a residuum which in general character, except so far as the particles may have been affected by the acid, is exactly like the material which forms the claylike partition. We are thus readily led to the conclusion that on the floors of the deeper seas there is constantly descending, in the form of a very slow shower, a mass of mineral detritus. Where organic life belonging to the species which secrete hard shells or skeletons is absent, this accumulation, proceeding with exceeding slowness, gradually accumulates layers, which take on a shaly character. Where limestone-making animals abound, they so increase the rate of deposition that the proportion of the mineral material in the growing strata is very much reduced; it may, indeed, become as small as one per cent of the mass. In this case we may say that the deposit of limestone grew a hundred times as fast as the intervening beds of shale.

The foregoing considerations make it tolerably clear that the sea floor is in receipt of two diverse classes of sediment--those of a mineral and those of an organic origin. The mineral, or inorganic, materials predominate along the shores. They gradually diminish in quantity toward the open sea, where the supply is mainly dependent on the substances thrown forth from volcanoes, on pumice in its massive or its comminuted form--i.e., volcanic dust, states of lava in which the material, because of the vesicles which it contains, can float for ages before it comes to rest on the sea bottom. Variations in the volcanic waste contributed to the sea floor may somewhat affect the quantity of the inorganic sediments, but, as a whole, the downfalling of these fragments is probably at a singularly uniform rate. It is otherwise with the contributions of sediment arising from organic forms. This varies in a surprising measure. On the coral reefs, such as form in the mid oceans, the proportion of matter which has not come into the accumulation through the bodies of animals and plants may be as small as one tenth of one per cent, or less. In the deeper seas, it is doubtful whether the rate of animal growth is such as to permit the formation of any beds which have less than one half of their mass made up of materials which fell through the water.

In certain areas of the open seas the upper part of the water is dwelt in by a host of creatures, mostly foraminifera, which extract limestone from the water, and, on dying, send their shells to the bottom. Thus in the North Atlantic, even where the sea floor is of great depth beneath the surface, there is constantly accumulating a mass of limy matter, which is forming very massive limestone strata, somewhat resembling chalk deposits, such as abundantly occur in Great Britain, in the neighbouring parts of Europe, in Texas, and elsewhere. Accumulations such as this, where the supply is derived from the surface of the water, are not affected by the accidents which divide beds made on the bottom in the manner before described. They may, therefore, have the singularly continuous character which we note in the English chalk, where, for the thickness of hundreds of feet, we may have no evident partitions, except certain divisions, which have evidently originated long after the beds were formed.

We have already noted the fact that, while the floors of the deeper seas appear to lack mountainous elevations, those arising from the folding of strata, they are plentifully scattered over with volcanic cones. We may therefore suppose that, in general, the deposits formed on the sea floor are to a great extent affected by the materials which these vents cast forth. Lava streams and showers represent only a part of the contributions from volcanoes, which finally find their way to the bottom. In larger part, the materials thrown forth are probably first dissolved in the water and then taken up by the organic species; only after the death of these creatures does the waste go to the bottom. As hosts of these creatures have no solid skeleton to contribute to the sea floor, such mineral matter as they may obtain is after their death at once restored to the sea.

Not only does the contribution of organic sediment diminish in quantity with the depth which is attained, but the deeper parts of the ocean bed appear to be in a condition where no accumulations of this nature are made, and this for the reason that the water dissolves the organic matter more rapidly than it is laid down. Thus in place of limestone, which would otherwise form, we have only a claylike residuum, such as is obtained when we dissolve lime rocks in acids. This process of solution, by which the limy matter deposited on the bottom is taken back into the water, goes on everywhere, but at a rate which increases with the depth. This increase is due in part to the augmentation of pressure, and in part to the larger share of carbonic dioxide which the water at great depths holds. The result is, that explorations with the dredge seem to indicate that on certain parts of the deeper sea floors the rocks are undergoing a process of dissolution comparable to that which takes place in limestone caverns. So considerable is the solvent work that a large part of the inorganic waste appears to be taken up by the waters, so as to leave the bottom essentially without sedimentary accumulations. The sea, in a word, appears to be eating into rocks which it laid down before the depression attained its present great depth.

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 _Sargassum_ 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 passes 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 shores the currents of the sea which, except the massive 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 shore, 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 quantity in a pound of the fluid often amounting to the fiftieth part of its weight. He will thus perceive that along the shore 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 associate 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 shore belt is due to the fact that the waters are there subjected to a constant process of aëration 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 shell, 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.

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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 accumulations, 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 mass commonly becomes much more solid, passing 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 masses, 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 shifting 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 illustration the first, or translatory motion, may be compared to that which takes place when a bell is carried along upon a locomotive or a ship; 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 masses 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 masses 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 stopcock. Unless the plumber provides a cushion 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 cushion 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 mass 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 glass, 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 passing through dense crystalline strata encounters a mass 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 passed 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 mass, 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 illustrations. 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 masses down steep slopes, which is induced by the shaking. Everywhere on the land the agents of decay and erosion tend to bring heavy masses 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 accumulated 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 masses of loose material. Then for weeks they poured forth torrents of mud and the _débris_ 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 mass, 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 constitute 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 multitude, 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.