Chapter IX The Rocks and Their Order
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.
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.
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 débris 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 passing 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 shore the front of the undulation is retarded 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 shore, 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 shore, vessels, however well anchored, 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 ships 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 shore, 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 shore.
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 shipping. 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 ship. 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 crushing effect of these quakings which pass 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 diminishing 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 assaults. 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 accumulated 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 Washington 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 massive 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 asserted, 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 passing 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 assault 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 massive type, retain their places and are the fashions 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 Japanese, 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 passes 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 Ætna, 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 accumulated 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 passage. 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 mass 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 attitude 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 accumulated 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 retarded 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 massive 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 latitude 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 quantities of rock, either in the form of detached columns or as detrital accumulations 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 passed away.
The shores 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.
Around the northern Atlantic we almost everywhere find the glacial waste here and there accumulated near the margin of the sea in the complicated sculptured outlines which are assumed by kame sands and gravels. From a study of these features just above the level of high tide, the writer has become convinced that the North Atlantic district has long been exempt from the assaults of other waves than those which are produced during heavy storms. At the present time the waves formed by earthquakes appear to be of destructive violence only on the west coast of South America, where they roll in from a region of the Pacific lying to the south of the equator and a few hundred miles from the shore of the continent, which appears to be the seat of exceedingly violent shocks. A similar field occurs in the Atlantic between the Lesser Antilles and the Spanish peninsula, but no great waves have come thence since the time of the Lisbon earthquake. The basin of the Caribbean and the region about Java appear to be also fields where these disturbances may be expected, though in each but one wave of this nature has been recorded. Therefore we may regard these secondary results of a submarine earthquake as seldom phenomena.
Duration of Geological Time
Although it is beyond the power of man to conceive any such lapses of time as have taken place in the history of this earth, it is interesting, and in certain ways profitable, to determine as near as possible in the measure of years the duration of the events which are recorded in the rocks. Some astronomers, basing their conclusions on the heat-containing power of matter, and on the rate at which energy in this form flows from the sun, have come to the conclusion that our planet could not have been in independent existence for more than about twenty million years. The geologist, however, resting his conclusions on the records which are the subject of his inquiry, comes on many different lines to an opinion which traverses that entertained by some distinguished astronomers. The ways in which the student of the earth arrives at this opinion will now be set forth.
By noting the amount of sediment carried forth to the sea by the rivers, the geologist finds that the lands of the earth—those, at least, which are protected by their natural envelopes of vegetation—are wearing down at a rate which pretty certainly does not exceed one foot in about five thousand years, or two hundred feet in a million years. Discovering at many places on the earth’s surface deposits which originally had a thickness of five thousand feet or more, which have been worn down to the depths of thousands of feet in a single rather brief section of geological time, the student readily finds himself prepared to claim that a period of from five to ten million years has often been required for the accomplishment of but a very small part of the changes which he knows to have occurred on this earth.
As the geologist follows down through the sections of the stratified rocks, and from the remains of strata determines the erosion which has borne away the greater part of the thick deposits which have been exposed to erosion, he comes upon one of those breaks in the succession, or encounters what is called an unconformity, as when horizontal strata lie against those which are tilted. In many cases he may observe that at this time there was a great interval unrepresented by deposits at the place where his observations are made, yet a great lapse of time is indicated by the fact that a large amount of erosion took place in the interval between the two sets of beds.
Putting together the bits of record, and assuming that the rate of erosion accomplished by the agents which operate on the land has always been about the same, the geologist comes to the conclusion that the section of the rocks from the present day to the lowest strata of the Laurentian represents in the time required for their formation not less than a hundred million years; more likely twice that duration. To this argument objection is made by some naturalists that the agents of erosion may have been more active in the past than they are at present. They suggest that the rainfall may have been much greater or the tides higher than they now are. Granting all that can be claimed on this score, we note the fact that the rate of erosion evidently does not increase in anything like a proportionate way with the amount of rainfall. Where a country is protected by its natural coating of vegetation, the rain is delivered to the streams without making any considerable assault upon the surface of the earth, however large the fall may be. Moreover, the tides have little direct cutting power; they can only remove detritus which other agents have brought into a condition to be borne away. The direct cutting power of the tidal movement does not seem to be much greater in the Bay of Fundy, where the maximum height of the waves amounts to fifty feet, than on the southern coast of Massachusetts, where the range is not more than five. So far as the observer can judge, the climatal conditions and the other influences which affect the wear of rocks have not greatly varied in the past from what they are at the present day. Now and then there have been periods of excessive erosion; again, ages in which large fields were in the conditions of exceeding drought. It is, however, a fair presumption that these periods in a way balance each other, and that the average state was much like that which we find at present.
If after studying the erosive phenomena exhibited in the structure of the earth the student takes up the study of the accumulations of strata, and endeavours to determine the time required for the laying down of the sediments, he finds similar evidence of the earth’s great antiquity. Although the process of deposition, which has given us the rocks visible in the land masses, has been very much interrupted, the section which is made by grouping the observations made in various fields shows that something like a maximum thickness of a hundred and fifty thousand feet of beds has been accumulated in that part of geologic time during which strata were being laid down in the fields that are subjected to our study. Although in these rocks there are many sets of beds which were rapidly formed, the greater part of them have been accumulated with exceeding slowness. Many fine shales, such as those which plentifully occur in the Devonian beds of this country, must have required a thousand years or more for the deposition of the materials that now occupy an inch in depth. In those sections a single foot of the rock may well represent a period of ten thousand years. In many of the limestones the rate of accumulation could hardly have been more speedy. The reckoning has to be rough, but the impression which such studies make upon the mind of the unprejudiced observer is to the effect that the thirty miles or so of sedimentary deposits could not have been formed in less than a hundred million years. In this reckoning it should be noted that no account is taken of those great intervals of unrecorded time, such as elapsed between the close of the Laurentian and the beginning of the Cambrian periods.
There is a third way in which we may seek an interpretation of duration from the rocks. In each successive stage of the earth’s history, in different measure in the various ages, mountains were formed which in time, during their exposure to the conditions of the land, were worn down to their roots and covered by deposits accumulated during the succeeding ages. A score or more of these successively constructed series of elevations may readily be observed. Of old, it was believed that mountain ranges were suddenly formed, but there is, however, ample evidence to prove that these disturbed portions of the strata were very gradually dislocated, the rate of the mountainous growth having been, in general, no greater in the past than it is at the present day, when, as we know full well, the movements are going on so slowly that they escape observation. Only here and there, as an attendant on earthquake shocks or other related movements of the crust, do we find any trace of the upward march which produces these elevations. Although not a subject for exact measurements, these features of mountain growth indicate a vast lapse of time, during which the elevations were formed and worn away.
Yet another and very different method by which we may obtain some gauge of the depths of the past is to be found in the steps which have led organic life from its lowest and earliest known forms to the present state of advancement. Taking the changes of species which have occurred since the beginning of the last ice epoch, we find that the changes which have been made in the organic life have been very small; no naturalist who has obtained a clear idea of the facts will question the statement that they are not a thousandth part of the alterations which have occurred since the Laurentian time. The writer is of the opinion that they do not represent the ten thousandth part of those vast changes. These changes are limited in the main to the disappearance of a few forms, and to slight modifications in those previously in existence which have survived to the present day. So far as we can judge, no considerable step in the organic series has taken place in this last great period of the earth’s history, although it has been a period when, as before noted, all the conditions have combined to induce rapid modifications in both animals and plants. If, then, we can determine the duration of this period, we may obtain a gauge of some general value.
Although we can not measure in any accurate way the duration of the events which have taken place since the last Glacial period began to wane, a study of the facts seems to show that less than a hundred thousand years can not well be assumed for this interval. Some of the students who have approached the subject are disposed to allow a period of at least twice this length as necessary for the perspective which the train of events exhibits. Reckoning on the lowest estimate, and counting the organic changes which take place during the age as amounting to the thousandth part of the organic changes since the Laurentian age, we find ourselves in face once again of that inconceivable sum which was indicated by the physical record.
Here, again, the critics assert that there may have been periods in the history of the earth when the changes of organic life occurred in a far swifter manner than in this last section of the earth’s history. This supposition is inadmissible, for it rests on no kind of proof; it is, moreover, contraindicated by the evident fact that the advance in the organic series has been more rapid in recent time than at any stage of the past. In a word, all the facts with which the geologist deals are decidedly against the assumption that terrestrial changes in the organic or the inorganic world ever proceed in a spasmodic manner. Here and there, and from time to time, local revolutions of a violent nature undoubtedly occur, but, so far as we may judge from the aspect of the present or the records of the past, these accidents are strictly local; the earth has gone forward in its changes much as it is now advancing. Its revolutions have been those of order rather than those of accident.
The first duty of the naturalist is to take Nature as he finds it. He must avoid supposing any methods of action which are not clearly indicated in the facts that he observes. The history of his own and of all other sciences clearly shows that danger is always incurred where suppositions as to peculiar methods of action are introduced into the interpretation. It required many centuries of labour before the students of the earth came to adopt the principle of explaining the problems with which they had to deal by the evidence that the earth submitted to them. Wherever they trusted to their imaginations for guidance, they fell into error. Those who endeavour to abbreviate our conception of geologic time by supposing that in the olden days the order of events was other than that we now behold are going counter to the best traditions of the science.
Although the aspect of the record of life since the beginning of the Cambrian time indicates a period of at least a hundred million years, it must not be supposed that this is the limit of the time required for the development of the organic series. All the important types of animals were already in existence in that ancient period with the exception of the vertebrates, the remains of which have apparently now been traced down to near the Cambrian level. In other words, at the stage where we first find evidence of living beings the series to which they belong had already climbed very far above the level of lifeless matter. Few naturalists will question the statement that half the work of organic advance had been accomplished at the beginning of the Cambrian rocks. The writer is of the opinion that the development which took place before that age must have required a much longer period than has elapsed from that epoch to the present day. We thus come to the conclusion that the measurement of duration afforded by organic life indicates a yet more lengthened claim of events, and demands more time than appears to be required for the formation of the stratified rocks.
The index of duration afforded by the organic series is probably more trustworthy than that which is found in the sedimentary strata, and this for the reason that the records of those strata have been subjected to numerous and immeasurable breaks, while the development of organic life has of necessity been perfectly continuous. The one record can at any point be broken without interrupting the sequences; the other does not admit of any breaches in the continuity.
The Moon
Set over against the earth—related to, yet contrasted with it in many ways—the moon offers a most profitable object to the student of geology. He should often turn to it for those lessons which will be briefly noted.
In the beginning of their mutual history the materials of earth and moon doubtless formed one vaporous body which had been parted from the concentrating mass of the sun in the manner noted in the sketch of the history of the solar system. After the earth-moon body had gathered into a nebulous sphere, it is most likely that a ring resembling that still existing about Saturn was formed about the earth, which in time consolidated into the satellite. Thenceforth the two bodies were parted, except for the gravitative attraction which impelled them to revolve about their common centre of gravity, and except for the light and heat they might exchange with one another.
The first stages after the parting of the spheres of earth and moon appear to have been essentially the same in each body. Concentrating upon their centres, they became in time fluid by heat; further on, they entered the rigid state—in a word, they froze—at least in their outer parts. At this point in their existence their histories utterly diverge; or rather, we may say, the development of the earth continued in a vast unfolding, while that of the moon appears to have been absolutely arrested in ways which we will now describe.
With the naked eye we see on the moon a considerable variation in the light of different parts of its surface; we discern that the darker patches appear to be rudely circular, and that they run together on their margins. Seeing this little, the ancients fancied that our satellite had seas and lands like the earth. The first telescopes did not dispel their fancies; even down to the early part of this century there were astronomers who believed the moon to be habitable; indeed, they thought to find evidence that it was the dwelling place of intelligent beings who built cities, and who tried to signal their intellectual kindred of this planet. When, however, strong glasses were applied to the exploration, these pleasing fancies were rudely dispelled.
Seen with a telescope of the better sort, the moon reveals itself to be in large part made up of circular depressions, each surrounded by a ringlike wall, with nearly level but rough places between. The largest of these walled areas is some four hundred miles in diameter; thence they grade down to the smallest pits which the glass can disclose, which are probably not over as many feet across. The writer, from a careful study of these pits, has come to the conclusion that the wider are the older and the smaller the last formed. The rude elevations about these pits—some of which rise to the height of ten thousand feet or more—constitute the principal topographic reliefs of the lunar surface. Besides the pits above mentioned, there are numerous fractures in the surface of the plains and ringlike ridges; on the most of these the walls have separated, forming trenches not unlike what we find in the case of some terrestrial breaks such as have been noted about volcanoes and elsewhere. It may be that the so-called canals of Mars are of the same nature.
The most curious feature on the moon’s surface are the bands of lighter colour, which, radiating from certain of the volcanolike pits—those of lesser size and probably of latest origin—extend in some cases for five hundred miles or more across the surface. These light bands have never been adequately explained. It seems most likely that they are stains along the sides of cracks, such as are sometimes observed about volcanoes.
The eminent peculiarity of the moon is that it is destitute of any kind of gaseous or aqueous envelope. That there is no distinct atmosphere is clearly shown by the perfectly sharp and sudden way in which the light of a star disappears when it goes behind the moon and the clear lines of the edge of the satellite in a solar eclipse. The same evidence shows that there is no vapour of water; moreover, a careful search which the writer has made shows that the surface has none of those continuous down grades which mark the work of water flowing over the land. Nearly all of the surface consists of shallow or deep pits, such as could not have been formed by water action. We therefore have not only to conclude that the moon is waterless, but that it has been in this condition ever since the part that is turned toward us was shaped.
As the moon, except for the slight movement termed its “libration,” always turns the same face to us, so that we see in all only about four sevenths of its surface, it has naturally been conjectured that the unseen side, which is probably some miles lower than that turned toward us, might have a different character from that which we behold. There are reasons why this is improbable. In the first place, we see on the extreme border of the moon, when the libration turns one side the farthest around toward the earth, the edge of a number of the great walled pits such as are so plenty on the visible area; it is fair to assume that these rings are completed in the invisible realm. On this basis we can partly map about a third of the hidden side. Furthermore, there are certain bands of light which, though appearing on the visible side, evidently converge to some points on the other. It is reasonable to suppose that, as all other bands radiate from walled pits, these also start from such topographic features. In this way certain likenesses of the hidden area to that which is visible is established, thus making it probable that the whole surface of the satellite has the same character.
Clearly as the greater part of the moon is revealed to us—so clearly, indeed, that it is possible to map any elevation of its surface that attains the height of five hundred feet—the interpretation of its features in the light of geology is a matter of very great difficulty. The main points seem to be tolerably clear; they are as follows: The surface of the moon as we see it is that which was formed when that body, passing from the state of fluidity from heat, formed a solid crust. The pits which we observe on its surface are the depressions which were formed as the mass gradually ceased to boil. The later formed of these openings are the smaller, as would be the case in such a slowing down of a boiling process.
As the diameter of the moon is only about one fourth of that of the earth, its bulk is only about one sixteenth of that of its planet; consequently, it must have cooled to the point of solidification ages before the larger sphere attained that state. It is probable that the same changeless face that we see looked down for millions of years on an earth which was still a seething, fiery mass. In a word, all that vast history which is traceable in the rocks beneath our feet—which is in progress in the seas and lands and is to endure for an inconceivable time to come—has been denied our satellite, for the reason that it had no air with which to entrap the solar heat and no water to apply the solar energy to evolutionary processes. The heat which comes upon the moon as large a share for each equal area as it comes upon the earth flies at once away from the airless surface, at most giving it a temporary warmth, but instituting no geological work unless it be a little movement from the expansion and contraction of the rocks. During the ages in which the moon has remained thus lifeless the earth, owing to its air and water, has applied a vast amount of solar energy to geological work in the development and redevelopment of its geological features and to the processes of organic life. We thus see the fundamental importance of the volatile envelopes of our sphere, how absolutely they have determined its history.
It would be interesting to consider the causes which led to the absence of air and water on the moon, but this matter is one of the most debatable of all that relates to that sphere; we shall therefore have to content ourselves with the above brief statements as to the vast and far-acting effects which have arisen from the non-existence of those envelopes on our nearest neighbour of the heavens.
Methods in Studying Geology
So far as possible the preceding pages, by the method adopted in the presentation of facts, will serve to show the student the ways in which he may best undertake to trace the order of events exhibited in the phenomena of the earth. Following the plan pursued, we shall now consider certain special points which need to be noted by those who would adopt the methods of the geologist.
At the outset of his studies it may be well for the inquirer to note the fact that familiarity with the world about him leads the man in all cases to a certain neglect and contempt of all the familiar presentations of Nature. We inevitably forget that those points of light in the firmament are vast suns, and we overlook the fact that the soil beneath our feet is not mere dirt, but a marvellous structure, more complicated in its processes than the chemist’s laboratory, from which the sustenance of our own and all other lives is drawn. We feel our own bodies as dear but commonplace possessions, though we should understand them as inheritances from the inconceivable past, which have come to us through tens of thousands of different species and hundreds of millions of individual ancestors. We must overlook these things in our common life. If we could take them into account, each soul would carry the universe as an intellectual burden.
It is, however, well from time to time to contemplate the truth, and to force ourselves to see that all this apparently simple and ordinary medley of the world about us is a part of a vast procession of events, coming forth from the darkness of the past and moving on beyond the light of the present day. Even in his professional work the naturalist of necessity falls into the commonplace way of regarding the facts with which he deals. If he be an astronomer, he catalogues the stars with little more sense of the immensities than the man who keeps a shop takes account of his wares. Nevertheless, the real profit of all learning is in the largeness of the understanding which it develops in man. The periods of growth in knowledge are those in which the mind, enriched by its store, enlarges its conception while it escapes from commonplace ways of thought. With this brief mention of what is by far the most important principle of guidance which the student can follow, we will turn to the questions of method that the student need follow in his ordinary work.
With almost all students a difficulty is encountered which hinders them in acquiring any large views as to the world about them. This is due to the fact that they can not make and retain in memory clear pictures of the things they see. They remember words rather than things—in fact, the training in language, which is so large a part of an education, tends ever to diminish the element of visual memory. The first task of the student who would become a naturalist is to take his knowledge from the thing, and to remember it by the mental picture of the thing. In all education in Nature, whether the student is guided by his own understanding or that of the teacher, a first and very continuous aim should be to enforce the habit of recalling very distinct images of all objects which it is desired to remember. To this end the student should practise himself by looking intently upon a landscape or any other object; then, turning away, he should try to recall what he has beheld. After a moment the impression by the sight should be repeated, and the study of the memory renewed. The writer knows by his own experience that even in middle-aged people, where it is hard to breed new habits, such deliberate training can greatly increase the capacity of the memory for taking in and reproducing images which are deemed of importance. Practice of this kind should form a part of every naturalist’s daily routine. After a certain time, it need not be consciously done. The movements of thought and action will, indeed, become as automatic as those which the trained fencer makes with his foil.
Along with the habit of visualizing memories, and of storing them without the use of words, the student should undertake to enlarge his powers of conceiving spaces and directions as they exist in the field about him. Among savages and animals below the grade of man, this understanding of spacial relations is very clear and strong. It enables the primitive man to find his way through the trackless forest, and the carrier pigeon to recover his mate and dwelling place from the distance of hundreds of miles away. In civilized men, however, the habit of the home and street and the disuse of the ancient freedom has dulled, and in some instances almost destroyed, all sense of this shape of the external world. The best training to recover this precious capacity will now be set forth.
The student should begin by drawing a map on a true scale, however roughly the work may be done, of those features of the earth about him with which he is necessarily most familiar. The task may well be begun with his own dwelling or his schoolroom. Thence it may be extended so as to include the plan of the neighbouring streets or fields. At first, only directions and distances should be platted. After a time to these indications should be added on the map lines indicating in a general way contours or the lines formed by horizontal planes intersecting the area subject to delineation. After attaining certain rude skill in such work, the student may advantageously make excursions to districts which he can see only in a hurried way. As he goes, he should endeavour to note on a sketch map the positions of the hills and streams and the directions of the roads. A year of holiday practice in such work will, if the tasks occupy somewhere about a hundred hours of his time, serve greatly to extend or reawaken what may be called the topographic sense, and enable him to place in terms of space the observations of Nature which he may make.
In his more detailed work the student should select some particular field for his inquiry. If he be specially interested in geologic phenomena, he will best begin by noting two classes of facts—those exhibited in the rocks as they actually appear in the state of repose as shown in the outcrops of his neighbourhood, and those shown in the active manifestations of geological work, the decay of the rocks and the transportation of their waste, or, if the conditions favour, the complicated phenomena of the seashores.
As soon as the student begins to observe, he should begin to make a record of his studies. To the novice in any science written, and particularly sketched, notes are of the utmost importance. These, whether in words or in drawings, should be made in face of the facts; they should, indeed, be set down at the close of an observation, though not until the observer feels that the object he is studying has yielded to him all which it can at that time give. It is well to remark that where a record is made at the outset of a study the student is apt to feel that he is in some way pledged to shape all he may see to fit that which he has first written. In his early experience as a teacher, the writer was accustomed to have students compare their work of observation and delineation with that done by trained men on the same ground. It now seems to him best for the beginner at first to avoid all such reference of his own work to that of others. So great is the need of developing independent motive that it is better at the outset to make many blunders than to secure accuracy by trust in a leader. The skilful teacher can give fitting words of caution which may help a student to find the true way, but any reference of his undertakings to masterpieces is sure to breed a servile habit. Therefore such comparisons are fitting only after the habit of free work has been well formed. The student who can afford the help of a master, or, better, the assistance of many, such as some of our universities offer, should by all means avail himself of this resource. More than any other science, geology, because of the complexity of the considerations with which it has to deal, depends upon methods of labour which are to a great extent traditional, and which can not, indeed, be well transmitted except in the personal way. In the distinctly limited sciences, such as mathematics, physics, or even those which deal with organic bodies, the methods of work can be so far set forth in printed directions that the student may to a great extent acquire sound ways of work without the help of a teacher.
Although there is a vast and important literature concerning geology, the greater part of it is of a very special nature, and will convey to the beginner no substantial information whatever. It is not until he has become familiar with the field with which he is enabled to deal in the actual way that he can transfer experience thus acquired to other grounds. Therefore beyond the pleasing views which he may obtain by reading certain general works on the science, the student should at the outset of his inquiry limit his work as far as possible to his field of practice, using a good text-book, such as Dana’s Manual of Geology, as a source of suggestions as to the problems which his field may afford.
The main aim of the student in this, as in other branches of inquiry, is to gain practice in following out the natural series of actions. To the primitive man the phenomenal world presents itself as a mere phantasmagoria, a vast show in which the things seen are only related to each other by the fact that they come at once into view. The end of science is to divine the order of this host, and the ways in which it is marshalled in its onward movement and the ends to which its march appears to be directed. So far as the student observes well, and thus gains a clear notion of separated facts, he is in a fair way to gather the data of knowledge which may be useful; but the real value of these discernments is not gained until the observations go together, so as to make something with a perspective. Until the store of separate facts is thus arranged, it is merely crude material for thought; it is not in the true meaning science, any more than a store of stone and mortar is architecture. When the student has developed an appetite for the appreciation of order and sources of energy in phenomena, he has passed his novitiate, and becomes one of that happy body of men who not only see what is perceived by the mass of their fellows, but are enabled to look through those chains of action which, when comprehended, serve to rationalize and ennoble all that the senses of man, aided by the instruments which he has devised, tell us concerning the visible world.
Notes
1 Some astronomers, particularly the distinguished Professor Newcomb, hold that the sun can not have been supplying heat as at present for more than about ten million years, and that all geological time must be thus limited. The geologist believes that this reckoning is far too short.
2 See Newcomb’s Popular Astronomy, p. 234. Harper Brothers, New York.
3 The present writer has made an extended and careful study of marsh and swamp phenomena, and is very familiar with the aspect of these fields in the nighttime. He has never been able to see any sign of the Jack-o’-lantern light. Looking fixedly into any darkness, such as is afforded by the depths of a wood, the eye is apt to imagine the appearance of faint lights. Those who have had to do with outpost duty in an army know how the anxious sentry, particularly if he is new to the soldier’s trade, will often imagine that he sees lights before him. Sometimes the pickets will be so convinced of the fact that they see lights that they will fire upon the fiction of the imaginations. These facts make it seem probable that the Jack-o’-lantern and his companion, the Will-o’-the-wisp, are stories of the overcredulous.
4 This principle of the spheroidal form in falling fluids is used in making ordinary bird shot. The melted lead drops through sievelike openings, the resulting spheres of the metal being allowed to fall into water which chills them. Iron shot, used in cutting stone, where they are placed between the saw and the surface of the rock, are also made in the same manner. The descending fluid divides into drops because it is drawn out by the ever-increasing speed of the falling particles, which soon make the stream so thin that it can not hold together.
5 In some relatively rare cases salt deposits are formed in lagoons along the shores of arid lands, where the sea occasionally breaks over the beach into the basin, affording waters which are evaporated, leaving their salt behind them.
6 In certain fields of permanent snow, particularly near their boundaries, some very lowly forms of vegetable life may develop on a frozen surface, drawing their sustenance from the air, and supplied with water by the melting which takes place during the summertime. These forms include the rare phenomenon termed red snow.
7 It is reported that one of these natural bridges of Carter County has recently fallen down. This is the natural end of these features. As before remarked, they are but the remnants of much more extensive roofs which the processes of decay have brought to ruin.
8 I venture to use this word in place of the phrase “lava-yielding” for the reason that the term is needed in the description of volcanoes.
9 In part the excellent record of Vesuvius is due to the fact that since the early Christian centuries the priests of St. Januarius, the patron of Naples, have been accustomed to carry his relics in procession whenever an eruption began. The cessation of the outbreak has been written down to the credit of the saint, and thus we are provided with a long story of the successive outbreaks.