Chapter IX (1)

THE MOVEMENTS AND DEFORMATIONS OF THE EARTH’S BODY
(DIASTROPHISM).

The body of the earth is subject to an infinite variety of movements, ranging from the almost inconceivably rapid to the almost imperceptibly slow, and from the almost immeasurably minute to the enormously massive; but, for practical treatment, they fall mainly into two couplets: (1) the minute and rapid, and (2) the slow and massive. Sudden movements of local masses, giving rise to intense vibrations, are put in the first class. There are innumerable minute and slow movements, but unless they rise to appreciable magnitude by long continuance, they are neglected.

MINUTE AND RAPID MOVEMENTS.

The crust of the earth is in a state of perpetual tremor. For the most part, these tremors are too minute to be sensible, but are revealed by delicate instrumental devices. Some of them are but the declining stages of sensible vibrations, but others are minute from their inception. Many of them spring from the ordinary incidents of the surface, and claim attention chiefly as obstacles to the study of more significant oscillations. Winds, waves, waterfalls, the tread of animals, the rumble of traffic, the blasts of mines, the changes of temperature, the variations in atmospheric pressure, the weighting of rainfall and the lightening of evaporation, the rupture of rock or ice or frozen earth, and many other processes, make their contributions to local and minute movements. For the greater part, these vibrations are superficial in origin, and are soon damped beyond recognition by dispersal and by the inelastic and discontinuous nature of the looser material of the surface. When a temporary rigid crust is formed by freezing, as in winter, these surface vibrations are transmitted with much less loss, and the distances at which the rumble of winter traffic is heard, is a good illustration of the function of continuity and solidity in the conveyance of vibrations.

_Earthquakes._

When the tremors spring from sources within the earth itself and are of appreciable violence, they are recognized as earthquakes. The sources of earthquake tremors are various. The most prevalent is probably the fracture of rocks and the slipping of strata on each other in the process of faulting. The interpretation of movements of this class has now been so far perfected that the length and depth of the fault, the amount of the slip, and the direction of the hade are capable of approximate estimation.[224] To the same class belong the movements due to slumping. They are illustrated by the sliding and arrest of great masses of sediment along the steep fronts of deltas, and of the accumulations of deep-sea oozes on steep submarine slopes. Such slumping is, in reality, superficial faulting. Seismic tremors often attend volcanic eruptions, and are then probably attributable to the sudden fracture and displacement of rock by the penetration of lava, or by rapid and unequal heating. They are perhaps also due sometimes to the sudden generation or cooling of steam in underground conduits, crevices, and caverns, the action possibly being in some cases of the “water-hammer” type. In rare instances, probably, the bursting of beds overlying pent-up non-volcanic gases may give origin to earthquakes. A more superficial source of earthquake vibrations is the collapse of the roofs of subterranean caverns.

Seismic vibrations seem to be in part compressional, in part distortional, in part (on the surface) undulatory, and in part irregular. The distortional are especially significant, as they seem to imply a solid medium of transmission.

=Points of origin, foci.=—It is probable that nearly or quite all earthquake movements start within the upper ten miles of the crust, and most of them within the upper five. Some of the earlier estimates indeed placed the points of origin as deep as 20 or 30 miles, but in these cases the necessary corrections, discussed below, were neglected. Most of the recent and more accurate estimates fall within the limits given.

The method of estimating the depth of the centers of disturbance consists in observing the directions of throw or thrust of bodies at the surface, and in regarding these as representing the lines of emergence of the earthquake-waves. By plotting these lines of emergence, and projecting them backwards to their underground crossings, a first approximation to the location of the focus is reached (the lines _EF′_, Fig. 446). From the nature of the case, the observations of the angles of emergence cannot be very accurate, but an effort is made to limit the error by making the number of observations great.

Two systematic corrections are to be applied to all such estimates, the one for varying elasticity and density, and the other for varying continuity. Both reduce the estimated depth. In making the correction for varying elasticity, it must be noted that the velocity of vibrations varies directly as the square root of the elasticity, and inversely as the square root of the density. The velocity is also accelerated by increase of temperature. The elasticity, temperature, and density all increase with depth. Theoretically, the increase of velocity due to the increasing elasticity and temperature of increasing depths, overbalances the retardation due to increasing density, and recent observations on the transmission of seismic waves through deep chords of the earth have confirmed this conclusion. The path of the vibration will, therefore, be curved toward the surface, as pointed out by Schmidt and illustrated in Fig. 446, taken from his discussion.[225] From this it is clear that the focus is not so deep as implied by the simple backward projection of the lines of emergence.

A second correction must be made for the differences of continuity of the upper rock in the vertical and horizontal directions. In the outer part of the earth, the continuity in horizontal directions is interrupted by vertical fissures. Were these not usually filled with water, they would soon kill the horizontal component of the seismic wave, and the residual portion would be directed almost vertically to the surface, for the width of the fissures is almost always greater than the amplitude of the seismic vibrations. The water restores the continuity, in a measure, but not perfectly, for the elasticity of water is much less than that of rock. It is clear that in horizontal movement there must be a constant transfer from rock to water and from water to rock, and this must retard, as well as partially destroy, the vibrations. In a vertical direction, however, the rocks rest firmly upon one another, and this gives measurable continuity, the only change being from one layer or kind of rock to another. It seems certain, therefore, that the vertical component of the seismic wave will be less damped and less retarded in transmission than the horizontal. It will, therefore, reach the surface sooner and will have the greater effect on bodies at the surface, not only for the reasons given, but also because it emerges more nearly in the line of least resistance and of freest projection. On this account, a second correction must be added to the correction for elasticity, and this must further reduce appreciably the first estimate of the depth of the focus.

Observation shows that in some way a seismic wave becomes separated in transmission into portions of different natures and speeds, but their interpretation is yet uncertain. These separated portions probably consist of the compressional, the distortional, and the undulatory waves, and perhaps of refractions and reflections of these (see Fig. 448).

A most important recent achievement is the detection and investigation of seismic tremors that appear to have come _through_ the earth. The transmission of such waves promises to reveal much relative to the nature of the deep interior, when enough data are gathered to warrant conclusions. The rate of propagation in the central parts is found to be greater than in the outer parts, implying high elasticity within.

=The amplitude of the vibrations.=—From the very disastrous effects of severe earthquakes, it is natural to infer that the distinctive oscillations must have large amplitude, but in fact it is the suddenness of the vibration, rather than its length, that is effective. Instrumental investigations indicate that the oscillations, after they have left their points of origin, are usually only a fraction of a millimeter in amplitude; at most they seldom exceed a few millimeters. A sudden shock with an amplitude of 5 or 6 millimeters is sufficient to shatter a chimney. It is true that estimates assigning amplitudes of a foot or more have been made, but their correctness is open to serious doubt. It should be understood that it is the length of oscillation of the particles of the subsurface rock transmitting the vibrations that is referred to, not the movement of the free surface, or of objects on the surface. The throw at and on the surface is much greater. Just as a slight, quick tap of a hammer on a floor is sufficient to make a marble lying on it bound several inches, so a sufficiently sudden rise of the surface of the earth, though but a fraction of an inch, may project loose bodies many feet.

=Destructive effects.=—The interpretation of the disastrous results of earthquake shocks has, therefore, its key in the suddenness and strength of rather minute vibrations of the earth-matter, but it is also dependent on the freedom of motion of the bodies affected. The rocks of the deeper zones, where the matter is sensibly continuous, transmit the seismic vibrations without appreciable disruptive effect, so far as known, though the origin of crevices has been assigned to this cause; but bodies at the surface are fractured, overturned, and hurled from their places. The reason is doubtless this: Within a great mass firmly held in place by cohesion and pressure on all sides, the forward motion of a particle develops an equal elastic resistance, and it is quickly thrown back again and the wave passes on. At the surface, where bodies are freer to move, the stroke of the vibration projects the body, and so, instead of vibratory resilience, the chief energy is converted into mass-motion. The tap of a hammer sends an almost imperceptible vibration along the floor, but this vibration may throw a glass ball, beneath which it runs, into the air. So the minute vibrations of earth-matter may travel miles from their origin through continuous substance with little result, and then so suddenly thrust a loose body on the surface, or the base of a column, or the foundation of a house, as to rack it with differential strains, or even to hurl it to destruction. So, too, earth-waves striking the sea-border may thrust the waters off shore by their sudden impact, and the reaction may develop a wave which overwhelms the coast. Such waves may doubtless arise from a sudden stroke of seismic vibrations on the sea-bottom. The great gaping fissures that sometimes open during earthquakes occur oftenest where the surface on one side is less well supported than on the other, as on a slope, or near a bluff-face or a river-channel. When in such situations the earth is once suddenly forced in the direction of least resistance, it is not always met by sufficient elastic resistance to throw it back. Sometimes, however, there is an elastic return, and the fissure closes forcibly an instant after it is opened.

=Direction of throw.=—Immediately above the point of origin, technically the _epicentrum_ or _epi-focal point_, bodies are projected upwards. When crushing takes place in such a case, it is due to the upthrust or to the return downfall. At one side of the epicentrum the thrust is oblique in various degrees, and is usually more destructive, if not too far from the epicentrum. The destructiveness commonly increases for a certain distance from the epi-focal point, and then diminishes. Under ideal conditions, the greatest effects are found where the vibration emerges at an angle of about 45°, but various influences modify this result. Lines drawn through points of equal effect (isoseismals) are not usually regular circles or ellipses about the epicentrum, as they would be under ideal conditions. The various divergencies represent differences of effective elasticity, of surface, and of other influences. As most earthquakes originate from lines, planes, or masses, rather than points, there are doubtless differences of intensity of vibration at different points on the lines, planes, or areas of origin, and these differences introduce inequalities in propagation and in surface effects.

=Rate of propagation.=—The progress of a seismic wave varies very greatly. Both experimental tests and natural observations give very discordant results. At present, they justify only the broad statement that the velocity of propagation varies from several hundreds to several thousands of feet per second at the surface. The rate seems to be greater for strong vibrations than for weak ones, and hence it is faster near the origin than farther away. The strength of a vibration dies away, theoretically, according to the inverse square of the distance from the point of origin. Practically there is to be added to this the partial destruction of the vibrations by conversion into other forms of motion.

=Sequences of vibrations.=—Near the source, the main shocks are apt to come suddenly and to be followed by minor tremors. At a distance there are usually “preliminary” vibrations followed by the main tremors, and these by others of gradually diminishing value. This development is assigned to different rates of propagation, and to refractions and reflections not unlike the prolongation of thunder (see Fig. 448). This deployment of the vibrations is notably developed in the shocks that pass long distances through the earth. The vibrations of the first phase are regarded as compressional, those of the second as distortional, while the largest oscillations which arise still later perhaps come around the surface, and may be undulatory, though their nature is not yet determined.

There is often, however, a true succession of original shocks caused by a succession of slips or ruptures at the source. Sometimes these are exceedingly persistent, running through days, weeks, or even months. In such cases a slow faulting is probably in progress, and little slips and stops follow in close succession. In one instance as many as 600 shocks in ten days have been reported.[226]

=Gaseous emanations.=—Vapors and gas frequently issue from earthquake rents, and are popularly made to serve as causes, but they are usually merely the earth gases that are permitted to escape by the rending of the ground, or are forced out by readjustment of the shaken beds. Like other subterranean gases, they are often sulphurous, and they are sometimes hot, especially in volcanic regions. Where the shocks are connected with eruptions, the gases may be truly volcanic.

=Distribution of earthquakes.=—Over large portions of the globe, severe earthquakes are exceedingly rare, but in certain regions they are unfortunately frequent. For the most part, these are volcanic districts, but this is by no means a universal relation. Earthquakes and volcanoes are only in part associates. In general, it may be said that earthquakes are frequent where geologic changes are in rapid progress, as along belts of young mountains, where the stresses are not yet adjusted, or at the mouths of great streams, where deltas are accumulating, or about volcanoes, where temperatures and strains are changing, or on the great slopes, particularly the submarine slopes, where readjustments in response to inequalities of surface stress are in progress. Not a few, however, occur where the special occasion is not at all obvious.

_The Geologic Effects of Earthquakes._

Earthquakes are of much less importance, geologically, than many gentler movements and activities. Disastrous as they sometimes are to human affairs, they leave few distinct and readily identifiable marks which are more than temporary.

=Fracturing of rock.=—During the passage of notable earthquake waves, the solid rock is probably often fractured (see p. 509), though where it is covered by deep soil the fractures are rarely observable at the surface. Elsewhere the crevices are readily seen, especially if they gape. In a few instances surface-rock has been seen to be thoroughly shattered after the passage of an earthquake, as in the Concepcion earthquake of 1835.[227] Joints which were before closed are often opened during an earthquake. Thus in northern Arizona, not far from Canyon Diablo, there is a crevice traceable for a considerable distance, which is said to have been opened during an earthquake. Locally, it gaps several feet. Other notable earthquake fissures have been recorded in India,[228] Japan, and New Zealand. During an earthquake which shook the South Island of New Zealand in 1848, “a fissure was formed averaging 18 inches in width, and traceable for a distance of 60 miles, parallel to the axis of the adjacent mountain chain.”[229] The development of fractures or the opening of joints is sometimes accompanied by faulting. This was the case in Japan during the earthquake of October 28, 1891, when the surface on one side of a fissure, which could be traced for 40 miles, sank 2 to 20 feet. In this case there was also notable horizontal displacement, the east wall of the fissure being thrust locally as much as 13 feet to the north.[230]

=Changes of surface.=—Circular surface openings or basins are sometimes developed during earthquakes. This was the case during the Charleston earthquake of 1886,[231] and similar effects have been noted elsewhere. These openings often serve as avenues of escape for ground-water, gases, and vapor. They are commonly supposed to be the result of the collapse of caverns, or other subterranean openings, the collapse often causing the forcible ejection of water. Such openings are likely to be formed only where the surface material is incoherent. Sandstone dikes (p. 514) may perhaps be associated in origin with earthquakes.

Earthquakes are likely to dislodge masses of rock in unstable positions, as on slopes or cliffs. They may also occasion slumps and landslides.[232]

=Effects on drainage.=—The fracturing of the rock may interfere with the movement of ground-water. After new cracks are developed, or old ones opened or closed, the movement of ground-water adapts itself to the new conditions. It follows that springs sometimes cease to flow after an earthquake, while new ones break out where there had been none before. The character of the water of springs is sometimes changed, presumably because it comes from different sources after the earthquake. Joints may be so widened as to intercept rivulets, and the waters thus intercepted may cause the further enlargement of the opening. Illustrations of this sort are furnished by the earthquakes of the Mississippi valley (Lat. 36° to 38°) in 1811–12. Where faults accompany earthquakes, they occasion ponds or falls where they cross streams. Illustrations of both were furnished by the Chedrang River of India after the earthquake of 1897.[233]

=Effects on standing water.=—Some of the most destructive effects of earthquakes are felt along the borders of the sea. Thus the great sea-wave of the Lisbon earthquake (1755) and that of the earthquake which affected the coasts of Ecuador and Peru in 1868 are examples. Such waves have been known to advance on the land as walls of water 60 feet in height. They are most destructive along low coasts, for here the water may sweep much more extensively over the land. The great loss of life during an earthquake has usually been the direct result of the great waves. Lakes are also affected by earthquakes, their waters sometimes rising and falling for several hours after the initial disturbance, but lake-waves are much feebler than those of the sea, and are not often destructive.

Earthquake shocks are sometimes remarkably destructive to the life of lakes and seas. Thus during the Indian earthquake of 1897, “fishes were killed in myriads as by the explosion of a dynamite cartridge ... and for days after the earthquake, the river (Sumesari) was choked with thousands of dead fish ... and two floating carcasses of Gangetic dolphins were seen which had been killed by the shock.”[234] This wholesale destruction of life is of interest, since the surfaces of layers of rock, often of great age, are sometimes covered with fossils of fish or other animal forms, so numerous and so preserved as to indicate that the animals were killed suddenly and in great numbers, and their bodies quickly buried. It has been suggested that such rock surfaces may be memorials of ancient earthquake shocks.[235]

=Changes of level.=—Permanent changes of level sometimes accompany an earthquake. Thus after the earthquake of 1822 “the coast of Chili for a long distance was said to have risen 3 or 4 feet.”[236] Similar results have occurred on the same coast at other times, and on other coasts at various times. Depression of the surface is perhaps even more common than elevation. Thus on the coast of India all except the higher parts of an area 60 square miles in extent were sunk below the sea during an earthquake in 1762. Widespread depression in the vicinity of the Mississippi in Missouri, Arkansas, Kentucky, and Tennessee accompanied the earthquakes of 1811 and 1812. Some of the depressed areas were converted into marshes, while others became the sites of permanent lakes. Reelfoot Lake, mainly in Tennessee, is an example. Change of level is involved wherever there is faulting, and faulting is probably rather common in connection with earthquakes.

Changes of level are not confined to the land. Where earthquake disturbances affect the sea-bottom in regions of telegraph cables, the cables are often broken. In such cases notable changes have sometimes been discovered and recorded when the cables were repaired. Striking examples are furnished by the region about Greece.[237] In one instance (1873) the repairing vessel found about 2000 feet of water where about 1400 feet existed when the cable was laid. In another instance (1878) the bottom was “so irregular and uneven for a distance of about two miles, that a detour was made and the cable lengthened by five or six miles.” In still another case (1885) the repairing vessel found a “difference of 1500 feet between the bow and stern soundings.” These records point to sea-bottom faulting on a large scale.

It is probably no nearer the truth to say that changes of level result from earthquakes than to say that earthquakes result from changes of level. The two classes of phenomena are probably to be referred to a common cause.[238]

SLOW MASSIVE MOVEMENTS.

It is a far cry from the intense and inconceivably rapid oscillations of the earthquake, to the excessively slow subsidences of continents, or even the slow wrinkling of mountain folds. Not infrequently rivers wear down their channels across a mountain range as fast as it rises athwart them. The movements of continents are even more deliberate. But, far apart as these contrasted movements are, in rate and method, they are associated in ultimate causation, and the earthquake shock is often merely an incident in the formation of a mountain range or in the subsidence of a continent.

The great movements are usually classed (1) as continent-making (epeirogenic) and (2) mountain-making (orogenic). They may also be classed as (1) vertical movements and (2) horizontal movements, and dynamically, as (1) thrust movements and (2) stretching movements. It is to be understood that these distinctions are little more than analytical conveniences, for continental movements are often at the same time mountain-making movements; vertical movements are usually involved in horizontal movements, and stretching usually takes part in the processes in which thrust predominates, and _vice versa_. But where one phase greatly preponderates, it may conveniently give name to the whole.

=Present movements.=—Critical observations on seacoasts show that some shores are slowly rising and some slowly sinking _relative to the ocean-level_. We do not certainly know what their movements are relative to the center of the earth; very possibly all may be sinking, one set faster than the other, the ocean-surface also going down at an intermediate rate. Theoretically, all might possibly be rising, one set faster than the other, the ocean also rising at an intermediate rate, though this is extremely improbable. One set may be actually rising relative to the center of the earth, and the other sinking, while the ocean-level is stationary, or nearly so. This is the way in which we are accustomed to interpret them. A general shrinkage of the earth, however, is probably going on, carrying down land-surface and sea-surface. It has been urged by Suess[239] that the general shrinkage is so great that the local upward warpings and foldings never equal it, and that the real movements are all downward, though in different degrees. This is probably the general fact at least. Over against this is the popular disposition to regard earth movements generally as “upheavals.” There is also a predilection for regarding the rigid land as moving and the mobile sea-level as fixed. In reality, the sea is an extremely adaptive body that settles into the irregular hollows of the lithosphere, and is shifted about with every warping of the latter. Whatever change affects the capacity of its depressions affects also the sea-level. If they are increased, the sea settles more deeply into them; if they are decreased, the sea spreads out more widely on the borders of the land. The one thing that gives a measure of stability to the sea-level is the fact that all the great basins are connected, and so an _average_ is maintained. A warping down in one part of the sea-bottom may be offset by an upward warping somewhere else in the 72% of the earth’s surface covered by the ocean, and so it is only the _sum total of all changes_ in the sea-bottoms and borders that effects the common level. Thus it happens that, notwithstanding its instability and its complete subordination to the lithosphere, the sea-level is the most convenient basis of reference, and has become the accepted datum-plane. If there were some available mode of measuring the distance of points from the center of the earth, it would give absolute data and absolute terms, and would reveal much that is now uncertain respecting the real movements of the surface. For convenience, however, since absolute terms are impracticable, the ordinary language of geology, which represents movements as upward and downward, according to their relations to the sea-level or to the average surface, will be employed. Notwithstanding this concession to convenience in the use of terms, it is of the greatest importance to form, and to constantly retain, true fundamental views.

=Fundamental conceptions.=—The existence of any land at all is dependent on the inequalities of the surface and of the density of the lithosphere, for if it were perfectly spheroidal and equidense, the hydrosphere would cover it completely to a depth of about two miles. Not only are inequalities necessary to the existence of land, but these inequalities must be _renewed from time to time_, or the land area would soon, geologically speaking, be covered by the sea. The renewal has been made again and again in geological history by movements that have increased the inequalities in the surface of the lithosphere. With each such movement, apparently, the oceans have withdrawn more completely within the basins, and the continents have stood forth more broadly and relatively higher, until again worn down. This renewal of inequalities appears to have been, in its great features, a _periodic movement_, recurring at long intervals. In the intervening times, the sea has crept out over the lower parts of the continents, moving on steadily and slowly toward their complete submersion, which would inevitably have been attained if no interruption had checked and reversed the process. These are the great movements of the earth, and in them lies, we believe, the soul of geologic history and the basis for its grand divisions. The reasons for this will appear as the history is followed, and its most potential agencies are seen unfolding themselves. At the same time, there have been numerous minor surface movements in almost constant progress. While these two classes of movements have been associated, and are perhaps due in the main to the same causes, they are sufficiently different in some of their dynamic aspects to be separated in treatment.

_Nearly Constant Small Movements._

Innumerable gentle warpings have affected nearly every portion of the surface of the globe at nearly all stages of its history. Not only during the periods of great movements were there countless minor and gentler movements, but at times of relative quiescence there were slow swellings and saggings of the surface of the lithosphere. They sometimes affected small areas and sometimes large ones, and they were sometimes of upward phase and sometimes of downward. They were the immediate agencies in locating and controlling the deposition of stratified rocks, though they rest back on the great movements for their working conditions. Very slow sinkings of sea-borders have permitted deposition to go on in shallow water for long periods without being interrupted by the local filling of the sea. Very slow swellings of land tracts, relative or absolute, have permitted erosion to supply material for such sedimentation for long periods without exhausting the sources. Very slow upward warpings in one region and downward warpings in another have shifted the borders of the land and sea, and with them the areas of erosion and deposition. Thus have arisen overlaps and unconformities of strata and diversities in their distribution from stage to stage. Such movements may have amounted to a few inches, or a few feet, or a few fathoms per century. Downward movements have sometimes affected a considerable section of a continent, letting in a shallow _epicontinental_ sea upon it, such, for example, as the North Sea upon the northwestern border of the continent of Europe, and Hudson Bay upon the northeastern part of North America. Similar movements seem to have extended the seas even more widely upon the surface of the land in times past, as attested by the great transgressions of the ocean-borders and the great epicontinental spread of strata. Notwithstanding their great breadths, the epicontinental seas were generally shallow. Similar gentle warpings of upward phase rescued the bottoms of shallow seas from submersion, and inaugurated erosion; or they bowed base-leveled lands upward, and rejuvenated their streams and inaugurated a new cycle of denudation. Often they connected continents previously separated by shallow straits, and thus inaugurated inter-continental migrations of land life, while they stopped inter-oceanic migration.

The gentleness and frequency of these movements is attested by the character of the sediments and by their relations to one another, as will be seen in the study of the sedimentary series.

=Reciprocal features.=—These minor warpings show a notable tendency to be reciprocal. If one area is bowed up, another near by is bowed down. If the continents settle, the oceans rise on their borders. If the land is cut down, the sea is filled up. There is an important phase of this deserving especial note. Certain tracts have been slowly bowed upwards into long land swells, the streams being rejuvenated and degradation hastened. Adjacent tracts have been slowly bowed downwards into long parallel troughs which received the wash from the adjacent swells, and thus became tracts of exceptional sedimentation. Such a tract of parallel swell and sag, if our interpretation be correct, developed along the Atlantic border of North America in the Paleozoic era. By the slow upward warping of the swells, the feeding-grounds of the streams were maintained, and the sags were filled about as fast as they sank. Thus a great depth of sediment was laid down in the course of an era measured by millions of years. So in other regions, especially near the borders of the continents, there have been similar reciprocal movements, giving at once feeding-grounds for the streams and lodgment-grounds for the sediments, side by side in parallel belts. It is a common view that these belts of deep sedimentation were the forerunners of mountain formation, and that they determined the formation of the mountains. In view of the grounds for doubting the efficiency of so superficial an agency in mountain formation, which will appear as we go on, it may be well to hold this view in abeyance, and to dwell on the reciprocal nature of the action, in which the upward bowing that gave the feeding-grounds is as vital a factor as the sagging that accommodated the sedimentation. It is important to recognize that in so far as the crust was weak enough to yield to these gentler forces, it was not strong enough to accumulate the great stresses necessary to form mountain ranges, and further, that in so far as the stresses were eased by the gentle warping, they could not be accumulated for the later work of mountain-folding. It is nevertheless probable that the conditions which located the gentle swelling and sagging also located the mountain-folding.

_The Great Periodic Movements._

=Mountain-forming movements.=—Along certain tracts, usually near the borders of the continents, and at certain times, usually separated by long intervals, the crust was folded into gigantic wrinkles, and these constitute the chief type of mountains, though not the only type. The characteristic force in this folding was lateral thrust. The strata were not only arched, but often closely folded, and sometimes intensely crumpled. In extreme cases, like the Alps, the folds flared out above, giving overturn dips and reversed strata, as illustrated in the chapter on Structural Geology, pp. 501–511. In these cases there was an upward as well as a horizontal movement, for the folds themselves were lifted; but the horizontal thrust so much preponderated, and was so much the more remarkable, that the upward movement was overshadowed. It is well to note, however, that these mountain ranges are crumpled _outward_ and not _inward_, as might be expected if they resulted simply from the shrinkage of the under side of a thin shell. The folds are sometimes nearly upright and symmetrical, and sometimes inclined and asymmetrical, as illustrated in the chapter referred to. Where the folds lean, the inference has been drawn that the active thrust came from the side of the gentler slope, the folds being pushed over toward the resisting side, and this seems to be commonly true. The original attitude of the beds, however, has much to do with the character of the folds.[240] By a slight change in the mode of thrust, sheets of paper may be so pushed as to lean forward or backward at pleasure. The leaning of the folds seems, therefore, a doubtful criterion for determining the direction of the active movement. Mountains of the thrust type usually consist of a series of folds nearly parallel to each other, the whole forming an anticlinorium.

=Distribution of folded ranges.=—The prevailing location of this class of mountains is so generally near the borders of the continents that the relation is probably significant. Dana[241] long ago called attention to the fact that the greatest mountain ranges stand opposite the greatest ocean-basins, and he connected the elevation of the one with the depression of the other. One of the most notable exceptions to this relation is the complex system of southern Europe, from the Pyrenees to the Caucasus, and another is the Altai and connected ranges (Fig. 449). The Urals and not a few minor ranges are also exceptions. It is probably better to regard the crumpled tracts as lying on the borders of great segments of the earth that acted essentially as units, and to regard the relationship to the sea as a coincidence that is only in part causal.[242]

=Plateau-forming movements.=—Another leading phase of crustal movement is the settling or rising of great blocks of the crust, as though by vertical rather than horizontal force. The western plateau of North America and the great plateau of Thibet are gigantic examples. The American plateau embraces numerous blocks which, while they have been elevated together, are individually tilted in their own fashion. At the surface, they are separated by fault-planes, but below, some of them, and perhaps most of them, pass into flexures. Most of these flexures are of the monoclinal type (p. 516), which dynamically means much the same as a fault; but some of them may be of the compressive type, without inconsistency with vertical fault-relief above. Research has not yet covered thoroughly any great plateau, and knowledge of this class of movements is less complete than that of folding by lateral thrust, and it has a less ample place in the literature of the subject. The plateau-forming movements are, however, much more massive than the mountain-folding movements, and stand next in magnitude to the continent-forming movements. Plateaus may be regarded as smaller platforms superposed on the continental platforms.

In the ocean-basins, there appear to be raised platforms of the plateau type, and there are remarkable “deeps” that have the aspect of anti-plateaus.

=Continent-forming movements.=—True continent-forming movements appear to have antedated the earliest known sediments. As far back as we can read the sedimentary record, the continents seem to have been well established, and there is little evidence that they have since been fundamentally changed. It is true that some very eminent geologists have rather freely connected formations on one continent with formations having similar faunas on an opposite continent, by a hypothetical conversion of the intervening ocean-bottoms into land or shallow water; but most such faunal relations can be explained almost equally well by migration around the coasts, or at most by mere ridge-connections. The paucity, if not total absence, of abysmal deposits in the strata of the continents, taken with the persistence of terrestrial and coastal faunas, leaves little room for assigning an interchange of position between abysmal depths and continental elevations, and _vice versa_. Dynamic considerations also offer grave difficulties. The doctrine of the persistence of continents probably ought not to be pushed so far as to exclude shallow water, or even land, connections between South America, Antarctica, Australia, India, and South Africa, directly or indirectly, at certain stages of geological history. Without forming final conclusions as to the measure of the change which the continents have suffered during known geological history, it is safe to conclude that the continents and ocean-basins were in the main formed very early in the earth’s history, and that subsequent changes have consisted chiefly in the further sinking of the basins and the further protrusion of the land, save as the latter has been cut down by erosion. Incidentally, the ocean-basins have probably been extended and the continents restricted. On the other hand, the continents have been built out on their borders by wash from the land, and the waters of the ocean have been somewhat lifted by the deposition of sediment in their basins. It is estimated that the cutting away of the present continents, and the deposition of the material in the ocean-basins, would raise the sea-level about 650 feet. (R. D. George.)

=Relations of these movements in time.=—The folding movements seem to have had extraordinary prevalence in the earliest ages, for the Archean rocks are almost universally crumpled, and often in the most intricate fashion. There is no sign that the folding was then limited to the borders of the continents; it seems rather to have affected the whole continental surface. After the beginning of the well-known sedimentary series, crumpling appears to have taken place chiefly at long intervals, thus marking off great time-divisions, and to have been confined at any given stage to certain tracts, chiefly on the borders of great segments of the earth’s crust.

Concerning the plateau-forming movements in the past, knowledge is very meager, as the detection of plateaus of ancient times is more difficult than the detection of folds. Gentle warpings have apparently been in progress at all times.

=Relations of vertical to horizontal movements.=—The downward movements are unquestionably the primary ones, and the horizontal ones are secondary and incidental. The fundamental feature is doubtless central condensation actuated by gravity, and the master movements are the sinkings of the ocean-basins. The great periodic movements that made mountains and plateaus, and changed the capacity of the ocean-basins, probably started with the sinking of part or all of the ocean-bottoms. In the greater periodic movements, probably all the basins participated more or less, but some seem to have been more active than others. For example, in the last great mountain-making period, the Pacific basin seems to have been more active than the Atlantic, while in the similar great event at the close of the Paleozoic, the opposite seems to have been true. The squeezing up of the continents doubtless took place simultaneously with the settling of the basins. The true conception is perhaps that the ocean-basins and continental platforms are but the surface forms of great segments of the lithosphere, all of which crowd toward the center, the stronger and heavier segments taking precedence and squeezing the weaker and lighter ones between them. The area of the more depressed or master segments is almost exactly twice that of the protruding or squeezed ones. This estimate includes in the latter about 10,000,000 square miles now covered with shallow water. The volume of the hydrosphere is a little too great for the true basins, and it runs over, covering the borders of the continents. The amount of the overflow fluctuates from time to time, and may be neglected in a study of the movements and deformations of the lithosphere.

=The squeezed segments.=—The great protruding segments show a tendency toward rude triangularity. They are (1) the Eurasian, now strongly ridged on the south and east, and relatively flat on the northwest; (2) the African, rather strongly ridged on the east, but less abruptly elevated on the west and north; (3) the North American, now strongly ridged on the west, more gently on the east, and relatively flat at the north and in the interior; (4) the South American, strongly ridged on the west and somewhat on the northeast and southeast.

The foregoing form the major group. The minor group embraces (5) the Antarctic segment, not as yet sufficiently known to be well defined, and (6) the Australian, broadly reniform rather than triangular. To these are perhaps to be added (7) the largely submerged platform that stretches from Sumatra and Java on the southwest to the Philippines on the northeast, and is attached to India on the northwest; and (8) Greenland, which, though closely associated with North America, is partially separated by a rather deep depression.

=The depressed or master segments.=—The great sunken segments show a tendency to assume roughly polygonal, rather than triangular, forms. This accords with the primary place assigned them, since, in a spherical surface divided into larger and smaller segments, the major parts should be polygonal while the minor residual segments are more likely to be triangular. The major segments are (1) the Pacific, (2) the Indian, (3) the North Atlantic, and (4) the South Atlantic. These form the principal group, while (5) the Arctic deeps (not including the shallow epicontinental portions), (6) the Mediterranean, (7) the Caribbean, and (8) the chain of deep pits between the Philippine ridge and the Bornean platform, constitute a subordinate group.

Each member of the minor group is an irregular chain of depressed pits rather than a single continuous deep, unless the Arctic depression, of which little is now known, proves an exception. They lie between the greater segments at what may be conceived to be points of _critical working relations_, and are accompanied by small elevated blocks. The Caribbean, the Mediterranean, and the Bornean regions are the seats of the greatest present volcanic and related activities.

In a general view, there are then four great sunken quadrilaterals and four great elevated triangles, with minor attendants in each class. Lest fondness for simplicity and symmetry lead too far, we must hasten to observe that the _dimensions_ are not alike in either class. The Pacific segment is more than twice the size of any other basin segment, and four times that of the North Atlantic. The Eurasian triangle is more than twice the average size of the other land segments, and nearly three times that of the South American. Nor is there any large common divisor of approximate accuracy. This is not at all strange if the earth be regarded as a body of somewhat heterogeneous composition which naturally shrank in rather irregular segments. On the other hand, this irregularity is somewhat strange if the earth has evolved from a very homogeneous and symmetrical, primitive, fluid state. It is also a serious consideration in any theory that appeals to crystalline form, or analogy, as in the doctrine of a tetrahedral earth.

Roughly approximated in millions of square miles, the major depressed segments are as follows: the Pacific, 60, the Indian, 27, the South Atlantic, 24, and the North Atlantic, 14, leaving 8 for minor depressions. The elevated segments are Eurasian, 24, African, 12, North American, 10, and South American, 9, leaving 10 for the minor blocks.

If these segments be regarded as the great integers of body-movement, two-thirds of them taking precedence in sinking and the other third in suffering distortion, it is easy to pass to the conception of sub-segments, moving somewhat differently from the main segments, so as to aid in their adjustment to one another, and thus to the conception of plateaus and deeps. It is easy also to pass to the conception of mutual crowding and crumpling at the edges of these segments, accompanied by fracture and slipping. These conceptions perhaps represent the true relations between the massive movements of the abysmal and continental segments, as well as the less massive plateau-forming movements and the mountain-forming distortions. The mountains and plateaus are probably the incidental results of the great abysmal and continental readjustments.

The great movements are probably to be attributed to stresses that gradually accumulated until they overcame the rigidity of the thick massive segments involved, and forced a readjustment. In accumulating these stresses, some local yielding on weak lines and at special points was an inevitable incident in distributing more equably the accumulating stresses. So, also, the first great readjustments probably left many local strains and unequal stresses which gradually eased themselves by warpings, minor faultings, etc., so that some minor movements were a natural sequence of the great movements. But there were doubtless many local and superficial causes, such as irregular gains and losses of heat, regional loading and unloading, solution, hydration, etc., that have caused local or regional movement, and which have little to do with the great deformations of the earth’s body. As implied above, the gentle, nearly constant movements probably fall mainly into a different category from the great periodic movements. Both will be considered further.

=The differential extent of the movements.=—Between the highest elevation of the land and the lowest depth of the ocean, there is a vertical range of nearly twelve miles. There may have been higher elevations, relatively, in past times, but probably not deeper depressions; and so, if we assume that the surface was once perfectly spheroidal, this may be taken as a maximum expression of _differential_ movement, not _absolute_ vertical movement. From the Thibetan plateau, where a considerable area exceeds three miles in height, to the Tuscarora deep, where a notable tract exceeds five miles in depth, the range is eight miles, which may fairly represent the vertical range of rather massive differential movement. From the average height of the continents to the average abysmal bottoms of the oceans the range is nearly three miles, which may be taken as the differential movement of the great segments. Under certain hypotheses of the origin and early history of the earth, to be sketched later, the surface is not assumed to have been perfectly spheroidal originally, and hence the present irregularities do not necessarily imply so great differential movement.

If the protruding portions of the lithosphere were graded down and the basins graded up to a common level, this level would lie about 9000 feet below the ocean-surface. This equated level is the best basis of reference for relative segmental movements. Referred to this datum plane, the continents, having an area about half as great as that of the ocean depths, have been squeezed up relatively about two miles, and the basins have sunk about one mile from the ideal common plane. The total downward movement, representing the total shrinkage of the earth, is quite unknown from observation. It is probably very much greater than the differential movement, as will appear from theoretical considerations as we go on.

The extent of the _lateral_ movements has a peculiar interest, for it bears theoretically on the shrinkage of the earth. Every mile of descent of the crust represents 6 miles (6.28) shortening of the circumference. If the vertical movements were limited to the _relative_ ones just named, the mile of basin descent would give but little more than 6 miles of surplus circumference for lateral thrust and crumpling. How far does this go in explaining the known facts? By measuring the folds of the Alps, Heim has estimated the shortening represented by them to be 74 miles.[243] Claypole estimated the shortening for the Appalachians in Pennsylvania, not including the crystalline belt on the east, at 46 miles;[244] McConnel placed that of the Laramide range in British America at 25 miles,[245] and LeConte that of the Coast range in California at 9 to 12 miles.[246] These estimates must be corrected for the thickening and thinning of the beds in the process of folding, for the composite character of the folds, and for the effects of shearing and faulting. These will in part tend to increase and in part to decrease the estimates. The first effect of horizontal thrust is to close up all crevices and compact the beds as much as they will stand without bending. A part of the unusual thickness which the beds of folded regions commonly show is probably due to this edgewise compression. In experiments on artificial strata made to illustrate foldings (Fig. 449_a_), the thickening of the layers is a very appreciable part of the process, though probably natural beds do not thicken in equal proportion. After the beds have been closely folded and the thrust is athwart them, they are thinned and stretched on the limbs of the fold. How far this and other causes of extension offset initial compression is undetermined, and is differently estimated. It seems highly probable from the nature of the case that the edgewise compression which resulted from sustaining the full stress before the beds bent, was much greater than the crosswise compression on the limbs of the folds, which came into action only after the stress had been largely satisfied by folding.

Whatever the correction, and whatever the probable errors of the above estimates, the amount of shortening involved in folding is large. The estimates given are merely those for certain periods of folding, and represent only that portion of the compression of the circumference which was concentrated in a given mountain range. The whole shortening of a circumference is to be found by adding together all the transverse foldings on a given great circle, following it about the globe at right angles to a given folded tract. In so doing, it will be seen that the belt does not usually cross more than one or two strongly folded tracts of the same age, from which it is inferred that the shortening on each great circle was largely concentrated in a few tracts running at large angles to each other, to accommodate the shrinkage of the globe in all directions. If the folding in a main range crossing any great circle is doubled, it will probably represent roughly the shortening for that entire circle for that age. If one is disposed to minimize the amount of folding, the estimate may perhaps be put roundly at 50 miles, on an entire circumference, for each of the great mountain-making periods. If, on the other hand, one is disposed to give the estimates a generous figure so as to put explanations to the severest test, he may perhaps fairly place the shortening at 100 miles, or even more. For the whole shortening since Cambrian times, perhaps twice these amounts might suffice, for while there have been several mountain-making periods, only three are perhaps entitled to be put in the first order, that at the close of the Paleozoic, that at the close of the Mesozoic, and that in the late Tertiary. The shortening in the Proterozoic period was considerable, but is imperfectly known. The Archean rocks suffered great compression in their own times, and probably shared in that of all later periods, and if their shortening could be estimated closely, it might be taken as covering the whole. Assuming the circumferential shortening to have been 50 miles during a given great mountain-folding period, the appropriate radial shrinkage is 8 miles. For the more generous estimate of 100 miles, it is 16 miles. If these estimates be doubled for the whole of the Paleozoic and later eras, the radial shortening becomes 16 and 32 miles, respectively.

THE CAUSES OF MOVEMENT.

_General Considerations._

The volume of the earth is at all times dependent on two sets of antagonistic forces, (1) the attractive or centripetal, consisting of gravity and the molecular and sub-molecular attractions, and (2) the resistant forces—which are not necessarily centrifugal—consisting of heat and the resistant molecular and sub-molecular forces.

1. _The centripetal agencies._