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.