Evidences of oscillation—Examples of oscillation—Temple of Jupiter Serapis—Observations of Darwin—Causes of oscillation.

Evidences of oscillation.—By earth oscillations are meant those slow and quiet changes in the relative level of the sea and land which geologists speak of as elevations or subsidences. These movements are especially characteristic of volcanic and earthquake-shaken countries.

As evidences of elevations we appeal to phenomena like raised beaches, sea-worn caves, raised coral reefs, and the remains of other dead organisms like barnacles, and the borings of lithodomous shells in and on the rocks of many coasts high above the level of the highest tides. As a proof that subsidence has taken place, there is the evidence afforded by submerged forests, the prolongation of certain valleys beneath the bed of the ocean, the formation of coral islands, the peculiar distribution of the plants and animals which we find in many countries, and the submergence of works of human construction. Inasmuch as these phenomena are discussed so fully in many treatises on physical geology, the references to them here will be made as brief as possible. Elevations and depressions which have taken place at the time of large earthquakes in a paroxysmal manner have already been mentioned. The movements referred to in this chapter, although generally taking place with extreme slowness, in certain instances, by an increase in their rapidity, have approached in character to earth pulsations. In most instances it would appear that the upward movement of the ground, which may be likened to a process of tumefaction, goes on so gently that it only becomes appreciable after the lapse of many generations.

Examples of movements.—Lyell estimated that the average rate of rise in Scandinavia has been about two and a half feet per century. At the North Cape the rise may have been as much as five or six feet per century. Observations made at the temple of Jupiter Serapis, between October 1822 and July 1838, showed that the ground was sinking at the rate of about one inch in four years. Since the Roman period, when this temple was built, the ground has sunk twenty feet below the waves. Now the floor of the temple is on the level of the sea. Lyell remarks that if we reflect on the dates of the principal oscillations at this place there appears to be connection between the movements of upheaval and a local development of volcanic heat, whilst periods of depression are concurrent with periods of volcanic quiescence.[155]

As examples of movements even more rapid than those at the Temple of Jupiter Serapis we refer to an account of the earthquakes in Vallais (November 1755), when the ground about a mountain at a small distance from Brigue sank about a thumb’s-breadth every twenty-four hours. This took place between December 9 and February 26.[156]

Another remarkable example of earth movement is given in the account of the earthquake at Scarborough, on December 29, 1737, when the head of the spa water well was forced up in the air about ten yards high. At this time the sands on the shore are said to have risen so slowly that people came out to watch them.[157]

Two other examples of rapid earth movement are taken from Professor Rossi’s ‘Meteorologia Endogena.’ Professor D. Seghetti, writing to Professor Rossi, says that a few lustres ago (one lustre = twenty years) Mount S. Giovanni hid the towns Jenne and Subiaco from each other. From Subiaco the church at Jenne is now visible, which a few years ago was invisible. The people at Jenne also can see more than formerly. The supposition is that the side of Mount S. Giovanni is lowered. This fact corresponds to a fact stated by Professor Carina, who says that forty or fifty years ago from Granaiola you could not see either the church of S. Maria Assunta di Citrone or the church of S. Pietro di Corsena. Now you can see both.[158]

For a remarkable example illustrating the connection between seismic activity and elevation we are indebted to the patient labours of Darwin, who carefully investigated the evidences of elevation which are visible upon the western coasts of South America. These evidences, consisting of marks of erosion, caves, ancient beaches, sand dunes, terraces of gravel, &c., were traced between latitudes 45° 35' to 12° 5', a distance north and south of 2,075 geographical miles, and there is but little doubt that they extend much farther. As deduced from observations upon upraised shells alone, a summary of Mr. Darwin’s observations are contained in the following table:—

	Feet
At Chiloe the recent elevation has been	350
„ Concepcion „ „	625 to 1,000
„ Valparaiso „ „	1,300
„ Coquimbo „ „	252
„ Lima „ „	85

Shells, similar to those clinging to uplifted rocks, which are evidences of these elevations, still exist in the neighbouring seas, and in the same proportionate numbers as they are found in the upraised beds. In addition to this, Mr. Darwin shows us that at Lima, during the Indo-human period, the elevation has been at least eighty-five feet. At Valparaiso, during the last 220 years, the rise was about nineteen feet, and in the seventeen years subsequent to 1817 the rise has been ten or eleven feet, a portion only of which can be attributed to earthquakes. In 1834 the rise there was apparently still in progress.

At Chiloe there has been a gradual elevation of about four feet in four years. These, together with numerous other examples, testify to the gradual but, as compared with other parts of the globe, exceedingly rapid rise of the ground upon the western shores of South America.[159] The most important point to be noticed is that this district of rapid elevation is one of the most earthquake-shaken regions of the world. And further, judging from Darwin’s remarks, in those portions of it where the movements have been the most extensive, and at the same time probably the most rapid, the seismic disturbances appear to have been the most noticeable.

Similar remarks may be applied to Japan, it being in those districts where evidences of recent elevation are abundant that earthquakes are numerous. Thus, in the bay of Yedo, where we have borings of lithodomi in the tufaceous cliffs ten feet above high-water mark, which, inasmuch as the rock in which they are found is soft and easily weathered, indicate an exceedingly rapid elevation, earthquakes are of common occurrence.

From the evidences of elevation which we have upon the South American coast, Japan, and in other countries, it appears that these movements are intermittent, there being periods of rest, when sea cliffs are denuded, and perhaps even periods of subsidence. There is also evidence to show that, although these movements have been gradual from time to time, they have been aided by starts occasioned by earthquakes.

As to whether earthquakes are more numerous during periods of elevation, or of subsidence, or during the intermediate periods of rest, we have no evidence.

Sudden displacements which occasionally accompany earthquakes might, it was said, sometimes be regarded as the cause of an earthquake, and sometimes as the effect.

The slow elevations here referred to may be looked upon as being one of the more important factors in the production of earthquakes. By various causes the rocky coast is bent until, having reached the limit of its elasticity, it snaps, and, in flying back like a broken spring, causes the jars and tremors of an earthquake.

If this is the case, then the number of earthquakes felt in a district which is being elevated may possibly be a function of the rate of elevation.

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