The phenomenon was not unlike that of a swell produced by a distant storm. It would seem possible that in some cases pulsations producing phenomena like the ‘seiches’ of Switzerland might have their origin beneath the ocean, or deep down beneath the earth’s crust. Perhaps, instead of commencing with the ‘snap and jar’ of an earthquake, they may commence as a heaving or sinking of a considerable area, which may be regarded as an uncompleted effort in the establishment of an earthquake or a volcano.
From what has now been said it would seem that earth pulsations are phenomena with a real existence, and that some of these are attributable to earthquakes. On the other hand, certain earthquakes are attributable to earth pulsations. Some of the phenomena which have been brought forward have only a possible connection with these movements, and they yet require investigation. Elastic tides in the earth’s crust have for long been realities in the minds of physicists. These, however, are due to lunar and solar influences, and are regular in their action. The tidal-like movements called pulsations are of greater magnitude, and their goings and comings are irregular.
CHAPTER XXI.
EARTH OSCILLATIONS.
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]