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.