Of movements like these, especially along the western shores of South America, Darwin, who paid so much attention to this subject, has given many examples. In 1822, the shore near Valparaiso was suddenly lifted up, and Darwin tells us that he heard it confidently asserted ‘that a sentinel on duty, immediately after the shock, saw a part of a fort which previously was not within the line of his vision, and this would indicate that the uplifting was not vertical.’[70] That the large areas of land should be shifted permanently in horizontal directions, as well as vertically, we should anticipate from the observations which we are able to make upon large fissures which are caused by earthquakes.
Another remarkable example of sudden movement in the rocky crust is that which took place during the earthquakes of 1811–12 in the valley of the Mississippi, near to the mouth of the Ohio, which was convulsed to such a degree, that lakes, twenty miles in extent, were formed in the course of an hour. This country, which is called the ‘sunk country,’ extends some seventy to eighty miles north and south, and thirty miles east and west.[71]
In the ‘Gentleman’s Magazine’ we read of the little territory of Causa Nova, in Calabria, being sunk twenty-nine feet into the earth by an earthquake, without throwing down a house. The inhabitants, being warned by a noise, escaped into the fields, and only five were killed.[72]
Other examples of these permanent dislocations of strata are to be found in almost every text-book on geology.
Geological changes produced.—Passing over the accounts of earth movements which are more or less fictitious, and confining our attention to the well authenticated facts, we see at once the important part which earthquakes have played as agents working geological changes. Even in the nineteenth century long tracts of coast, as in Chili and New Zealand, have been raised, whilst other areas, like the Delta of the Indus, have been sunk. Sir H. Bartle Frere, speaking about the disturbance which took place in his latter region in 1819, remarks that all the canals drawn from the Fullalee River ceased to run for about three days, probably indicating a general upheaval of the lower part of the canal. In consequence of the earthquakes in former times it is not unlikely that water-courses have ceased to flow, water has decreased in wells, and districts have been depopulated.[73]
Sometimes these changes have taken place gradually and sometimes with violence. Mountains have been toppled over, valleys have been filled, cities have been submerged or buried.
With the records of these convulsions before us, we see that seismic energy yet exhibits a terrible activity in changing the features of the globe.
Reason of these movements.—To formulate a single reason for these catastrophes would be difficult. Where they are of the nature of landslips, or materials have been dislodged from mountain sides, the cause is evidently the sudden movement of this ground acting upon strata not held together in a sufficiently stable condition. A similar explanation may be given for the sudden elevations or depressions of strata in a district removed from the centre where the disturbance had its origin. The seismic effort exhibits itself in a certain area round its origin as a sudden push, and by this push, strata are fractured and caused to move relatively to each other.
At or near to the origin of an earthquake it might be argued that it was the sudden falling of rocky strata towards a position of stable equilibrium that caused the shaking, and in such a case the movements referred to may be regarded as the cause rather than the effect of an earthquake.
A subject closely connected with the sudden dislocation of strata, is the production of secondary or consequent earthquakes, due to the disturbance of ground in a critical state (see p. 248).