OCR Text |
Show CONCEPCION· March, 1835. disturbance, some little way distant. If the waves sent off from the paddles of a steam-vessel be watched breaking on the sloping shore of a still river, the water will be seen first to retire two or three feet, and then to return in little breakers, precisely analogous to those consequent on an earthquake. From the oblique direction in which the waves are sent off from the paddles, the vessel has proceeded a long way ahead, before the undulation reaches the shore; and hence it is at once manifest, that this movement bears no relation to the actual displacement of the fluid from the bulk of the vessel. Indeed, it seems a general circumstance, that in all cases where the equilibrium of an undulation is thus destroyed, the water is drawn from the resisting surface to form the advancing breaker.* Considering then a wave produced by an earthquake as an ordinary undulation proceeding from some point or line in the offing, we can see the cause, first of its occurrence some time after the shock; secondly, of its affecting the shores of the mainland and of outlying islets in a uniform manner-namely, the water retiring first, and then returning in a mountainous breaker; and lastly, of its size being modified (as appears to be the case) by the form of the neighbouring coast. For instance Talcuhano and Callao are situated at the head of great shoaling bays, and they have always suffered from this phenomenon; whereas, the town of Valparaiso, which is seated close on the border of a profound ocean, though shaken by the severest earthquakes, has never been overwhelmed by one of these terrific deluges. Qn this view, we have only to imagine, in the case of Concepcion, a point of disturbance in the bottom ef the sea in a south-west direction, whence the wave was seen to travel, and where the land was elevated to a greater height than any other part,-and the whole phenomenon will be explained. It is probable that near every coast, the chief line of dis- * I am indebted to Mr. Whewell for explaining to me the probable movements on the shore, of an undulation of which the equilibrium has been destroyed. March 1835. VOLCANIC PHENOMENA. turbance wo~d b: situated at that distance in the offing, where the flmd whiCh was most agitated, from overlying the shallow bottom near the land, joined on to that part which covered the depths (but slightly moved) of the ocean. In all distant parts of the coast the small oscillations of the sea, both at .the moment of the great shock, and during the lesser followmg ones, would be confounded with the undulation propagated from the focus of disturbance, and hence the series of movements would be undistinguishable. The most remarkable effect (or perhaps speaking more correctly, cause) of this earthquake was the permanent elevation of the land. Captain FitzRoy having twice visited the island of Santa Maria, for the purpose of examining every circumstance with extreme accuracy, has brought a mass of evidence in proof of such elevation, far more conclusive than that on which geologists on most other occasions place im, plicit faith. The phenomenon possesses an uncommon degree of interest, from this particular part of the coast of Chile having previously been the theatre of several earthquakes of the worst class. It is almost certain, from the altered soundings, together with the circumstance of the bottom of the bay near Penco, consisting of hard stone, t~at there has been an uplifting to the amount of four fathoms, smce the famous convulsion of 17 51. With this additional instance fresh before us, we may assume as probable, accord~ ing to the principles laid down by Mr. Lyell,* other small successive elevations, and may fearlessly maintain that the pro~ blem of the raised shells, t recorded by Ulloa, is explained. Some of the consequences which may be deduced from the phenomena connected with this earthquake are most important in a geological point of view; but in the present work I cannot do more than simply allude to the results. Althouo-h • • 0 It 1s known that earthquakes have been felt over enormous • Lyell's Geology, book ii., chap. xvi. t I saw these shells in very great quantities on the flanks of the island of Quiriquina. |