OCR Text |
Show 380 CONCEPCION. March, 1835. spaces, and strange subterranean noises likewise heard over nearly equal areas, yet few cases are on record of volcanoes, very far distant from each other, bursting out at the same moment of time. In this instance, however, at the same hour when the whole country around Concepcion was permanently elevated, a train of volcanoes situated in the Andes, in front of Chiloe, instantaneously spouted out a dark column of smoke, and during the subsequent year continued in uncommon activity. It is, moreover, a very interesting circumstance, that, in the immediate neighbourhood, these eruptions entirely relieved the trembling ground, although at a little distance, and in sight of the volcanoes, the island of Chiloe was strongly affected. To the northward, a volcano burst out at the bottom of the sea adjoining the island of Juan Fernandez, and several of the great chimneys in the Cordiller-a uf central Chile commenced a fresh period of activity. We thus see a permanent elevation of the land, renewed activity through habitual vents, and a submarine outburst, forming parts of one great phenomenon. The extent of country throughout which the subterranean forces were thus unequivocally displayed, measures 700 by 400 geographical miles. From several considerations, which I have not space here to enter on, and especially from the number of intermediate points whence liquefied matter was ejected, we can scarcely avoid the conclusion, however fearful it may be, that a vast iake of melted matter, of an area nearly doubling in extent that of the Black Sea, is spread out beneath a mere crust of solid land. The elevation of the land to the amount of some feet during these earthquakes, appears to be a paroxysmal movement, in a series of lesser and even insensible steps, by which the whole west coast of South America has been raised above the level of the sea. In the same manner, the most violent explosion from any volcano is merely one in a series of lesser eruptions: and we have seen that both these phonomena, which are in so many ways related, are parts of one common action, only modified by local circum- March, 1835. VOLCANIC PHENOMENA, 381 stances. With respect to the cause of the paroxysmal convulsion in particular portions of the great area which is simultaneously affected, it can be shown to be extremely probable, that it is owing to the giving way of the superincumbent strata, (and this giving way probably is a consequence of the tension from the general elevation) and their interjection by fluid rock-one step in the formation of a mountain chain. On this view we are led to conclude, that the unstratified mass forming the axis of any mountain, has been pumped in when in a fluid state, by as many separate strokes as there were earthquakes. For instance, in the case of Concepcion, during the few months subsequent to the great shock, upwards of three hundred tremours of the ground were felt, each of which indicated a fresh fracture, and injection of the fluid stone. It is a case precisely analogous to what happens in all bad eruptions, which are invariably followed by a succession of smaller ones : the difference is, that in the volcano the lava is ejected, while in the formation of a mountain chain it is injected. This view of the extremely gradual elevation of a line of mountains, will alone explain the difficulty (which, as far as I am aware, has never been attempted to be solved) of the axis consisting of rock which has become solid under the pressure of the superincumbent strata, while yet these same strata, in their present inclined and vertical positions, cannot possibly cover more than a small portion of that axis. |