Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

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 superin- cumbent strata, (and this giving way probably is a conse- quence 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 sub- sequent 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.