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Show 130 GEOLOGY OF THE HIGH PLATEAUS. cases, for want of ability to show the contrarj^ it may be accounted a sufficient explanation, and in general it cannot be questioned, that in most volcanoes this identical action plays a more or less important part. Scoria, pumice, and volcanic dust have unquestionably this origin; but the whole of the extravasation is not so accomplished. The outpour of lava is a very different matter. It is comparatively calnr*and quiet in its flow, like water welling forth from a spring; sometimes boiling, bubbling, and spurting a little, but never boisterous or obstreperous. It continues its flow for days and sometimes weeks, but at length ceases and comes to rest. A careful examination of the details of volcanic eruptions leaves the impression that they are pressed up by the weight of rocks which overlie their reservoirs, and that their extravasation is merely a hydrostatic problem of the simplest order. The conception of a liquid inclosed in a cavity beneath the surface and opening to the outer air through a stand-pipe requires some discussion when we come to apply it to volcanic eruptions. Our conceptions of the constrained motion of liquids are derived from experiments upon small quantities of them in small vessels; but when we come to such enormous volumes as are disgorged by volcanoes, a consideration arising from mere magnitude enters into the schemeâ€"a consideration which has no bearing in relation to small volumes. This is the strength of the receptacle. It is a well-known principle in mechanics that the relative strength of a body is inversely proportional to its size. Thus, where we have similar bodies subject to forces which are proportional to their own masses, the resistance to detrusion is proportional only to the square of their linear dimensions. It is this relation which limits the span of an arch or the length of a truss. Now, if we could conceive the contents of one of these subterranean lava reservoirs to be suddenly annihilated, so great must be their dimensions that the rocks above would instantly sink into the cavity, just as the rocks above a coal-mine do on small provocation. A small cavity, on the other hand, might persist. Now, the point I wish to illustrate is that the strength of the retaining-walls of a lava reservoir are relatively so weak, in consequence of the large dimensions, that their effect is very nearly the same as it would be if the lava were overlaid by another liquid with which it could not commingle. It is the |