Now, the districts thus classed together are not only related by geographical proximity, but have some real subterranean as well as apparent superficial connection. Humboldt and Darwin speak con fidently of the great volcanic regions of the Andes as one grand system of subterranean activity ; though the manifestations of this at the surface offer local peculiarities, both as to time and circumstances. Mr. Darwin has been led, by the investigatjon of the volcanoes and earthquakes of the Cordilleras of the Andes, to regard them all as depending primarily on the disturbance of a vast internal sea of melted rock, spread below a Large part of South America.
Other conclusions, equally on a large scale, which have been drawn by M. de Beaumont from other classes of phenomena, have a direct bearing on this subject. M. de Beaumont has inferred that the principal mountain-ranges throughout the world have their several geological dates determinable by comparing the positions of the disturbed and the undisturbed strata in and around them : that to each great period of fractures in the earth'a crust belongs a certain prevalent direction in which those fractures happened; and though this view may be subject to particular objections and restrictions, there is this great truth in it, that the several systematic fractures which it professes to refer to one certain geological date have each an assignable date. Thia date being assigned. we find that the earth's cruet has in ancient geological times been broken by lines of fracture or bent into flexurea, 10, 60, 100, of several hundred miles long, and this often (there are many examples in the British Isles) with no unusual exhibition of really volcanic rock on the line, and even with little appearance of unerupted granite or sienite. These great fractures traverse nearly all regions, with no special reference to active or extinct volcanoes, and it is clear that they are due to a general cause, which has been in operation through all past geological periods, and which produced effects exactly comparable in kind, if greater in degree, to those now performed by modern earth quakes. But if we consider the account of the effects of the great Lisbon earthquake in 1755, which extended over Europe, changing momentarily the level of the land, raising waves 60 feet high at Cadiz, and 18 feet at Madeira, and causing sensible disturbance in the West Indies and Loch Fyne ; or the narratives of the Chilian earthquakea in 1822 and 1835, the former of which raised the sea-shore fur 100 and the latter rent and shattered the entire provinces of Canqueres and Concepcion in every direction—it will remain very doubtful in our minds whether the internal power to which earthquakes owe their force has really decreased, or the violence of the earthquake been moderated and relieved by the intermitting action of volcanoes. Mr. Darwin speaks confidently of earthquakes and volcanic eruptions in South America as parts of the same phenomenon, now one and then the other, or both together, but at different points, relieving the pressure on the " internal sea of molten rock ;" and this view, which is the largest, appears at the same time the simplest and beat founded of all the postulates for a general theory of volcanoes.
This able writer has indeed by a simple inference brought us at once to the basie of this theory. He has inferred that the primary
shock of an earthquake is caused by a violent rending of the strata, which on the coast of Chili and Peru seems generally to occur at the bottom of the neighbonring sea.
Here then we take our basis of a general theory of volcanic actions. The earth's crust is subject to fractures, and has always been subject to fractures on a great scale: below the surface of the earth is now, and was in ancient geological periods, an internal sea of molten rock ; this sea is agitated and thrown bodily from its place by the rending of the strata : a stare of translation (not an ordinary undulation) is generated in the liquid mass [Neves stn Tines], which passes rapidly onwards and moves the land on its crest, in a given direction : this is the earthquake. A portion of the melted rock Is forced by the general pressure into cavities of the rocks, or spread out in irregular sheets on the bed of the sea ; these are the dykea and interposed beds of Plutoniu rock : to some part of the internal hot fluid, water finds access, and the steam which is generated and confined supports local columns of melted rock, in particular fissures of the earth's crust, till the lava finds vent and flows to the surface, or is driven up in dust and scone' by the violent extrication of the vapour : this is the local volcanic action. As to the composition of that internal sea of melted rock, we may admit it to contain unoxidised metalloids, if by this means we can better explain the peculiar chemical nature of the pro ducts which come to the surface; and thus we find at last only one condition remaining to be satisfied, namely, the condition of a continual and regressive destruction of the equilibrium of the internal masses of the earth, which causes the violent rending of the strata ante cedent to earthquakes and volcanoes. On this point we need not enlarge. The general progress of geological and physical science has rendered it very probable that the disturbance of the equilibrium of the earth's internal masses, which has at so many geological epochs been exalted to an intensity equal to sink and raise hun dreds of miles square, and to fold into complicated contortions the seemingly solid crust of the globe, is due simply to a slow change and gradual diminution of the earth's internal heat. Great fractures, Plutonic rocks, and volcanic accumulations, are of all geological ages ; but as our existing land is, in respect of a very large part of its surface, of very recent date, and volcanic cones of loose materials cannot with stand the wasting action of the sea, it is no wonder that the antiquity of volcanoes if judged only by the relation of volcanic products visible on the land to the stratified crust of the earth, appears much inferior to that of the Plutonic rocks, which were formed among the strata of every age, under circumstances which admitted of their being preserved But if we more closely study this matter. and compare marine volcanic sediments, such as have been spread by the waves round the base of Sciacca, or Sabrina, with the " trappean" sandstones described by Sir R. I. Murchison interposed amongst the Silurian strata, we shall perceive that local volcanic excitement consequent on general changes in the internal condition of the earth is a phenomenon of all geological periods.