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Show THE PEOXIMATE CAUSE OF EEUPTIONS. 127 veniejit season to explode. It presents the case as a problem of energy acquired by some secondary forces, of which we are at present ignorant. There is one general assumption which satisfies all the main requisites of volcanism. It is this: Volcanic phenomena are hroughi about by a local increase of temperature within certain subterranean horizons. This, indeed, is not a solution of the problem, for it throws us back instantty upon the ulterior question, What has caused the increase of temperature f All my efforts to find an answer to this ulterior question have utterly failed. But the proximate idea is suggested on every hand, and its reality takes deeper root in conviction the more it is contemplated. Around it the broader facts take form and coherence. It explains their secondary character as contradistinguished from the primordial. It explains the cyclical phases of volcanism; their beginning in a recent epoch of the world's secular history; their growth, decay, and extinction. It explains their intermittent characterâ€" why eruptions are repetitive instead of continuous. It explains the explosive and energetic character of the phenomena ; and, lastly, it explains the lithological order of the eruptions, as will presently be shown. But there is another and alternative assumption. We may suppose the deeply-seated rocks in regions of high temperature to undergo changes, one result of which is to lower their melting-points. This is not so strange as it might at first seem, for its accomplishment is conceivably within known physical laws. A relief of pressure is one conceivable mode. Probably another would be the absorption of water under great pressure and at high temperature It can hardly be doubted that a rock charged with water and so confined that the water cannot readily escape is more fusible than the same rock in an anhydrous condition. The fact that lavas bring to the surface considerable quantities of water may be held to be evidence that water does find access to them from above. The only alternative view is that water formed a part of their original constitution. This is undoubtedly the case on the view that lavas are remelted metamor-phic rocks; for the metamorphics all contain water, partly mechanically held and partly as water of combination in hydrous minerals. The amount of contained water is variable, but ordinarily more than one per cent, and sometimes much more This quantity, however, probably falls far below |
