Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/254

Rh ‘.240 to allow him to assume tl1at all the causes and modes of geo- logical change have been definitively ascertained. On the earth itself there may remain for future discovery evidence of former operations by heat, magnetism, chemical change, or otherwise, which may explain many of the phenomena with which geology has to deal. Of the inﬂuences, so many and profound, which the sun exerts 11pon our planet, we can as yet only dimly perceive a little. Nor can we tell what other cosmical inﬂuences may have lent their aid in the evolution of geological changes. In the present state of our knowledge, all the geological energy upon and within the earth must ultimately be traced back to our parent sun. There is, however, a certain pro- priety and convenience in distinguishing between that part of it which is due to the survival of some of the original energy of the planet, and that part which arises from the present supply of energy received day by day from the sun. in the former case we have to deal with the interior of the earth and its reaction upon the surface; in the latter we deal with the surface of the earth, and to some extent with its reaction on the interior. This distinction allows of a broad treatment of the subject under two divisions :— I. I13/pogene or Platonic Action——the changes within the earth caused by original internal heat and by chemical action. II. Epigene or ;Stt7_‘fCtC€ Act-z'on—the changes produced on the superﬁcial parts of the earth, chieﬂy by the circulation of air and water set in motion by the sun’s heat. DIVISION I.——HYPOGENE ACTION. .172 I-nguz'ry into the Geological C’/zanges in Progress beneath the S-mgface of the Emtk. In the discussion of this branch of the subject we must carry in our minds the conception of a globe still intensely hot in its interior, radiating heat into space, and conse- quently contracting in bulk. Portions of molten rocks from inside are from time to time poured out at the surface. Sudden shocks are generated by which destructive earth- quakes are propagated to and along the surface. Wide geographical areas are pushed up or allowed to sink down. In the midst of these movements very remarkable changes are produced upon the rocks of the crust; they are shat- tered, fractured, squeezed, crumpled, rendered crystalline, and even fused. Section I.—Volcanoes and Volcanic Action. The term volcanic action (vulcanism or vulcanicity) embraces all the phenomena connected with the expulsion of heated materials from the interior of the earth to the surface. Among these phenomena there are some of an evanescent character, while others leave permanent proofs of their existence. It is naturally to the latter that the geologist gives the chief attention, for it is by their means that he can trace the former phases of volcanic activity in regions where, for many ages, there have been no volcanic eruptions. In the operations of existing volcanoes he can observe only the superﬁcial manifestations of volcanic action. But, examining the rocks of the earth's crust, he discovers that in the lapse of ages, amid the many terrestrial revolu- tions which geology reveals, the very roots of former volcanoes have been laid bare, displaying subterranean phases of vulcanism which could not be studied in any modern volcano. Hence an acquaintance only with active volcanoes will not give us acomplete knowledge of volcanic action. It must be supplemented and enlarged by an investigation of the traces of former volcanoes preserved in the crust of the earth. The openings by which the heated materials from the interior reach the surface include volcanoes (with their GEOLOGY [nI. DYNAMICAL. accompanying orifices). hot-springs, and gas-springs. A volcano may be deﬁned as a conical eminence, composed wholly or mainly of materials which have been ejected from below, and which have accumulated at the surface round the vent of eruption. As a rule it presents at its summit a cup—shaped cavity termed the crater, at the bottom of which is the top of the main funnel or pipe whereby the communication is maintained with the heated interior. A volcano, when of small size, may consist merely of one diminutive cone; when of the largest dimensions, it forms a huge mountain, with many subsidiary cones and many lateral ﬁssures or pipes, from which the heated volcanic products are given out. Volcanoes may break through any kind of geological formation. In Auvergne, in the Miocene period, they burst through the granitic and gneissose plateau of central France. In Lower Old Red Sandstone times they pierced eontorted Silurian rocks in central Scotland. In late Tertiary and post-Tertiary ages they found their way through soft marine strata, and formed the huge piles of Etna, Sonnna, and Vesuvius. On the banks of the llhine, at Bonn and elsewhere, they have penetrated some of the older alluvia of that river. In many instances, also, newer volcanoes have appeared on the sites of older ones. In Scotland the Carboniferous volcanoes have risen on the sites of those of the Old Iled Sandstone, those of the Permian period have broken out among the earlier Carboniferous eruptions, while the Miocene lavas have been injected into all these older volcanic masses. Again, the newer pug/s of Auvergne were sometimes erupted through much older and already greatly denuded basalt-streams. Somma and Vesuvius have arisen out of the great Neapolitan plain of marine tuﬁ'. In central Italy, also, newer cones have been thrown up upon the great Roman plain of more ancient volcanic debris. It is usual to class volcanoes as active, dormant, and extinct. This arrangement, however, often presents con- siderable difﬁculty in its application. An active volcano cannot of course be mistaken, for even when not in actual eruption it shows, by its abundant evolution of steam and l1ot vapours, that it might break out into activity at any moment. But it is in many cases impossible to decide whether a volcano should be called extinct or only dormant. The volcanoes of Silurian age in Wales, of Carboniferous age in Ireland, of Permian age in the Hartz, of Miocene age in the Hebrides, are certainly all extinct. But the Miocene volcanoes of Iceland are still represented there by Skaptar- J 'o'kull, Hecla, and their neighbours. Somma, in the first century of the Christian era, would have_ been naturally regarded as an extinct volcano. Its ﬁres had never been known to have been kindled within human tradition, its vast crater was a wilderness of wild vines and brushwood, haunted, no doubt, by wolf and wild-boar. Yet in a few days, in the autumn of the year 79 the half of the crater walls was blown out by a terriﬁc series of explosions, the present Vesuvius was then formed within the limits of the earlier crater, and since that time volcanic action has been intermittently exhibited up to the present day. Some of the intervals of quietude, however, have been so consider- able that the mountain might then again have been claimed as an extinct volcano. Thus, in the 131 years between 1500 and 1631, so completely had eruptions ceased that the crater had once more become choked with copsewood. A few pools and springs of very salt and hot water remained as memorials of the former condition of the mountain. But this period of quiescence closed with the eruption of 1631, —the most powerful of all the known explosions of Vesu- vius, except the great one of 79. In the island of Ischia, Mont Epomeo was last in eruption in the year 1302, its previous outburst having taken place,