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Show 268 EOCENE PERIOD. [Ch. XIX. When rivers are dispossessed of their channels by lava, they usually flow between the mass of lava and one side of the or1· g·m a1 valley . They then eat out a passage, par.t ly throu(ol' h the volcanic and partly through the older format10n ; but as the soft tertiary marls in Auvergne give way more readily than the basalt, it is usua1ly at the expense of the former that the enlarging and deepening of the new valley is effected, and all the remaining lava is then left on one side, in the manner represented in the above wood-cut. Age of the more modern lavas.-The only organic remains found as yet in the ancient alluviums appear to belong to the Miocene period; but we have heard of none discovered in the gravel underlying the newest lavas,- those which either occupy the channels of the existing rivers or are very slightly elevated above them. We think it not improbable that even these may be of Miocene date, although the conjecture will appear extremely rash to some ~vho are aware that the cones and craters whence the lavas 1ssue, are often as fresh in their aspect as the majority of the cones of the forest zone of Etna. The brim of the crater of the Puy de Pariou, near Clermont, is so sharp, and has been so Jittle blunted by time, that it scarcely affords room to stand upon. 'This and other cones in an equally remarkable state of integrity have stood, we conceive, uninjured, not in spite of their loose porous nature, as some O'Coloo·ists mi(J'ht think, but in consequence of it. No rills can b b b collect where all the rain is instantly absorbed by the sand and scorire, as we have shown to be the case on Etna (see above, p. 102), and nothing but a waterspout breaki~g directly upon the Puy de Pariou could carry away a portiOn of the hill, so long as it is not rent by earthquakes or engulphed. d' Attempt to di·vide Volcanos into ante-diluvi~n and po~t- tluvian.- The opinions above expressed are entirely at vanance with the doctrines of those writers who have endcav~u~·~d to arrange all the volcanic cones of Europe under two dlVlsiOns, Ch, XIX.] AGE OF AUV.ERGNE VOLCANOS, 269 those of ante-diluvian and those of post-diluvian origin. To the former they attribute such hills of sand and scorire as exhibit on theil' surface evident signs of aqueous denudation; to the latter, such as betray no marks of having been exposed to such aqueous action. According to this classification almost all the minor cones of Central France must be called postdiluvian; although, if we receive this term in its ordinary acceptation as denoting posteriority of date to the Noachian delugeJ we are forced to suppose that all the volcanic eruptions occurred within a period of little more than twenty centuries, or between the era of the flood, which happened about 4000 years ago, and the earliest historical records handed down to us respecting the former state of Central France. Dr. Daubeny has justly observed, that had any of these I<,rench volcanos been in a state of activity in the age of Julius Cresar, that general, who encamped upon the plains of Auvergne, and laid siege to its principal city, (Gergovia, near Clermont,) could hardly have failed to notice them. Had there been even any record of their existence in the time of Pliny or Sidonius ApollinarisJ the one would scarcely have omitted to make mention of it in his Natural History, nor the other to introduce some allusion to it among the descriptions of this his native province. This poet's residence was on the borders of the Lake Aidat, which owed its very existence to the damming up of a river by one of the most modern lava currents*. The ruins of several Roman bridges and of the Roman baths at Royat confirm the conclusion that no sensible a~ter~tion has taken place in the physical geography of the d1stnct, not even in the chasms excavated throuO'h the newest 1 . b a vas smce ages historically remote. We have no data at present for presuming that any one of the Auvergne cones has been produced within the last 4000 or 5000 years; and the * Daubeny on Volcanos, p. 14. |