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Show 266 EOCENE PERIOD· (Ch. XIX, carried away by the force of running water, the cone whence the lava issued escaped destruction, because it stood upon a platform of gneiss several hundred feet above the level of the valley in which the force of running water was exerted. It is needless to multiply examples, or the Vivarais would supply many others equally striking. Among many we m.ay instance the cone of J aujac, and its lava current*, which is a counterpart of that near Pranal last mentioned. Lavas and Alluviums of different Ages.-VV e have seen that on the flanks of Etna, since the commencement of the present century, several currents of lava have flowed at the bottom of the Val del Bove, at the foot of precipices formed of more ancient lavas and tuffs. So we find in Auvergne that some streams of melted matter have flowed in v~lleys, the sides of which consist partly of older lavas. These are often seen capping the hills in broad sheets, resting sometimes on granite, sometimes on fresh-water strata. Many of the earlier lavas of Auvergne have flowed out upon the platform of granite before aU the existing valleys had been excavated; others again spread themselves in broad sheets over the horizontal lacustrine deposit, when these had been covered with gravel, probably soon after the drainage of the lakes. Great vicissitudes in the physical geography of the country must have taken place since the flowing of these ancient lavas; and it is evident that the changes were gradual and successive, caused probably by the united agency of running water and subterranean movements. We frequently observe one mass of lava capping a hill, and a second at a lower eleva· tion, forming a terrace on the side of a valley ; or sometimes occupying the bed of a river. It is a most interesting fact that we almost invariably find in these cases beds of gravel underlying the successive currents of lava, as in Catalonia before described (pp. 189, 190). Occasionally, when the highest platform of lava is 700 or 800 feet above the lowest, we cannot fail to be struck with the won- * See Scrope's Central France, rlate 14. Ch. XIX.] ALLUVIUMS OF DIFFERENT AGES, 267 derful alterations effected in the d ram. age o f t h e countr · the first current flowed '. for the mo s t el evate d a 11 uvJ•U msy msmucset No. 61, Lavas of Auvergne resting on alluviums o"dil¥'-·ent , • }] 'J .v•• ages. orlgma y. h. ave been accumulated in tl1 e 1o west 1e vels of the then ex1stmg surface. As some geo1 o gi. sts 11 ave referred a1lm ost all the superficial gravels to one era' and h ave supposed t 1em to be the result of one sudden catastrophe, the he-uomena of Auvergne above alluded t o at.e very I. mportPa nt The flo~s of volcanic matter have preserved portions of tl • surfahc e m. .t he. state in which they existed at s uccess1. ve pen.o dIse s~ t at It IS Impossible to confound together the alluviums of drfferent ages. The reader will see at once by rei! the wo d ( 1erence to o -cut No. 61) that a considerable interval of f ocdcu rred between the formation of the uppermost b ed of graIvrneel ~n that next below it; during which interval the uppermost :v~ was poured out and a valley excavated, at the bottom of tl hiCh the· second bed of gravel accumulated · I n l'k I e manner le pourmg out of a second current of lava a d .t' I deepenin f h ll ' n a 1art 1er g o t e va ey, took place between the date of the slecond gravel and that of the modern alluvium which n fill t le channel of the river *. ow s * For localities in Central France whe 1 alluviums at different ele f b re avas or sheets of basalt repose on MM. LeGrand d' A ussi ;I:~~~s. a o~e the present valleys, consult the works of Jobert, Bouillet, and otl~ers, OSler, amond, Scrope, Bertrand de Doue, Croizet, |