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Show 258 EOCENE PERIOD. [Ch. XIX. of the period under consideration, and shall then proceed to show that there are in the same country volcanic rocks of much newer date, some of which appear to be referrible to the Miocene era. Volcanic ·rocks associated with Lacust·rine inAuve·rgne.-The first locality to which we shall call the reader's attention is Pont du Chateau near Clermont, where a section is seen in a precipice on the right banlc of the river Allier*. Beds of volcanic tufF alternate with a fresh-water limestone, which is in some places pure, but in others spotted with fragments of volcanic matter, as if it were deposited while showers of sand and scorire were projected from a neighbouring vent t- This limestone contains the Helix Ramondi and other shells of Eocene species. It is immaterial to our present argument whether the volcanic sand was showered down from above, or drifted to the spot by a river, for the latter opinion must presuppose the country to have been covered with volcanic ejections during the Eocene period. Another example occurs in the Puy de l\Iarmont, ncar Veyres, where a fresh-water marl alternates with volcanic tuff containing Eocene shells. The tuff or breccia in this locality is precisely such as is known to result from volcanic ashes falling into water, and subsiding together with ejected fragments of marl and other stratified rocks. 'These tuffs and marls are highly inclined, and traversed by a thick vein of basalt which, as it rises in the hill, divides into two branches. Gergovia.-The hill of Gergovia near Clermont affords a third example. W c agree with 1\IM. Dufrenoy and Jobert that there is no alternation here of lava and fresh-water strata, in the manner supposed by some other observers!; but the position and contents of some of the tuffs prove them to have been derived from volcanic eruptions which occunecl. during the deposition of the Eocene formations. "' This place, and all the others in A uv<'rgne, mentioned in this chapter, were examined by the author, in company with Mr. Murchison, in 1828. t See Scro11e's Central F1·ance1 p. 21, l Scrope1 ibid, p. 7, ·Ch.XIX.] VOLCANIC ROCKS-AUVERGN~. 259 The bottom of the hill consists of slightly inclined beds of white and greenish marls, more than three hundred feet in thickne~s, ~hich are ~ntersected by a dike of basalt, which may b~ studied In the ravme above the village of Merdogne. The d1ke here cuts through the marly strata at a considerable angle, No. 60. ,I I Ill Ill ,I II I I I I 'I I I J,l IIIII I/ Ill ;I Ill :Ill Bnsnltlc capping. ----- White nod yellow mo.rl. -~======----========~ nnWdghrietee n marls. Hill of Gergovia. producing, in general, great alteration and confusion in them for some distance from the point of contact. Above the white and green marls, a series of beds of limestone and marl containin()' f h ' b res -water shells, are seen to alternate with volcanic tuff. In tl~e lowest part of this division, beds of pure marl alternate With compact fissile ~~ff resembling some of the subaqueous tuffs of Italy_and S1c1ly called peperinos. Occasionally fragments of scor1re are visible in this rock. Still higher is seen another ~roup_ of some thickness, consisting exclusively of tuff, upon whiCh he other marly strata intermixed with volcanic matter. 'l'here are many points in Auvergne where igneous rocks have b~en forced by subsequent injection through clays and marly hii_Jestones, in such a manner that the whole has become blended m one confused and brecciated mass, between wllich and th: basalt there is sometimes no very distinct line of demarcatiOn. In the cavities of such mixed rocks we often find calcedony and crystals of mesotype, stilbite and arragonite. To s 2 |
