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Show 234 EOCENE PERIOD. [Ch. XVII. we never meet with calcareous rocks covered by a considerable thickness of quartzose sand or green marl, and the uppermost marls and sands are more cal~areous than the lower. }'rom the resemblance of the Eocene limestones of Auvergne to the Italian travertins, we may conclude that they were derived from the waters of mineral springs,-such springs as now exist in Auvergne, and which rising up through the granite precipitate travertin. r.rhey are sometimes thermal, but this character is by no means constant. We suppose that, when the ancient lake of the Limagne £rst beO'an to be filled with sediment, no volcanic action had pro- o duced lava and scorire on any part of the surface of Auvergne. No pebbles, therefore, of lava were transported into the lake,no fragments of volcanic rocks imbedded in the conglomerate. But at a later period, when a considerable thickness of sandstone and marl had accumulated, eruptions broke out, and lava and tuff were alternately deposited, at some spots, with the lacustrine strata. Of this we shall give proofs in the 19th chapter. It is not improbable that cold and thermal springs, holding different mineral ingredients in solution, increased in number durinO' the successive convulsions attending this development 0 of volcanic agency, and thus carbonate and sulphate of lime, silex, and other minerals, were produced. Hence these mine. rals predominate in the uppermost strata. The subterranean movements may then have continued until they altered the relative levels of the country and caused the waters of the lakes to be drained off, and the farther accumulation of regular fresh-water strata to cease. The occurrence of these convulsions anterior to the Miocene epoch, and prolonged during a succession of after-ages, may explain why no fresh-water formations more recent than the Eocene are now found in this country. We may easily conceive a similar series of events to give rise to analogous results in any modern basin, such as that of Lake Superior, for example, where numerous rivers and torrents are carrying down the detritus of a chain of mountains into the Ch. XVII.] LACUSTRINE STRATA-PUY. EN VELAY. 235 lake. The transported materials must he arranged according to their size and weight, the coarser ncar the shore, the finer at a greater distance from land ; but in the gravelly and sandy beds of Lake Superior no pebbles of modern volcanic rocks can be included) sinee there are none of these at present in the district. If the igneous action should break out in that country and produce lava, scorire, and thermal springs, the deposition of gravel, sand, and marl, might still continue as before ; but in addition, there would then be an intermixture of volcanic gravel and tuff, and rocks precipitated from the waters of mineral springs. Although the fresh-water strata of the Limagne approach generally to a horizontal position, the proofs of local disturbance are sufficiently numerous and violent to allow us to suppose great changes of level since the Eocene period. We are unable to assign a northern barrier to the ancient lake, although we can still trace its limits to the east, west, and south, where they were formed of bold granitic eminences. But we need not be surprised at our inability to restore the physical geography of the country after so great a series of volcanic eruptions. It is by no means improbable that one part of the district may have been moved upwards bodily, while the others remained at rest, or even suffered a movement of depression. Puy en Velay .-In the department of the Haute Loire, a fresh-water formation, very analogous to that of Auvergne, is situated in the basin of the Loire, and is exposed in the valley in which stands the town of Le Puy. Since the deposition of the lacustrine strata, there have been so many volcanic eruptions in this country, and such immense quantities of lava and scorire poured out upon the surface, that the aqueous rocks are almost buried and concealed. We are indebted, however, to the researches of M. Bertrand de Doue for having distinctly ascertained the succession of strata, and we have had opportunities of verifying his observations during a 'risit to Le Puy. In this basin we find, as in AuvergneJ two great divisions, consisting of grits and marls; th~ former composed of quartzose |