denudation to which the freshwater formation of the Limagne has been subjected, and which has left (as M. Ramond expressed it) only a few detached hills as relics of a former plain elevated many hundred feet above that now existing, may well be believed to have been accompanied by a corresponding destruction of superficial rocks at the northern end of the plain. I have attempted to show, not unsuccessfully, I hope, that a large proportion, if not the whole, of the degradation sustained since the tertiary period, as well by the granitic platform itself and its surrounding secondary zone, as by the freshwater strata, was effected by the slow and gradual but long-continued erosive force of the ordinary meteoric agents of denudation, rain, torrents, and river-floods, co-operating, as in this district they most probably did, at least during its eruptive periods, with frequent earthquake shocks, and perhaps a general elevation of the southern portion of the platform. I cannot conclude the detailed description I have attempted to give of this interesting country, better than by quoting the eloquent summary of its characteristic features which Sir Charles Lyell has given.* 66 We are here presented with the evidence of a series of events of astonishing magnitude and grandeur, by which the original form and features of the country have been greatly changed, yet never so far obliterated but that they may still, in part at least, be restored in imagination. Great lakes have disappeared-lofty mountains have been formed, by the reiterated emission of lava, preceded and followed by showers of sand and scoria-deep valleys have been subsequently furrowed out through masses of lacustrine and volcanic origin-at a still later date, new cones have been thrown up in these valleys-new lakes have been formed by the damming up of rivers-and more than one creation of quadrupeds, birds, and plants [Miocene, Pliocene, and Post-Pliocene] have followed in succession; * Manual, p. 127, ed. 1855. yet the region has preserved from first to last its geographical identity; and we can still recall to our thoughts its external condition and physical structure before these wonderful vicissitudes began, or while a part only of the whole had been completed. There was first a period when the spacious lakes, of which we still may trace the boundaries, lay at the foot of mountains of moderate elevation, unbroken by the bold peaks and precipices of Mont Dor, and unadorned by the picturesque outline of the Puy de Dome, or of the volcanic cones and craters now covering the granitic platform. During this earlier scene of repose deltas were slowly formed; beds of marl and sand, several hundred feet thick, deposited; siliceous. and calcareous rocks precipitated from the waters of mineral springs; shells and insects imbedded, together with the remains of the crocodile and tortoise; the eggs and bones of water birds, and the skeletons of quadrupeds, some of them belonging to the same genera as those entombed in the Eocene gypsum of Paris. To this tranquil condition of the surface succeeded the era of volcanic eruptions, when the lakes were drained, and when the fertility of the mountainous district was probably enhanced by the igneous matter ejected from below, and poured down upon the more sterile granite. During these eruptions, which appear to have taken place after the disappearance of the [Lower Miocene] fauna, and partly in the [Pliocene] epoch, the mastodon, rhinoceros, elephant, tapir, hippopotamus, together with the ox, various kinds of deer, the bear, hyæna, and many beasts of prey ranged the forest, or pastured on the plain, and were occasionally overtaken by a fall of burning cinders, or buried in flows of mud, such as accompany volcanic eruptions. Lastly, these quadrupeds became extinct, and gave place to [Post-Pliocene] mammalia, and these, in their turn, to species now existing. There are no signs, during the whole time required for this series of events, of the sea having intervened, nor of any denudation which may not have been accomplished by currents in the different lakes, or by rivers and floods accompanying repeated earthquakes, during which the levels of the district have in some places been materially modified, and perhaps the whole upraised relatively to the surrounding parts of France.” APPENDIX. CATALOGUE OF ORGANIC REMAINS, TABLE OF HEIGHTS, AND EXPLANATION OF THE MAPS AND ENGRAVINGS. APPENDIX. ORGANIC REMAINS OF THE TERTIARY AND POST-TERTIARY FORMATIONS OF CENTRAL FRANCE. VEGETABLE remains abound in the lower arenaceous beds of the lacustrine formations, as well as in the tuffs of the volcanic period, where they occasionally form beds of workable lignite. But in neither case as yet have they been scientifically determined. They chiefly consist of the leaves, fruits, and occasionally stems of dicotyledonous trees, or of reeds and other plants, the usual growth of marshy spots. The stems of charæ are very abundant as well as their seed-vessels. I am able to say little more of the fossil mollusks found in the same formations. The tertiary sandstones rarely contain shells, but some species of Cyrena have been found in them.* The associated or overlying limestones and marls abound in shells belonging to the genera Helix, Lymneus, Paludina, Bulimus, Cerithium, Cyrena, Unio, and Cypris. Some of the species mentioned by M. Bouillet† appear to be referable to an earlier period than that to which, on the best Palæontological authorities, we have considered the entire freshwater formation of Central France to belong, viz., the Lower Miocene. Sir Charles Lyell has recently touched upon this question in a supplement to his Manual. And the matter remains still open to further investigation. Two marine shells are said to have been discovered in a sandy stratum near Issoire, by MM. Bravard and Pomel, belonging to the genera Natica and Pleurotoma, and akin to some occurring in the Faluns of the lower basin of the Loire. Such a circumstance would seem to indicate either a reflux at some period of the waters of that river, or that the Miocene sea had actually ascended the Pomel, Bulletin, 2de Ser., vol. i. p. 579. † Bull., vol. vi. p. 99 and 255. |