entire granitic region proves it to have formed an island in the ancient ocean where those strata were deposited, it is quite possible that it may have sustained considerable absolute elevation since the formation of its tertiary beds, and especially during its eruptional era; indeed it would be unlikely that Central France should have remained unmoved during the elevation of portions of the neighbouring Alps and the basin of Switzerland from beneath the sea to a height of 5000 or 6000 feet. And again, with our knowledge of the repeated alternations of marine and freshwater deposits in the basin of Paris, into which the calcariferous waters of the Limagne overflowed, it is impossible to question the probability of occasional subsidence along the line of the intervening heights. I am however of opinion that whatever changes of relative level took place, they operated over wide superficial areas, since few or no traces of disturbance are visible in the sedimentary beds of the Limagne. The surfaces over which the basaltic currents flowed, and which, having been since protected by them from the rain-fall, preserve the highest remaining surfaces of the lacustrine formation (although at that time they must have been its lowest levels), exhibit just that gradual inclination away from the bordering heights, where the greater number were erupted, which might be expected to prevail in the bottom of a shallow but gradually deepening lake. No sudden faults or dislocations appear to indicate any relative changes of level within areas of moderate extent.* On the other hand, the vast amount of *This remark, however, must be confined to the Limagne. The freshwater strata of the Cantal have evidently suffered a certain amount of disturbance. M. Raulin describes the calcareous beds in the valley of the Alagnon as reaching a height of between 600 and 700 feet above the corresponding strata in the contiguous valley of the Cer; and some portions of these tertiary beds (viz. at Dienne) have been lifted up to an absolute elevation of 3680 feet-a position in which it is difficult to suppose they were deposited. 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.* "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. |