of Mont Dor and the Cantal, they must be referred to a similar origin; and we agree with Mr Scrope, that the sudden rush of large bodies of water down the sides of an elevated volcano, at its moments of eruption, sweeping away all the loose materials surrounding its crater, might be supposed to give rise to such breccias; and that the floods observed by Humboldt and others to descend from the trachytic volcanoes of America, countenance the hypothesis *.

The associated strata of alluvium and breccia terminate, in all the cases described by us, in abrupt escarpments towards the rivers, and they must once have extended much farther, but possibly not so far as to the opposite sides of the present valleys. For we have seen, that where the bottom of a valley, in Auvergne, or the Vivarais, has been filled with lava, the rivers have usually hollowed out a new passage through the granitic schists, as well as in part through the lava. So, if a valley become a lake, and be filled up to a great height with alluvial conglomerates and tufaceous breccias, it is highly improbable that the new excavation will coincide precisely in position with the original valley, though perhaps of equal width and depth; and especially if it happen that the ancient valley was bounded by tertiary marls, probably much softer than most of the newly imported matters.

For the same reason, therefore, that we so often see all that remains of an ancient basaltic current, exclusively confined to one and the same bank of a river, we may expect to find the relics of a lacustrine formation similarly situate.

To the hypothesis of lakes closed up by volcanic currents, it will, no doubt, be objected, that we are unable to point out any remnants of the supposed barriers. But how, we ask, is it probable, that such barriers could have survived the changes which this country has subsequently experienced in consequence of aqueous denudation, assisted, in all probability, by violent earthquakes? The present valleys are much deeper than those supposed to have been formerly converted into lakes. In the case of Mont Perrier, the excavation extends to the depth of between 50 and 100 feet in the fresh-water strata; and at St

Maurice, from 400 to 500 feet below the old alluvium and tufaceous breccia. At Monton, the depth of the valley below the tufaceous breccia is probably still greater; but here the high dip of the tertiary strata, and the proximity of basaltic dikes in the hill of Vayres, leave us in doubt how far volcanic disturbance may have co-operated with aqueous action. Now, we may demand, Is it reasonable to expect, that, if the present lakes of Aidat and Chambon were filled up, and their lacustrine deposites subsequently removed to as great an extent as those of Perrier and Monton, or as nearly annihilated as that of St Maurice,— or, if the valleys of the Couze de Chambon and Vayres had sunk below their present depth from 100 to 500 feet,-can it be expected that future geologists would still be able to trace the former outline of these lakes, or point out the position of their barriers? But it should be remembered, that there are other causes besides lava-streams, which frequently dam up the course of rivers in regions within the range of volcanic agency. The last great earthquakes in Sicily and Calabria, in 1783, gave rise to land-falls, wherein huge masses, more than a mile in length, bordering the coast or rivers, were suddenly precipitated into the adjoining sea or valley. In such cases, the barrier may last till the lake is filled up; but it will afterwards be more destructible, and will more readily admit of being entirely obliterated, than a lava-current. Two large lakes were thus occasioned in Calabria, the course of two rivers having been obstructed. In the case of Mont Perrier we are aware that several geologists consider a barrier wholly unnecessary. They endeavour to account for the phenomena in the same manner as Signor Lippi explained the successive alluviums, which certainly constitute a large portion of the covering under which Herculaneum is buried. The matter, it is said, washed down from a neighbouring volcano, is so copious, that rivers cannot remove what the floods carry down. To such an hypothesis, founded at least on plausible grounds, we reply, that so enormous a thickness of transported materials, severally distinct in their character, as are seen at Mont Perrier, could never have been lodged in so narrow a valley, if there had not been a permanent stoppage; for the alternating beds of rounded pebbles could not have been brought down by a river or flood, without the hollow

ing out or sweeping away of the subjacent pumiceous and other beds of lighter ingredients. Further, there would have been more confusion, and more unconformability, in the deposites, if they had subsided from turbulent waters rushing through this narrow valley, while it remained an open passage for the drainage of the higher country. But the waters of a temporary lake, on the contrary, would arrest the progress of the matter introduced into it, and allow it to spread over the bottom with some degree of regularity, the violent erosive force of the current being deadened, and each deposite being protected from destruction by the mass of water, while others were superimposed.

A lake, therefore, being regarded as alone capable of explaining in a satisfactory manner the phenomena of Perrier, it becomes an interesting matter of inquiry to decide at what epoch the valley was dammed up, and at what period, in that great succession of volcanic eruptions of which Auvergne has been the theatre, the animals, whose remains were buried in the lake, lived and perished in these regions.

Now, if the recent lava-stream, which has its course down the same valley, had flowed on as far as the village of Perrier, it would evidently have assisted us greatly in determining this relative date; for we should then have known how far the lacustrine formation had been eaten into when the "cheire" descended, and, consequently, whether the lake had long ceased to exist, before the more modern class of volcanoes in Auvergne had commenced their activity.

We have before stated, that this modern lava-current has actually flowed down the valley to a certain distance, viz. to the spot where the village of Ourseyre, below Besse, stands; but it stopped short at Sauriers, some miles before reaching the locality where the alluviums and breccias attest the former existence of the lake. This deficiency, however, is in some measure supplied by the existence, in the next adjoining valley of Nechers, of the current of Tartaret, one of the most recent in Auvergne, and which the geologist may look down upon from the summit of Mont Perrier; so that it requires no great effort of the imagination to suppose the lava transferred from the one valley to the other, and thereby to judge of the relations which a “cheire” of this age would have borne to the ancient alluviums, had it

flowed as far down the bed of the Couze d'Issoire, instead of that of the Couze de Chambon. This evidence, however, is of a nature best appreciable by those who have visited the district, and are familiar with the geological phenomena of Auvergne. To such it will appear clear, that one of the modern lava-streams of Auvergne, passing by Pardines, Perrier, and the Farm of Boulade, would merely share with the river the lowest level of the valley, and would not reach so high as the lowest beds of alluvium and trachytic breccia, which last would then hold the same relative position to the recent lava as does the ancient phonolitic current before mentioned, below Besse, to the modern "cheire" beneath.

The fossil remains of animals discovered near the Farm of Boulade, were imbedded in alluvial gravel and sand subjacent to a great mass of trachytic breccia. Now, the different varieties of this rock occurring at Mont Perrier are undistinguishable from those which enter into the composition of Mont Dor and the Cantal, where they are seen in the ridgy prolongations branching off from those mountains, to alternate with ancient volcanic products of different eruptions; and near the Cascade of Mont Dor, and at the foot of the Puy Gros, they are traversed by basaltic dikes. One of the masses of trachytic breccia at Perrier is no less than 60 feet in thickness, and encloses fragments of trachyte of enormous magnitude, cemented by a tufaceous base, as hard as the rock itself. Since these fragments are fully as angular and large as any of those in similar rocks laid open in the interior of Mont Dor, there is not a shadow of pretence for regarding those at Perrier as regenerated, and as having resulted from the breaking up of more ancient breccias.

And when we identify these breccias with those of Mont Dor, it must be recollected that such a conclusion is in perfect harmony with their height above the river at Perrier, which corresponds, as we before remarked, to that of lava-currents of the intermediate age in Auvergne. From such considerations, therefore, we must infer, that the alluvium, from whence the remains of quadrupeds were disinterred, near the Farm of Boulade, and at Perrier, was of very high antiquity; nor is this inference at variance with the character of these remains, since the individuals belonged for the most part, if not entirely, to extinct

species of the genera Mastodon, Elephant, Rhinoceros, Hippopotamus, Tapir, Bear, Hyæna, Stag, and others.* These quadrupeds, therefore, inhabited this district long before the more recent volcanic cones and lavas of Auvergne were in existence, ere yet the valleys had attained their present depth, and even before the fires of the ancient volcanoes of Mont Dor had become extinguished.

In concluding this article, we may observe, that Auvergne, Velay, and the Vivarais, throw peculiar light on the theory of valleys, because the volcanic rocks having been introduced upon the surface successively, and sometimes at intervals immensely distant from each other, have preserved portions of the surface in the state in which it existed at those several periods. Hence it becomes impossible to confound the effects of erosive action of one epoch with those of another. But for this circumstance, events the most remote in point of time,—the waste of floods or violent torrents of most distinct eras, would have been regarded as simultaneous. Thus the conglomerates, several hundred feet thick, on which rests nearly the whole series of alternating beds of trachyte, basalt, and scoriæ of Mont Dor, would have remained without any distinct line of separation from the latest alluvions; and the same. would, in many cases, have happened in Cantal, although we have now reason to conceive that the oldest lava of Etna is not more ancient, as compared with the latest, than are the several masses of transported materials last alluded to. If the debris of all these various periods were now strewed over the country at the various elevations where they are at present observed, and if we possessed not the means, which we now have, of pointing out their different ages, they would present the appearance of having been the result of one sudden and dreadful catastrophe, whereby rocks of different

• About forty species of quadrupeds will be described by MM. Croizet and Jobert, when their splendid and valuable work is completed, from the alluvions of Mont Perrier alone. Similar remains are not wanting, it appears, near Puy en Velay; for M. Bertrand Roux has just informed us, that he has obtained from a locality first pointed out by Dr Hibbert of Edinburgh, fossil bones and teeth belonging to the genera Rhinoceros, Hyæna, Stag and others, plates of which will shortly be published. Their geological position is very interesting, since they were found in volcanic scoriæ, covered by one of the most modern lava-currents of that district.