miles-originally, perhaps, indeed the whole plain of the Limagne, measuring forty miles by twenty-some idea may be formed of the countless myriads of minute animals belonging to one or two species only of mollusks which must have formerly lived and died on the bottom or shores of this extensive lake! The places where I have observed these characteristic beds best developed are on the hills of Gergovia above Romagnat, at the Puys Girou, de Jussat, de la Serre, de Monton, de Dallet, at Mont Chagny, Mont Jughat, and Les Côtes near Clermont; at. Davayat near Riom; at Aigueperse, Gannat, Mayet d'Ecole, St. Gerard-le-Puy, between Jaligny and la Palisse, at Pont Barraud, &c. It is unnecessary to suppose that the Phryganeæ or the mollusks all lived precisely on the spot where their remains are found. They may have multiplied in the shallows near the margin of the lake, or in the streams by which it was fed, and the cases have been drifted into the deep water, perhaps (as is suggested by Sir Charles Lyell) borne upon the masses of littoral reeds to which they were attached, when these were torn away from the banks by storms, and floated out by the winds and currents to great distances. Sometimes, instead of the Indusiæ, the calcareous incrustation appears to have enveloped the reeds themselves, grasses or mosses growing on the bank. At others the beds lose their stalagmitic character, and appear as a compact earthy limestone of an ochreous yellow colour, and contain large shells of the genera Helix, Planorbis, or Lymneus. The testaceous matter is often replaced by calcareous spar, occasionally by bitumen, which appears too in veins or crevices of this rock. Some are traversed by veins of flint or semi-opal. Indeed the whole rock becomes in some parts highly siliceous, breaking into sharp splinters with a conchoidal fracture. And

*

* Manual, p. 202, ed. 1856.

some resemble in mineral character the secondary lithographic limestone of Châteauroux. The calcareous marls often contain much gypsum, selenite filling the fissures of the rock and of the associated marls; and that so abundantly as to be extracted for commercial purposes-as at the Puys de Coran, de Millefleur, and the Butte de Montpensier.

In general it is in the more solid strata just described, rather than in the large intervening deposits of white, soft, foliated marls, that are found the remains not only of mollusks, but also of the vertebrated animals, of which a very copious catalogue has been published by M. Pomel.* Carnivora, Insectivora, Rodentes, Ungulata in numbers; and among them the anthracotherium, rhinoceros, dinotherium, cœnotherium, and palæocherus; Ruminants, crocodiles, tortoises, lacertæ, birds, serpents, fish, and batrachians are also frequent. All these, says M. Pomel, seem properly to belong to those great deposits of concretionary indusial limestone which form so important an element in the geognosy of the whole Limagne.† This is in fact what would be expected from the littoral origin we have already ascribed to the contents of these beds. M. Pomel declares the result of the examination of his catalogue to be that all the ossiferous deposits of the Limagne lake belong to the same geological and zoological period (the Lower Miocene), and cannot be divided into periods by reason of their palæontological characters, since the same species are found in the oldest as in the most recent beds of this formation. Whatever differences occur are such only as are found in the existing fauna, and may be referred to differences in geographical or climatic situation, in the habits or the greater or less extension of species, to accidents of deposition or fossilization. With

* This catalogue will be found in the Appendix.

Pomel, Catalogue des Vertèbres

Fossiles, découverts dans le Bassin de la Loire supérieure et de l'Allier. Paris, 1854, p. 151.

respect to the question of correspondence in point of age of the lacustrine deposits of the Limagne with other well-known tertiary formations, M. Pomel exhibits more doubt, but on the whole concludes from palæontological evidence that they may be considered as parallel to the fossiliferous beds of Mayence, more recent than the gypsum of the Paris basin, but older than the Faluns of Touraine-a classification which corresponds very closely with the opinion of Sir Charles Lyell, who, after hesitating for some time as to whether these Auvergne deposits belonged to the upper members of his Eocene formation, or to the lower ones of the Miocene, has, I have reason to believe, made up his mind to consider them as of the age of the latter division of the tertiary series."

These remarks refer, of course, to the lacustrine strata alone of the Limagne, not to certain later ossiferous alluvial deposits, such - as occur at the noted localities of Mont Perrier and Pardines, which belong to a period when the volcanos of this country had been long in activity, the lake certainly drained, its strata largely denuded, and deep and wide valleys channelled out through them. To these I shall revert in a later page.

No traces of rocks of a volcanic character occur in the conglomerates or sandstones which constitute the lower terms of this freshwater formation. But higher in the series a mixture is here and there to be observed of the products of neighbouring volcanic eruptions with the calcareous beds. In occasional instances fragments of basaltic lava, crystals of augite, scoriæ, and volcanic ashes are scattered through some of the undisturbed limestone-beds, and assume frequently a remarkable disposition, the heavier and larger fragments occupying the lower part of each stratum, the lighter and smaller stopping in

* See the Supplement to his fifth edition of the Manual (Murray, 1857).

the upper parts-suggesting the obvious idea that these fragments, after ejection from a volcanic vent, had fallen through the air into the lake at the time the stratum in which they occur was in the condition of very soft calcareous mud. Geologists will probably see in this fact some analogy to the arrangement of the layers of flint nodules in common chalk strata.* observed strata of this character on the mountain of Gergovia and several other points; but the most remarkable example may be seen on the banks of the Allier immediately above Pont.

I have

du Château. The river here flows at the foot of a line of cliff, which it is continually undermining, and thus exposes a regular vertical section rather more than 100 feet in height. (See the woodcut above.)

The upper surface of the cliff is horizontal, and formed to the depth of 8 or 10 feet of a bed of boulders exactly the same as those which the river still rolls along its bed below. The strata beneath are not completely horizontal, but have a slight curve rising upwards at each extremity of the exposed cliff, as if they had been deposited in a hollow, or perhaps subjected to some amount of disturbance since their deposition.

* In which case likewise "it seems as if there had been time for each successive accumulation of calcareous mud to become partially consolidated, and for a re-arrangement of its particles to

take place (the heavier silex sinking to the bottom), before the next stratum was superimposed."-Lyell's Manual, 5th ed.,

p. 244.

1. The lowest visible beds consist of a compact brownish grey limestone containing much fragmentary volcanic matter, generally arranged as above described in each stratum according to their respective gravities, the largest fragments at the bottom, the rest diminishing in size in proportion to their distance from thence. These beds dip rapidly below the level of the river, and are covered by

2. A bed about 4 feet thick of a coarse-grained limestone of an ochreous yellow colour and earthy fracture, without any visible admixture of volcanic matter. It contains the casts of numerous shells, Planorbes, Helices, Lymnei, the shells being often replaced by bitumen, which exudes likewise from fissures in the rock. This is a very common variety of the hard limestone beds frequent throughout the lacustrine series.

3. Above this is a considerable thickness of the soft white foliated marly limestone, characteristic of the formation, having a plain fracture, sometimes slightly conchoidal, strongly adhering to the tongue, with an earthy smell, its exposed surfaces occasionally presenting efflorescent carbonate of soda. It has frequently a tendency to desquamate in globular masses.

4. A single bed, 10 inches thick, of a very compact, hard, and fine-grained limestone, with a conchoidal fracture, of a deep bluish-black, probably coloured by volcanic ashes.

5. Numerous calcareo-volcanic strata, similar to No. 1; some of them being ribboned with different colours, yellow, brown, or bluish-black, determined, it seems, by the greater or less proportion of volcanic ashes contained in their several zones Where this proportion is considerable the rock generally has the more compact character and conchoidal fracture; and by still further increase of the volcanic element it passes into a kind of peperino or basaltic breccia, which often affects a globular concretionary structure.