of caddis-worms (the larvae of Phryganea), great heaps of which have been incrusted as they lay by carbonate of lime, and formed into a hard travertin or stalagmitic limestone. This rock is seen sometimes to form ranges of concretionary nodules, at others continuous beds, one over another, with layers of the foliated marls interposed.

It is well known that certain varieties of the Phryganea (or caddis-fly) are in the habit, when in their caterpillar state, of clothing their bodies with a cylindrical case composed entirely of minute river-shells of some single species-helices, mytili, planorbes, or other-united by glutinous filaments, and disposed in some sort of order around. These habitations are quitted when the insect's metamorphosis is completed; and on the banks of rivers or marshes frequented by them, heaps of such empty cases may be observed. If we suppose them in this state to be exposed to be incrusted by calcareous matter from the depositions of some neighbouring spring, they will assume precisely the appearance of the remarkable rock which we find in Auvergne, composing repeated strata of considerable bulk, alternating through a thickness of several hundred feet with the more ordinary marls. The surfaces of these beds are usually mammillated or botryoidal, and the calcareous matter enveloping the Indusiæ is arranged concentrically in the manner of a stalagmite. Where the bed is thinnest, the continuity is often interrupted or prolonged in separate nodular concretions of the same kind imbedded in loose sand. The minute shells surrounding the larva-cases are usually the Bulimus atomus of Brongniart, or a small Paludina. More than a hundred of these shells may be counted round a single tube, and ten or twelve tubes may be found packed together irregularly in a single cubic inch of the rock. When it is added that repeated strata of this kind eight or ten feet thick appear to have covered very many square

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.* I have 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

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.