I. LACUSTRINE FORMATION OF THE LIMAGNE D'AUVERGNE.

The largest of these lakes covered the surface now occupied by the wide and fertile valley of the Allier, called la Limagne d'Auvergne, extending from Brioude in the south to some distance beyond Moulins on the north, having an average width of near twenty miles. It was bounded laterally by two parallel granitic ranges, that of the Forez, which divides the waters of the Loire and Allier on the east, and that of the Monts Dômes, which separates the Allier from the Sioule on the west. An arm of the same lake also evidently reached some way up the valley of the Loire from the junction of that river and the Allier to the neighbourhood of Roanne. The lower levels of this great valleyplain are for the most part superficially covered with alluvium, composed chiefly of pebbles of granite, gneiss, trachyte, and basalt. The substratum, wherever it shows itself, consists of nearly horizontal strata of sand, sandstone, calcareous marl, clay, or limestone, none of which observe a fixed and invariable order of superposition. Several hills also composed of the same strata rise from the plain, generally connected more or less with the granite ranges on either side. They were very justly described by M. Ramond, writing in 1815,† as "scattered relics of a series of beds which once covered the actual surface of the valley, and constituted an ancient plain much above the level of the present one."

*

Many of these fragments have evidently been protected from the destruction that has involved the greater part of the formation by a covering of basalt; others owe their preservation to a

The compilers of the 'Carte Géologique de la France' class these alluvial beds into two divisions, one supposed to be of recent origin, the other referred to the Pliocene or upper Tertiary period, from its containing fewer

pebbles of basalt, and generally occurring at higher levels, than the existing river-beds. But these distinctions do not seem to me as yet well made out.

† Mémoire sur le Nivellement des Monts Dômes.

similar capping of horizontal strata of a hard and durable limestone of a stalagmitic character to be described hereafter. These hills are seldom found in immediate union with the granite ranges, but are in general separated from them by shallow transverse valleys, so that the precise junction of the two formations is not often observable. But where this can be seen the lowest arenaceous beds are usually found to lean against the granite, sometimes at a considerable angle.

The principal divisions of the lacustrine series may be classed as-1st, grit and conglomerate, associated with red, blue, and white marls and sandstones; 2ndly, green and white foliated marls; 3rdly, limestone or travertin, often oolitic.

1. The sandstones and conglomerates forming the first or lowest of these divisions were considered by M. Brongniart, who gave them the name of " Arkose," to be of secondary and marine formation, and to be much anterior to the lacustrine strata with which they are associated; and several of the French geologists have followed him in this view. As they rarely contain any definite organic remains, either alternative seems at first tenable. But on a closer examination there is no difficulty in recognising the occasional alternation of these sandstone-beds with the calcareous strata containing freshwater shells,* and I have therefore no doubt of their belonging to the lacustrine series. They usually are seen resting directly on the granite edges of the basin, from the detritus of which they are evidently derived. Some beds consist of a conglomerate of worn pebbles and fragments of granite, gneiss, mica-schist, porphyry, the rocks of the adjoining elevated district, but without the admixture of basaltic or any other volcanic rocks. These arenaceous strata are not continuous round the margin of the lake basin, being rather disposed here and there, like the independent deltas

*For example, in the hills called les Côtes and Chanturgue near Clermont.

which grow at the mouths of torrents, along the borders of existing lakes.*

Other beds consist of a quartzose grit formed of separate crystals of quartz, mica, and felspar, evidently formed in situ from the disintegrated materials of the granite on which they rest, and from which they are scarcely to be distinguished. The cement is generally siliceous, and the resulting rock is sometimes hard enough to be used for millstones. At other times the cementing matter is calcareous, and the stone more brittle. The calcareous matter sometimes is aggregated into nodular concretions, passing into solid beds of limestone resembling the Italian travertin or the deposits of mineral springs. It occasionally contains crystallized sulphate of barytes in veins, chalcedony, and bitumen. The most largely developed of these arenaceous beds are, however, those of red, blue, and yellow marls and sandstones, which present an aspect absolutely identical with the secondary new red sandstone and marl of England. Some of these strata are very friable, others sufficiently compact to be quarried for building-stone. They are, like the conglomerates above mentioned, with which they occasionally alternate, evidently derived for the most part from the degradation of the adjoining granite or gneiss, which is in fact seen to decompose into an alluvium very similar to these tertiary sands and marls. The calcareous element, where it occurs, was, no doubt, added from the interior of the primary crystalline rocks, whence even now issue many springs depositing large quantities of carbonate of lime, and which must have been the source of the far greater bulk of that material composing the marly strata generally superimposed to the sandstones.

2. Green and white foliated marls. Sir Charles Lyell observes

*Lyell, Man., p. 698.

that the same primary rocks which, by the partial destruction of their harder parts, gave rise to the quartzose grits and conglomerates before mentioned, would, by the reduction of the same materials into powder or fine mud, and the decomposition of their felspar, mica, and hornblende, produce aluminous clay, and, by admixture with carbonate of lime from the springs, calcareous marl. This fine sediment would naturally be carried out to a greater distance from the shore than the coarser materials which are usually found in the immediate neighbourhood of the granitic borders. These chalky marls certainly attain a thickness in some spots of 600 or 700 feet. They are for the most part either light yellowish green or white, and have very much the aspect of chalk, with, like it, a semiconchoidal fracture when the strata are sufficiently thick. Usually, however, they are thinly foliated-a character which arises from the innumerable thin shells, or carapace valves, of that minute animal called cypris, which is known to moult its integuments periodically, differing in this from the conchiferous mollusks. On other points flattened stems of charæ, or myriads of small paludinæ or other freshwater shells, may be observed by the microscope to occasion this foliation, which is carried to such a degree that twenty or thirty lamina may often be counted in the thickness of an inch.

3. Interstratified with the marls we find thick beds of an oolitic limestone resembling our Bath stone in colour and structure, and, like it, acquiring greater hardness on exposure to the air. At Gannat and elsewhere this rock contains land-shells and bones of quadrupeds and birds. At Chadrat the oolitic grains are so large as to deserve the name of pisolites, the small spheroids combining both the radiated and concentric structure.

But the most remarkable form assumed by this freshwater limestone is that called "Indusial," from the cases, or indusia,

of caddis-worms (the larvæ 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 Indusia 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