THIRD FRESHWATER FORMATION. 333 7. The superior sandstones and marine sands.This formation is composed of siliceous sand and sandstone, in beds frequently of great thickness and extent, but with the two surfaces seldom parallel. Both of them, and especially the upper, often present irregular prominences and hollows, of a rounded shape, which scarcely ever correspond; whence their very variable and unequal depth. The inferior portion of the beds, consists of a very pure sand quite destitute of fossils; the superior affords nodules of iron-ore disposed in horizontal layers.. This lower mass of the third sandstone deposit, is in many places terminated by a rock sandstone, limestone, or siliceous limestone, stored with sea shells, constituting a marked second marine formation above the chalk. It varies in colour, solidity, and even composition; and is evidently superior not only to the gypsum, but also to the extensive and very massive beds of sandstone, and sand without shells. There is therefore in the environs of Paris, three kinds of sandstone, sometimes very like each other in their mineralogical characters, but very different in position, and geological nature. On observing this last marine formation, placed in a position so different from the others, one cannot help reflecting on the singular circumstances which must have presided at the formation of the beds above described. 8. The third freshwater formation including the marls and the millstones.-This formation is distinguished from the two preceding by its position, and by some geognostic characters, which however are not sufficient of themselves, to designate it with certainty, when it stands insulated from the rest. But its position in the vicinity of Paris, so perfectly distinct from that of the second freshwater strata, leaves no doubt as to the difference between these two beds, separated as they are by a marine formation. This upper lacustrine, or fresh water formation, is composed in different districts of different rocks. In the plain of La Trappe, it consists of white, friable, or at least tender calcareous marls; in the environs of Epernon, of translucid, light-gray, or brown hornstones; in the mountain of Triel, Montreuil, &c., of opaque, white, or rose coloured jaspery flints; and on the plains of Meudon, Montmorency, Sanois, la Ferte-sons-Jonarre, &c., of porous or compact mill or buhr stones, reddish, grayish, or whitish in colour, sometimes without shells, sometimes replete with limnei, planorbes, potamides, helices, gyrogonites, silicified woods, and other organic remains, which must have lived in the fresh waters, or on the surface of the soil. The millstones rest immediately on the sands, which contain the sandstones. That formation consists of a ferro-argillaceous sand, greenish, reddish, or even whitish, clay marl, and millstones properly so called. These three substances do not seem to follow any fixed order in their superposition; the millstone is sometimes above, sometimes below, and sometimes in the middle; and the same thing may be said of the sand or clay marl. The buhr-stones are very rarely in continuous beds, but rather in angular pieces, as if resulting from thin beds, broken and enveloped in the clay marl, and in the iron-clay sand. The millstone is well known to be a silex, riddled throughout with a multitude of irregular cavities, garnished with siliceous network, disposed almost like the reticular tissue of bones, and daubed over with a red ochrey varnish. These cavities are often filled with clay-marl, or clayey sand. They do not communicate with each other. Another geological character of the proper buhr stones, namely of such as by the continuity of their mass are fittest for making flour millstones, is the absence of every organic body animal or vegetable, marine or fresh water. Sometimes these stones are covered CHARACTERISTIC FRESHWATER SHELLS. 335 only with vegetable mould, but often also we find above it, some compact buhr stones, flints, or marls, including fresh water shells or other organic remains (not marine), or diluvial rolled pebbles lying in a coarse grained sand. The most common associate of the millstones, is the freshwater limestone, nearly pure; the mixture of limestone and silex is next in abundance, and the great masses of freshwater siliceous stones are the rarest. Whether the limestone be marly or compact, it shows very often cylindrical cavities, irregular, and nearly parallel though sinuous. This freshwater limestone however hard when taken out of the quarry, sometimes falls down into a coarse powder, by the action of the weather; on which account it is chiefly used as a manure. But what characterises essentially this formation, is the presence of freshwater and land shells, almost all similar in genera to those now found in the French marshes or pools; such as limnei, planorbes, potamides, turbinated shells akin to the cerites, cyclostomes, helices, &c. We also find those small round grooved bodies, which seem to be the grains of a species of chara. (See the account of Loch Bakie, next section). It is remarkable that no bivalve shells occur in this formation, at least in the neighbourhood of Paris. The following testaceous mollusca occur : Cyclostoma elegans antiquum; potamides Lamarkii; planorbis rotundatus, cornu, Prevostinus; limneus corneus, fabulum, ventricosus inflatus; bulimus pygmeus, terebra; pupa Defrancii; helix Lemani, Desmarestina. There are also several vegetables of indeterminate genera. Among the determinate, are chara medicaginula, helicteres; and nymphæa arethusa (rhizoma, a subterranean stem). This freshwater formation is widely spread, not only over the environs of Paris, even thirty leagues to the south, but it is found in other districts of France. It has been recognised by M. Brogniart in Cantal, and in the department of Puy-de-Dome; and it occurs in many other places, but constantly with the same characters. It is matter of surprise, therefore, that it has hitherto been so little noticed by naturalists. (See next Section.) The great extent of this formation in the environs of Paris, and its presence in many other places, must make us admit the existence of great bodies of fresh water in the ancient state of the earth. Although we had no other examples of such extensive inland lakes, it would not seem more difficult to believe that they must have existed, than to admit the presence of the sea over the ground which constitutes our actual continents, along with so many other geological phenomena, no less inexplicable, though they cannot be contested. But in the case under consideration, we have still before us in the present state of the earth, examples of freshwater lakes, almost equal in length to France from north to south, and of immense breadth. We need merely look to a map of North America to be struck with the vast magnitude of lakes Superior, Michigan, Huron, Erie and Ontario. And in the operations of the petrifying calcareous waters now forming the travertino of Italy, we have an example of a great freshwater formation of rocky strata, as dense as any in the limestone beds of Paris. On reconsidering these beds from the chalk upwards, we conceive first of all a sea depositing on its bottom an immense mass of chalk, and mollusca of peculiar species. This precipitation of the chalk, and of its attendant shells suddenly stops; the sea retires, waters of another kind, very probably analogous to that of our freshwater lakes, succeed, and CUVIER'S REVIEW OF THE PARIS MINERAL BASIN. 337 up all the hollows of the marine formation are filled with clays, debris of land vegetables, and of freshwater shells. But soon another sea, producing new inhabitants, nourishing a prodigious quantity of testaceous mollusca, entirely different from those of the chalk, returns and covers the clay, its lignites, and their shells, to deposit on that basis, thick beds, composed in a great degree of the shelly-coverings of these new mollusca. By degrees, this production of shells diminishes, and also comes to an end; the sea withdraws, and the soil is again covered with lakes of freshwater. Alternate strata are formed of gypsum and marl, which envelope both the debris of the animals bred in the lakes, and the bones of those which lived on their banks. The sea comes back once more; it breeds at first some species of bivalve and turbinated shellfish, which disappear, and are replaced by oysters. An interval of time now elapses during which a great mass of sand is deposited. We are led to believe that no organized bodies lived at that period in this sea, or that their exuviæ have been completely destroyed; for none are to be found in the sand-bed. But the varied productions of this third sea re-appear, and we again observe on the summit of Montmartre, Romainville, the hill of Nanteuil le-Hadouin, &c. the same shells as were found in the marls placed over the gypsum, which though really different from those of the coarse-grained limestone, are still considerably like them. Lastly, the sea withdraws entirely for the third time. Lakes or marshes of freshwater take its place, and cover with the remains of their inhabitants, the tops of almost all the |