Fig. 156.Cypris unifasciata, a living species, greatly magnified.a.Upper part.b.Side view of the same.
Fig. 156.
Cypris unifasciata, a living species, greatly magnified.
Fig. 157.Cypris vidua, a living species, greatly magnified.[183-A]
Fig. 157.
Cypris vidua, a living species, greatly magnified.[183-A]
The entire thickness of these marls is unknown; but it certainly exceeds, in some places, 700 feet. They are, for the most part, either light-green or white, and usually calcareous. They are thinly foliated,—a character which frequently arises from the innumerable thin shells, or carapace-valves, of that small animal calledCypris; a genus which comprises several species, of which some are recent, and may be seen swimming swiftly through the waters of our stagnant pools and ditches. The antennæ, at the end of which are fine pencils of hair, are the principal organs of motion, and are seen to vibrate with great rapidity. This animal resides within two small valves, not unlike those of a bivalve shell, and moults its integuments periodically, which the conchiferous mollusks do not. This circumstance may partly explain the countless myriads of the shells ofCypriswhich were shed in the ancient lakes of Auvergne, so as to give rise to divisions in the marl as thin as paper, and that, too, in stratified masses several hundred feet thick. A more convincing proof of the tranquillity and clearness of the waters, and of the slow and gradual process by which the lake was filled up with fine mud, cannot be desired. But we may easily suppose that, while this fine sediment was thrown down in the deep and central parts of the basin, gravel, sand, and rocky fragments were hurried into the lake, and deposited near the shore, forming the group described in the preceding section.
Not far from Clermont, the green marls, containing theCyprisin abundance, approach to within a few yards of the granite which forms the borders of the basin. The occurrence of these marls so near the ancient margin may be explained by considering that, at the bottom of the ancient lake, no coarse ingredients were deposited in spaces intermediate between the points where rivers and torrents entered, but finer mud only was drifted there by currents. Theverticalityof some of the beds in the above section bears testimony to considerable local disturbance subsequent to the deposition of the marls; but such inclined and vertical strata are very rare.
Fig. 158.Vertical strata of marl, at Champradelle, near Clermont.A. Granite.B. Space of 60 feet, in which no section is seen.C. Green marl, vertical and inclined.D. White marl.
Fig. 158.
Vertical strata of marl, at Champradelle, near Clermont.
3.Limestone, travertin, oolite.—Both the preceding members of the lacustrine deposit, the marls and grits, pass occasionally into limestone. Sometimes only concretionary nodules abound in them; but these, where there is an increase in the quantity of calcareous matter, unite into regular beds.
On each side of the basin of the Limagne, both on the west at Gannat, and on the east at Vichy, a white oolitic limestone is quarried. At Vichy, the oolite resembles our Bath stone in appearance and beauty; and, like it, is soft when first taken from the quarry, but soon hardens on exposure to the air. At Gannat, the stone contains land-shells and bones of quadrupeds, resembling those of the Paris gypsum. At Chadrat, in the hill of La Serre, the limestone is pisolitic, the small spheroids combining both the radiated and concentric structure.
Indusial limestone.—There is another remarkable form of freshwater limestone in Auvergne, called "indusial," from the cases, orindusiæ, of caddis-worms (the larvæ ofPhryganea); great heaps of which have been incrusted, as they lay, by carbonate of lime, and formed into a hard travertin. The rock is sometimes purely calcareous, but there is occasionally an intermixture of siliceous matter. Several beds of it are frequently seen, either in continuous masses, or in concretionary nodules, one upon another, with layers of marl interposed. The annexed drawing (fig. 159.) will show the manner in which one of these indusial beds (a) is laid open at the surface, between the marls (b b), near the base of the hill of Gergovia; and affords, at the same time, an example of the extent to which the lacustrine strata, which must once have filled a hollow, have been denuded, and shaped out into hills and valleys, on the site of the ancient lakes.
Fig. 159.Bed of indusial limestone, interstratified with freshwater marl, near Clermont (Kleinschrod.)
Fig. 159.
Bed of indusial limestone, interstratified with freshwater marl, near Clermont (Kleinschrod.)
Fig. 160.Larva of recent Phryganea.[185-A]
Fig. 160.
Larva of recent Phryganea.[185-A]
Fig. 161.a. Indusial limestone of Auvergne.b. FossilPaludinamagnified.
Fig. 161.
We may often observe in our ponds thePhryganea(or Caddis-fly), in its caterpillar state, covered with small freshwater shells, which they have the power of fixing to the outside of their tubular cases, in order, probably, to give them weight and strength. The individual figured in the annexed cut, which belongs to a species very abundant in England, has covered its case with shells of a smallPlanorbis. In the same manner a large species of caddis-worm, which swarmed in the Eocene lakes of Auvergne, was accustomed to attach to its dwelling the shells of a small spiral univalve of the genusPaludina. A hundred of these minute shells are sometimes seen arranged around one tube, part of the central cavity of which is often empty, the rest being filled up with thin concentric layers of travertin. The cases have been thrown together confusedly, and often lie, as infig. 161.,at right angles one to the other. When we consider that ten or twelve tubes are packed within the compass of a cubic inch, and that some single strata of this limestone are 6 feet thick, and may be traced over a considerable area, we may form some idea of the countless number of insects and mollusca which contributed their integuments and shells to compose this singularly constructed rock. It is unnecessary to suppose that thePhryganeælived on the spots where their cases are now found; they may have multiplied in the shallows near the margin of the lake, or in the streams by which it was fed, and their cases may have been drifted by a current far into the deep water.
In the summer of 1837, when examining, in company with Dr. Beck, a small lake near Copenhagen, I had an opportunity of witnessing a beautiful exemplification of the manner in which the tubular cases of Auvergne were probably accumulated. This lake, called the Fuure-Soe, occurring in the interior of Seeland, is about twenty English miles in circumference, and in some parts 200 feet in depth. Round the shallow borders an abundant crop of reeds and rushes may be observed, covered with the indusiæ of thePhryganea grandisand other species, to which shells are attached. The plants which support them are the bullrush,Scirpus lacustris, and common reed,Arundo phragmitis, but chiefly the former. In summer, especially in the month of June, a violent gust of wind sometimes causes a current by which these plants are torn up by the roots, washed away, and floated off in long bands, more than a mile in length, into deep water. TheCyprisswarms in the same lake; and calcareous springs alone are wanting to form extensive beds of indusial limestone, like those of Auvergne.
4.Gypseous marls.—More than 50 feet of thinly laminated gypseous marls, exactly resembling those in the hill of Montmartre, at Paris, are worked for gypsum at St. Romain, on the right bank of the Allier. They rest on a series of green cypriferous marls which alternate with grit, the united thickness of this inferior group being seen, in a vertical section on the banks of the river, to exceed 250 feet.
General arrangement, origin, and age of the freshwater formations of Auvergne.—The relations of the different groups above described cannot be learnt by the study of any one section; and the geologist who sets out with the expectation of finding a fixed order of succession may perhaps complain that the different parts of the basin give contradictory results. The arenaceous division, the marls, and the limestone, may all be seen in some places to alternate with each other; yet it can, by no means, be affirmed that there is no order of arrangement. The sands, sandstone, and conglomerate, constitute in general a littoral group; the foliated white and green marls, a contemporaneous central deposit; and the limestone is for the most part subordinate to the newer portions of both. The uppermost marls and sands are more calcareous than the lower; and we never meet with calcareous rocks covered by a considerable thickness of quartzose sand or green marl. From the resemblance of the limestones to theItalian travertins, we may conclude that they were derived from the waters of mineral springs,—such springs as even now exist in Auvergne, and which may be seen rising up through the granite, and precipitating travertin. They are sometimes thermal, but this character is by no means constant.
It seems that, when the ancient lake of the Limagne first began to be filled with sediment, no volcanic action had yet produced lava and scoriæ on any part of the surface of Auvergne. No pebbles, therefore, of lava were transported into the lake,—no fragments of volcanic rocks embedded in the conglomerate. But at a later period, when a considerable thickness of sandstone and marl had accumulated, eruptions broke out, and lava and tuff were deposited, at some spots, alternately with the lacustrine strata. It is not improbable that cold and thermal springs, holding different mineral ingredients in solution, became more numerous during the successive convulsions attending this development of volcanic agency, and thus deposits of carbonate and sulphate of lime, silex, and other minerals were produced. Hence these minerals predominate in the uppermost strata. The subterranean movements may then have continued until they altered the relative levels of the country, and caused the waters of the lakes to be drained off, and the farther accumulation of regular freshwater strata to cease.
We may easily conceive a similar series of events to give rise to analogous results in any modern basin, such as that of Lake Superior, for example, where numerous rivers and torrents are carrying down the detritus of a chain of mountains into the lake. The transported materials must be arranged according to their size and weight, the coarser near the shore, the finer at a greater distance from land; but in the gravelly and sandy beds of Lake Superior no pebbles of modern volcanic rocks can be included, since there are none of these at present in the district. If igneous action should break out in that country, and produce lava, scoriæ, and thermal springs, the deposition of gravel, sand, and marl might still continue as before; but, in addition, there would then be an intermixture of volcanic gravel and tuff, and of rocks precipitated from the waters of mineral springs.
Although the freshwater strata of the Limagne approach generally to a horizontal position, the proofs of local disturbance are sufficiently numerous and violent to allow us to suppose great changes of level since the lacustrine period. We are unable to assign a northern barrier to the ancient lake, although we can still trace its limits to the east, west, and south, where they were formed of bold granite eminences. Nor need we be surprised at our inability to restore entirely the physical geography of the country after so great a series of volcanic eruptions; for it is by no means improbable that one part of it, the southern, for example, may have been moved upwards bodily, while others remained at rest, or even suffered a movement of depression.
Whether all the freshwater formations of the Limagne d'Auvergne belong to one period, I cannot pretend to decide, as large masses both of the arenaceous and marly groups are often devoid of fossils.Much light has been thrown on the mammiferous fauna by the labours of MM. Bravard and Croizet, and by those of M. Pomel. The last-mentioned naturalist has pointed out the specific distinction of all, or nearly all, the species of mammalia, from those of the gypseous series near Paris. Nevertheless, many of the forms are analogous to those of Eocene quadrupeds. TheCainotherium, for example, is not far removed from theAnoplotherium, and is, according to Waterhouse, the same as the genusMicrotheriumof the Germans. There are two species of marsupial animals allied toDidelphys, a genus also found in the Paris gypsum. TheAmphitragulus elegansof Pomel, has been identified with a Rhenish species from Weissenau near Mayence, called by KaupDorcatherium nanum; and other Auvergne fossils, e.g.,Microtherium Reuggeri, and a small rodent,Titanomys, are specifically the same with mammalia of the Mayence basin.
Cantal.—A freshwater formation, very analogous to that of Auvergne, is situated in the department of Haute Loire, near the town of Le Puy, in Velay, and another occurs near Aurillac, in Cantal. The leading feature of the formation last mentioned, as distinguished from those of Auvergne and Velay, is the immense abundance of silex associated with calcareous marls and limestone.
The whole series may be separated into two divisions; the lower, composed of gravel, sand, and clay, such as might have been derived from the wearing down and decomposition of the granitic schists of the surrounding country; the upper system, consisting of siliceous and calcareous marls, contains subordinately gypsum, silex, and limestone.
The resemblance of the freshwater limestone of the Cantal, and its accompanying flint, to the upper chalk of England, is very instructive, and well calculated to put the student upon his guard against relying too implicitly on mineral character alone as a safe criterion of relative age.
When we approach Aurillac from the west, we pass over great heathy plains, where the sterile mica-schist is barely covered with vegetation. Near Ytrac, and between La Capelle and Viscamp, the surface is strewed over with loose broken flints, some of them black in the interior, but with a white external coating; others stained with tints of yellow and red, and in appearance precisely like the flint gravel of our chalk districts. When heaps of this gravel have thus announced our approach to a new formation, we arrive at length at the escarpment of the lacustrine beds. At the bottom of the hill which rises before us, we see strata of clay and sand, resting on mica-schist; and above, in the quarries of Belbet, Leybros, and Bruel, a white limestone, in horizontal strata, the surface of which has been hollowed out into irregular furrows, since filled up with broken flint, marl, and dark vegetable mound. In these cavities we recognize an exact counterpart to those which are so numerous on the furrowed surface of our own white chalk. Advancing from these quarries along a road made of the white limestone, which reflects as glaring a light in the sun, as do our roads composed of chalk, we reach, atlength, in the neighbourhood of Aurillac, hills of limestone and calcareous marl, in horizontal strata, separated in some places by regular layers of flint in nodules, the coating of each nodule being of an opaque white colour, like the exterior of the flinty nodules of our chalk.
It will be remembered that the siliceous stone of Bilin, calledtripoli, is a freshwater deposit, and has been shown, by Ehrenberg, to be of infusorial origin (seep. 24.). What is true of the Bohemian flint and opal, where the beds attain a thickness of 14 feet, may also, perhaps, be found to hold good respecting the silex of Aurillac, which may also have been immediately derived from the minute cases of microscopic animalcules. But even if this conclusion be established, the abundant supply both of siliceous, calcareous, and gypseous matter, which the ancient lakes of France received, may have been connected with the subterranean volcanic agency of which those regions were so long the theatre, and which may have impregnated the springs with mineral matter, even before the great outbreak of lava. It is well known that the hot springs of Iceland, and many other countries, contain silex in solution; and it has been lately affirmed, that steam at a high temperature is capable of dissolving quartzose rocks without the aid of any alkaline or other flux.[189-A]
Travellers not unfrequently mention, in their accounts of India, Australia, and other distant lands, that they have seen chalk with flints, which they have assumed to be of the same age as the Cretaceous system of Europe. A hasty observation of the white limestone and flint of Aurillac might convey the same idea; but when we turn from the mineral aspect and composition to the organic remains, we find in the flints of the Cantal the seed-vessels of the freshwaterChara, instead of the marine zoophytes so abundantly imbedded in chalk flints; and in the limestone we meet with shells ofLimnea,Planorbis, and other lacustrine genera, instead of the oyster, terebratula, and echinus of the Cretaceous period.
Proofs of gradual deposition.—Some sections of the foliated marls in the valley of the Cer, near Aurillac, attest, in the most unequivocal manner, the extreme slowness with which the materials of the lacustrine series were amassed. In the hill of Barrat, for example, we find an assemblage of calcareous and siliceous marls; in which, for a depth of at least 60 feet, the layers are so thin, that thirty are sometimes contained in the thickness of an inch; and when they are separated, we see preserved in every one of them the flattened stems ofCharæ, or other plants, or sometimes myriads of smallPaludinæand other freshwater shells. These minute foliations of the marl resemble precisely some of the recent laminated beds of the Scotch marl lakes, and may be compared to the pages of a book, each containing a history of a certain period of the past. The different layers may be grouped together in beds from a foot to a foot and a half in thickness, which are distinguished by differences of composition and colour, the tints being white, green, and brown. Occasionally thereis a parting layer of pure flint, or of black carbonaceous vegetable matter, about an inch thick, or of white pulverulent marl. We find several hills in the neighbourhood of Aurillac composed of such materials, for the height of more than 200 feet from their base, the whole sometimes covered by rocky currents of trachytic or basaltic lava.[190-A]
Thus wonderfully minute are the separate parts of which some of the most massive geological monuments are made up! When we desire to classify, it is necessary to contemplate entire groups of strata in the aggregate; but if we wish to understand the mode of their formation, and to explain their origin, we must think only of the minute subdivisions of which each mass is composed. We must bear in mind how many thin leaf-like seams of matter, each containing the remains of myriads of testacea and plants, frequently enter into the composition of a single stratum, and how vast a succession of these strata unite to form a single group! We must remember, also, that piles of volcanic matter, like the Plomb du Cantal, which rises in the immediate neighbourhood of Aurillac, are themselves equally the result of successive accumulation, consisting of reiterated sheets of lava, showers of scoriæ, and ejected fragments of rock.—Lastly, we must not forget that continents and mountain-chains, colossal as are their dimensions, are nothing more than an assemblage of many such igneous and aqueous groups, formed in succession during an indefinite lapse of ages, and superimposed upon each other.
Subdivisions of the Eocene group in the Paris basin — Gypseous series — Extinct quadrupeds — Impulse given to geology by Cuvier's osteological discoveries — Shelly sands called sables moyens — Calcaire grossier — Miliolites — Calcaire siliceux — Lower Eocene in France — Lits coquilliers — Sands and plastic clay — English Eocene strata — Freshwater and fluvio-marine beds — Barton beds — Bagshot and Bracklesham division — Large ophidians and saurians — Lower Eocene and London Clay proper — Fossil plants and shells — Strata of Kyson in Suffolk — Fossil monkey and opossum — Mottled clays and sands below London Clay — Nummulitic formation of Alps and Pyrenees — Its wide geographical extent — Eocene strata in the United States — Section at Claiborne, Alabama — Colossal cetacean — Orbitoid limestone — Burr stone.
Subdivisions of the Eocene group in the Paris basin — Gypseous series — Extinct quadrupeds — Impulse given to geology by Cuvier's osteological discoveries — Shelly sands called sables moyens — Calcaire grossier — Miliolites — Calcaire siliceux — Lower Eocene in France — Lits coquilliers — Sands and plastic clay — English Eocene strata — Freshwater and fluvio-marine beds — Barton beds — Bagshot and Bracklesham division — Large ophidians and saurians — Lower Eocene and London Clay proper — Fossil plants and shells — Strata of Kyson in Suffolk — Fossil monkey and opossum — Mottled clays and sands below London Clay — Nummulitic formation of Alps and Pyrenees — Its wide geographical extent — Eocene strata in the United States — Section at Claiborne, Alabama — Colossal cetacean — Orbitoid limestone — Burr stone.
Fromwhat was said in the two preceding chapters, it has already appeared that we have in England no true chronological representative of the Miocene faluns of the Loire, and none of the Upper Eocene groupdescribed in the last chapter. But, when we descend to the middle and inferior divisions of the Eocene system of France, we find that they have their equivalents in Great Britain.
Gypseous series(2.a, Table,p. 175.).—Next below the upper marine sands of the neighbourhood of Paris, we find a series of white and green marls, with subordinate beds of gypsum. These are most largely developed in the central parts of the Paris basin, and, among other places, in the Hill of Montmartre, where its fossils were first studied by M. Cuvier.
The gypsum quarried there for the manufacture of plaster of Paris occurs as a granular crystalline rock, and, together with the associated marls, contains land and fluviatile shells, together with the bones and skeletons of birds and quadrupeds. Several land plants are also met with, among which are fine specimens of the fan palm or palmetto tribe (Flabellaria). The remains also of freshwater fish and of crocodiles and other reptiles, occur in the gypsum. The skeletons of mammalia are usually isolated, often entire, the most delicate extremities being preserved; as if the carcasses, clothed with their flesh and skin, had been floated down soon after death, and while they were still swoln by the gases generated by their first decomposition. The few accompanying shells are of those light kinds which frequently float on the surface of rivers, together with wood.
M. Prevost has therefore suggested that a river may have swept away the bodies of animals, and the plants which lived on its borders, or in the lakes which it traversed, and may have carried them down into the centre of the gulf into which flowed the waters impregnated with sulphate of lime. We know that the Fiume Salso in Sicily enters the sea so charged with various salts that the thirsty cattle refuse to drink of it. A stream of sulphureous water, as white as milk, descends into the sea from the volcanic mountain of Idienne on the east of Java; and a great body of hot water, charged with sulphuric acid, rushed down from the same volcano on one occasion, and inundated a large tract of country, destroying, by its noxious properties, all the vegetation.[191-A]In like manner the Pusanibio, or "Vinegar River," of Colombia, which rises at the foot of Puracé, an extinct volcano, 7,500 feet above the level of the sea, is strongly impregnated with sulphuric and muriatic acids and with oxide of iron. We may easily suppose the waters of such streams to have properties noxious to marine animals, and in this manner the entire absence of marine remains in the ossiferous gypsum may be explained.[191-B]There are no pebbles or coarse sand in the gypsum; a circumstance which agrees well with the hypothesis that these beds were precipitated from water holding sulphate of lime in solution, and floating the remains of different animals.
In this formation the relics of about fifty species of quadrupeds, including the generaPaleotherium,Anoplotherium, and others, have been found, all extinct, and nearly four-fifths of them belonging to a division of the orderPachydermata, which is now represented by only four living species; namely three tapirs and the daman of the Cape. With them a few carnivorous animals are associated, among which are a species of fox and gennet. Of theRodentia, a dormouse and a squirrel; of theInsectivora, a bat; and of theMarsupialia(an order now confined to America, Australia, and some contiguous islands), an opossum, have been discovered.
Of birds, about ten species have been ascertained, the skeletons of some of which are entire. None of them are referable to existing species.[192-A]The same remark applies to the fish, according to MM. Cuvier, and Agassiz, as also to the reptiles. Among the last are crocodiles and tortoises of the generaEmysandTrionyx.
The tribe of land quadrupeds most abundant in this formation is such as now inhabits alluvial plains and marshes, and the banks of rivers and lakes, a class most exposed to suffer by river inundations. Whether the disproportion of carnivorous animals can be ascribed to this cause, or whether they were comparatively small in number and dimensions, as in the indigenous fauna of Australia, when first known to Europeans, is a point on which it would be rash, perhaps, to offer an opinion in the present state of our knowledge.
Fig. 162.Paleotherium magnum.
Fig. 162.
Paleotherium magnum.
The Paleothere, above alluded to, resembled the living tapir in the form of the head, and in having a short proboscis, but its molar teeth were more like those of the rhinoceros (seefig. 163.).Paleotherium magnumwas of the size of a horse, 3 or 4 feet high. The annexed woodcut,fig. 162., is one of the restorations which Cuvier attempted of the outline of the living animal, derived from the study of the entire skeleton. When the French osteologist declared in the early part of the present century, that all the fossil quadrupeds of the gypsum of Paris were extinct, the announcement of so startling afact, on such high authority, created a powerful sensation, and from that time a new impulse was given throughout Europe to the progress of geological investigation. Eminent naturalists, it is true, had long before maintained that the shells and zoophytes, met with in many ancient European rocks, had ceased to be inhabitants of the earth, but the majority even of the educated classes continued to believe that the species of animals and plants now contemporary with man, were the same as those which had been called into being when the planet itself was created. It was easy to throw discredit upon the new doctrine by asking whether corals, shells, and other creatures previously unknown, were not annually discovered? and whether living forms corresponding with the fossils might not yet be dredged up from seas hitherto unexamined? But from the era of the publication of Cuvier's Ossements Fossiles, and still more his popular Treatise called "A Theory of the Earth," sounder views began to prevail. It was clearly demonstrated that most of the mammalia found in the gypsum of Montmartre differed even generically from any now existing, and the extreme improbability that any of them, especially the larger ones, would ever be found surviving in continents yet unexplored, was made manifest. Moreover, the non-admixture of a single living species in the midst of so rich a fossil fauna was a striking proof that there had existed a state of the earth's surface zoologically unconnected with the present order of things.
Fig. 163.Upper molar tooth ofPaleotherium magnumfrom Isle ofWight. (Owen'sBrit. Foss. p. 317.)Reduced one-third.
Fig. 163.
Upper molar tooth ofPaleotherium magnumfrom Isle ofWight. (Owen'sBrit. Foss. p. 317.)
Reduced one-third.
Grès de Beauchamp(2.b, Table,p. 175.).—In some parts of the Paris basin, sands and marls, called the Grès de Beauchamp, or Sables Moyens, divide the gypseous beds from the underlying Calcaire grossier. These sands contain more than 300 species of marine shells, many of them peculiar, but others common to the underlying marine deposit (No. 2.c.).
Calcaire grossier(2.c, Table,p. 175.).—The formation called Calcaire grossier consists of a coarse limestone, often passing into sand. It contains the greater number of the fossil shells which characterize the Paris basin. No less than 400 distinct species have been procured from a single spot near Grignon, where they are embedded in a calcareous sand, chiefly formed of comminuted shells, in which, nevertheless, individuals in a perfect state of preservation, both of marine, terrestrial, and freshwater species, are mingled together. Some of the marine shells may have lived on the spot; but theCyclostomaandLimneamust have been brought thither by rivers and currents, and the quantity of triturated shells implies considerable movement in the waters.
Nothing is more striking in this assemblage of fossil testacea than the great proportion of species referable to the genusCerithium(seefig. 164.). There occur no less than 137 species of this genusin the Paris basin, and almost all of them in the calcaire grossier. Now the livingCerithiainhabit the sea near the mouths of rivers, where the waters are brackish; so that their abundance in the marine strata now under consideration is in harmony with the hypothesis, that the Paris basin formed a gulf into which several rivers flowed, the sediment of some of which gave rise to the beds of clay and lignite before mentioned; while a distinct freshwater limestone, called calcaire siliceux, which will presently be described, was precipitated from the waters of others situated farther to the south.
Fig. 164.Cerithium cinctum.[194-A]
Fig. 164.
Cerithium cinctum.[194-A]
EOCENE FORAMINIFERA.Fig. 165.Calcarina rarispina, Desh.b. natural size.a,c. same magnified.Fig. 166.Spirolina stenostoma, Desh.B. natural size.A, C, D. same magnified.Fig. 167.Triloculina inflata, Desh.b. natural size.a,c,d, same magnified.Fig. 168.Clavulina corrugata, Desh.a. natural size.b,c. same magnified.
EOCENE FORAMINIFERA.
Fig. 165.Calcarina rarispina, Desh.
Fig. 166.Spirolina stenostoma, Desh.
Fig. 167.Triloculina inflata, Desh.
Fig. 168.Clavulina corrugata, Desh.
In some parts of the calcaire grossier round Paris, certain beds occur of a stone used in building, and called by the French geologists "Miliolite limestone." It is almost entirely made up of millions of microscopic shells, of the size of minute grains of sand, which all belong to the class Foraminifera. Figures of some of these are given in the annexed woodcut. As this miliolitic stone never occurs in the Faluns, or Miocene strata ofBrittany and Touraine, it often furnishes the geologist with a useful criterion for distinguishing the detached Eocene and Miocene formations, scattered over those and other adjoining provinces. The discovery of the remains of Paleotherium and other mammalia in some of the upper beds of the calcaire grossier shows that these land animals began to exist before the deposition of the overlying gypseous series had commenced.
Calcaire siliceux.—This compact siliceous limestone extends over a wide area. It resembles a precipitate from the waters of mineral springs, and is often traversed by small empty sinuous cavities. It is, for the most part, devoid of organic remains, but in some places contains freshwater and land species, and never any marine fossils. The siliceous limestone and the calcaire grossier occupy distinct parts of the Paris basin, the one attaining its fullest development in those places where the other is of slight thickness. They also alternate with each other towards the centre of the basin, as at Sergy and Osny; and there are even points where the two rocks are so blended together that portions of each may be seen in hand specimens. Thus, in the same bed, at Triel, we have the compact freshwater limestone, characterized by itsLimneæ, mingled with the coarse marine limestone, with its small multilocular shells, or "miliolites," dispersed through it in countless numbers. These microscopic testacea are also accompanied byCerithiaand other shells of the calcaire grossier. It is very extraordinary that in this instance both kinds of sediment must have been thrown down together on the same spot, yet each retains its own peculiar organic remains.
From these facts we may conclude, that while to the north, where the bay was probably open to the sea, a marine limestone was formed, another deposit of freshwater origin was introduced to the southward, or at the head of the bay; for it appears that during the Eocene period, as now, the ocean was to the north, and the continent, where the great lakes existed, to the south. From that southern region we may suppose a body of fresh water to have descended, charged with carbonate of lime and silica, the water being perhaps in sufficient volume to freshen the upper end of the bay. The gypseous series (2.a, Table,p. 175.), before described, was once supposed to be entirely subsequent in origin to the two groups, called calcaire grossier and calcaire siliceux. But M. Prevost has pointed out that in some localities they alternate repeatedly with both.
The gypsum, with its associated marl and limestone, is in greatest force towards the centre of the basin, where the calcaire grossier and calcaire siliceux are less fully developed. Hence M. Prevost infers, that while those two principal deposits were gradually in progress, the one towards the north, and the other towards the south, a river descending from the east may have brought down the gypseous and marly sediment.
It must be admitted, as highly probable, that a bay or narrow sea, 180 miles in length, would receive, at more points than one, the waters of the adjoining continent. At the same time, we must beprepared to find that the simultaneous deposition of two or more sets of strata in one basin, some freshwater and others marine, must have produced very complex results. But, in proportion as it is more difficult in these cases to discover any fixed order of superposition in the associated mineral masses, so also is it more easy to explain the manner of their origin, and to reconcile their relations to the agency of known causes. Instead of the successive irruptions and retreats of the sea, and changes in the chemical nature of the fluid, and other speculations of the earlier geologists, we are now simply called upon to imagine a gulf, into one extremity of which the sea entered, and at the other a large river, while other streams may have flowed in at different points, whereby an indefinite number of alternations of marine and freshwater beds would be occasioned.
Lits coquilliers(3.a, Table,p. 175.).—Below the calcaire grossier are extensive deposits of sand, in the upper parts of which some marine beds, called "lits coquilliers," occur, in which M. d'Archiac has discovered 200 species of shells. Many of these are peculiar, but the larger portion appear to agree with species of the calcaire grossier, so that the line of demarcation usually adopted between the French Lower and Middle Eocene formations, seems not to be very strongly drawn.Sands and plastic clay(3.b, Table,p. 175.)—At the base of the tertiary system in France are extensive deposits of sands, with occasional beds of clay used for pottery, and called "argile plastique." Fossil oysters (Ostrea bellovacina) abound in some places, and in others there is a mixture of fluviatile shells, such asCyrena cuneiformis(fig. 187.p. 204.),Melania inquinata(fig. 188.), and others, frequently met with in beds occupying the same position in the valley of the Thames. Layers of lignite also accompany the inferior clays and sands.
Immediately upon the chalk at the bottom of all the tertiary strata there is often a conglomerate or breccia of rolled and angular chalk flints, cemented by siliceous sand. These beds appear to be of littoral origin, and imply the previous emergence of some portions of the chalk, and its waste by denudation.
Fig. 169.Cardium porulosum. Parisand London basins.
Fig. 169.
Cardium porulosum. Parisand London basins.
The lower sandy beds of the Paris basin are often called the sands of the Soissonais, from a district so named 50 miles N.E. of Paris. One of the shells of the formation is adduced by M. Deshayes as an example of the changes which certain species underwent in the successivestages of their existence. It seems that different varieties of theCardium porulosumare characteristic of different formations. In the Lower Eocene of the Soissonais this shell acquires but a small volume, and has many peculiarities, which disappear in the lowest beds of the calcaire grossier. In these the shell attains its full size, and many distinctive characters, which are again modified in the uppermost beds of the calcaire grossier; and these last modifications of form are preserved throughout the whole of the "upper marine" (or Upper Eocene) series.[197-A]
The Eocene areas of Hampshire and London are delineated in the map (fig. 153.p. 174.).
The following table will show the succession of the principal deposits found in our island. The true place of the Bagshot sands, in this series, was never accurately ascertained till Mr. Prestwich published, in 1847, his classification of the English Eocene strata, dividing them into three principal formations, in which the Bagshot sands occupied the central place.[197-B]
Fig. 170.Lymnea longiscata.Freshwater Eocene strata, Isle of Wight.
Fig. 170.
Lymnea longiscata.
Freshwater Eocene strata, Isle of Wight.
Freshwater beds(2.a, Table,p. 175.).—In the northern part of the Isle of Wight, beds of marl, clay, and sand, and a friable limestone, containing freshwater shells, are seen, containing shells of the generaLymnea(seefig. 170.),Planorbis,Melanopsis,Cyrena, &c., several of them of the same species as those occurring in the Eocene beds of the Paris basin. Gyrogonites, also, or seed-vessels ofChara, exhibiting a similar specific identity, occur. At Headon Hill, on the western side of the island, where these beds are seen in the sea-cliffs, some of the strata contain a few marine and estuary shells, such asCytheræa,Corbula, &c., showing a temporary occupation of the area by brackish or salt water, after which the river or a lake seems again to have prevailed. Aspecies of fan-palm,Flabellaria Lamanonis, Brong., like one which characterizes the Parisian Eocene beds, has been recently detected by Dr. Mantell in this formation, in Whitecliff Bay, at the eastern end of the island.
Several of the species of extinct quadrupeds already alluded to as characterizing the gypsum of Montmartre have been discovered by Messrs. Pratt and Fox, in the Isle of Wight, chiefly at Binstead, near Ryde, asPalæotherium magnum,P. medium,P. minus,P. minimum,P. curtum,P. crassum, alsoAnoplotherium commune,A. secundarium,Dichobune cervinum, andChæropotamus Cuvieri. In Hordwell cliff, also on the Hampshire coast, several of these species, with other quadrupeds of new genera, such asPaloplotherium, Owen, have been met with; and remains of a remarkable carnivorous genus,Hyænodon. These fossils are accompanied by the bones ofTrionyx, and other tortoises, and by two land snakes of the genusPaleryx, Owen, from 3 to 4 feet long, also a species of crocodile, and an alligator. Among other fossils collected by Lady Hastings, Sir Philip Egerton has recognized the well-known gar or bony pike of the American rivers, a ganoid fish of the genusLepidotus, with its hard shining scales. The shells of Hordwell are similar to those of the freshwater beds of the Isle of Wight, and among them are a few specifically undistinguishable from recent testacea, asPaludina lentaandHelix labyrinthica, the latter discovered by Mr. S. Wood, and identified with an existing N. American helix.
The white and green marls of this freshwater series in Hampshire, and some of the accompanying limestones, often resemble those of France in mineral character and colour in so striking a manner, as to suggest the idea that the sediment was derived from the same region, or produced contemporaneously under very similar geographical circumstances.
Barton beds.—Both in the cliffs of Headon Hill and Hordwell, already mentioned, the freshwater series rests on a mass of pure white sand without fossils, and this is seen in Barton Cliff to overlie a marine deposit, in which 209 species of testacea have been found. More than half of these are peculiar; and, according to Mr. Prestwich, only 11 of them common to the London Clay proper, being in the proportion of only 5 per cent. On the other hand, 70 of them agree with thecalcaire grossiershells. As this is the newest purely marine bed of the Eocene series known in England, we might have expected that some of its peculiar fossils would be found to agree with the upper Eocene strata described in the last chapter, and accordingly some identifications have been cited with testacea, both of the Berlin and Belgian strata. It is nearly a century since Brander published, in 1766, an account of the organic remains collected from these cliffs, and his excellent figures of the shells then deposited in the British Museum are justly admired by conchologists for their accuracy.
Bagshot Sands(2.c, Table,p. 197.).—These beds, consisting chieflyof siliceous sand, occupy extensive tracts round Bagshot, in Surrey, and in the New Forest, Hampshire. They succeed next in chronological order, and may be separated into three divisions, the upper and lower consisting of light yellow sands, and the central of dark green sands and brown clays, the whole reposing on the London clay proper.[199-A]Although the Bagshot beds are usually devoid of fossils, they contain marine shells in some places, among whichVenericardia planicosta(seefig. 171.) is abundant, withTurritella sulciferaandNummulites lævigatus. (Seefig. 174.p. 200.)