A Manual of Elementary Geology Part 29

_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 its _Limneae_, mingled with the coa.r.s.e marine limestone, with its small multilocular sh.e.l.ls, or "miliolites,"

dispersed through it in countless numbers. These microscopic testacea are also accompanied by _Cerithia_ and other sh.e.l.ls 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 a.s.sociated 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 princ.i.p.al 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 be prepared 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 a.s.sociated mineral ma.s.ses, 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.

LOWER EOCENE, FRANCE.

_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 sh.e.l.ls. 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 sh.e.l.ls, such as _Cyrena 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.

[Ill.u.s.tration: Fig. 169. _Cardium porulosum_. Paris and 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 sh.e.l.ls of the formation is adduced by M. Deshayes as an example of the changes which certain species underwent in the successive stages of their existence. It seems that different varieties of the _Cardium porulosum_ are characteristic of different formations. In the Lower Eocene of the Soissonais this sh.e.l.l acquires but a small volume, and has many peculiarities, which disappear in the lowest beds of the calcaire grossier.

In these the sh.e.l.l 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]

ENGLISH EOCENE FORMATIONS.

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 princ.i.p.al 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 cla.s.sification of the English Eocene strata, dividing them into three princ.i.p.al formations, in which the Bagshot sands occupied the central place.[197-B]

Localities.

1. Upper Eocene. Wanting in Great Britain.

{ _a._ Freshwater and Headon Hill, Isle of { fluvio-marine beds. Wight; and Hordwell { Cliff, Hants.

2. Middle Eocene { _b._ Barton beds. Barton Cliff, Hants.

{ _c._ Bagshot and Bracklesham Bagshot Heath, Surrey; { sands and clays. Bracklesham Bay, { Suss.e.x.

{ _a._ London Clay Proper, Highgate Hill, { and Bognor beds. Middles.e.x; I. of { Sheppey; Bognor, 3. Lower Eocene { Suss.e.x.

{ _b._ Mottled and Plastic Newhaven, Suss.e.x; { clays and sands. Reading, Berks; { Woolwich, Kent.

[Ill.u.s.tration: 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 sh.e.l.ls, are seen, containing sh.e.l.ls of the genera _Lymnea_ (see fig. 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 of _Chara_, exhibiting a similar specific ident.i.ty, 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 sh.e.l.ls, such as _Cytheraea_, _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. A species 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, as _Palaeotherium magnum_, _P. medium_, _P. minus_, _P. minimum_, _P. curtum_, _P. cra.s.sum_, also _Anoplotherium commune_, _A. secundarium_, _Dichobune cervinum_, and _Chaeropotamus Cuvieri_. In Hordwell cliff, also on the Hampshire coast, several of these species, with other quadrupeds of new genera, such as _Paloplotherium_, Owen, have been met with; and remains of a remarkable carnivorous genus, _Hyaenodon_. These fossils are accompanied by the bones of _Trionyx_, and other tortoises, and by two land snakes of the genus _Paleryx_, 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 genus _Lepidotus_, with its hard shining scales. The sh.e.l.ls 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, as _Paludina lenta_ and _Helix 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 circ.u.mstances.

_Barton beds._--Both in the cliffs of Headon Hill and Hordwell, already mentioned, the freshwater series rests on a ma.s.s 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 the _calcaire grossier_ sh.e.l.ls. 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 sh.e.l.ls then deposited in the British Museum are justly admired by conchologists for their accuracy.

_Bagshot Sands_ (2. _c_, Table, p. 197.).--These beds, consisting chiefly of 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 sh.e.l.ls in some places, among which _Venericardia planicosta_ (see fig. 171.) is abundant, with _Turritella sulcifera_ and _Nummulites laevigatus_. (See fig. 174. p. 200.)

[Ill.u.s.tration: Fig. 171. _Venericardia planicosta_, Lamck.

_Cardita planicosta_, Deshayes.]

At Bracklesham Bay, near Chichester, in Suss.e.x, the characteristic sh.e.l.ls of this member of the Eocene series are best seen; among others, the huge _Cerithium giganteum_, so conspicuous in the calcaire grossier of Paris, where it is sometimes 2 feet in length. The volutes and cowries of this formation, as well as the lunulites and other corals, seem to favour the idea of a warm climate having prevailed, which is borne out by the discovery of a serpent _Palaeophis typhaeus_, exceeding, according to Mr.

Owen, 20 feet in length, and allied to the Boa, Python, Coluber, and Hydrus. The compressed form and diminutive size of certain caudal vertebrae indicate so much a.n.a.logy with Hydrus as to induce the Hunterian professor to p.r.o.nounce the extinct ophidian to have been marine.[199-B] He had previously combated with so much success the evidence advanced, to prove the existence in the Northern Ocean of sea-serpents in our own times, that he will not be suspected of any undue bias in contending for their former existence in the British Eocene seas. The climate, however, of the Middle Eocene period was evidently far more genial; and amongst the companions of the sea-serpent of Bracklesham was an extinct Gavial (_Gavialis Dixoni_, Owen), and numerous fish, such as now frequent the seas of warm lat.i.tudes, as the sword-fish (see fig. 172. p. 200.) and gigantic rays of the genus Miliobates. (See fig. 173.)

Out of 193 species of testacea procured from the Bagshot and Bracklesham beds in England, 126 occur in the French calcaire grossier. It was clearly, therefore, coeval with that part of the Parisian series more nearly than with any other. The _Nummulites laevigatus_ (see fig. 174.), a fossil characteristic of the lower beds of the calcaire grossier, is abundant at Bracklesham.

[Ill.u.s.tration: Fig. 172. Prolonged premaxillary bone or "sword" of a fossil sword-fish (_Caelorhynchus_). Bracklesham. Dixon's Fossils of Suss.e.x, pl. 8.]

[Ill.u.s.tration: Fig. 173. Dental plates of _Myliobates Edwardsi_.

Bracklesham Bay. Ibid. pl. 8.]

[Ill.u.s.tration: Fig. 174. _Nummulites_ (_Nummularia_) _laevigatus._ Bracklesham. Ibid. pl. 8.

_a._ section of the nummulite.

_b._ group, with an individual showing the exterior of the sh.e.l.l.]

_London clay proper_ (3. _a_, Table, p. 197.).--This formation underlies the preceding, and consists of tenacious brown and blueish grey clay, with layers of concretions called septaria, which abound chiefly in the brown clay, and are obtained in sufficient numbers from the cliffs near Harwich, and from shoals of the Ess.e.x coast, to be used for making Roman cement. The princ.i.p.al localities of fossils in the London clay are Highgate Hill, near London, the island of Sheppey, and Bognor in Hampshire. Out of 133 fossil sh.e.l.ls, Mr. Prestwich found only 20 to be common to the calcaire grossier (from which 600 species have been obtained), while 33 are common to the lits coquilliers (p. 196.), in which only 200 species are known in France.

We may presume, therefore, that the London clay proper is older than the calcaire grossier. This may perhaps remove a difficulty which M. Adolphe Brongniart has experienced when comparing the Eocene Flora of the neighbourhoods of London and Paris. The fossil species of the island of Sheppey, he observes, indicate a much more tropical climate than the Eocene Flora of France, which has been derived princ.i.p.ally from the "gypseous series." The latter resembles the vegetation of the borders of the Mediterranean rather than that of an equatorial region.

Mr. Bowerbank, in a valuable publication on the fossil fruits and seeds of the island of Sheppey, near London, has described no less than thirteen fruits of palms of the recent type _Nipa_, now only found in the Molucca and Philippine islands. (See fig. 175.) These plants are allied to the cocoa-nut tribe on the one side, and on the other to the _Panda.n.u.s_, or screw-pine. Species of cocoa-nuts are also met with, and other kinds of palms; also three species of _Anona_, or custard-apple; cucurbitaceous fruits, also (the gourd and melon family), are in considerable abundance.

Fruits of various species of _Acacia_ are in profusion; and, although less decidedly tropical, imply a warm climate.

[Ill.u.s.tration: Fig. 175. _Nipadites ellipticus._ Bow. Fossil palm of Sheppey.]

The contiguity of land may be inferred not only from these vegetable productions, but also from the teeth and bones of crocodiles and turtles, since these creatures, as Mr. Conybeare has remarked, must have resorted to some sh.o.r.e to lay their eggs. Of turtles there were numerous species referred to extinct genera, and, for the most part, not equal in size to the largest living tropical turtles. A snake, which must have been 13 feet long, of the genus _Palaeophis_ before mentioned, has also been described by Mr. Owen from Sheppey, of a different species from that of Bracklesham. A true crocodile, also, _Crocodilus toliapicus_, and another Saurian more nearly allied to the gravial, accompany the above fossils. A bird allied to the vultures, and a quadruped of the new genus _Hyracotherium_, allied to the Hyrax, Hog, and Chaeropotamus, are also among the additions made of late years to the palaeontology of this division.

[3 Ill.u.s.trations: FOSSIL Sh.e.l.lS OF THE LONDON CLAY.

Fig. 176. _Mitra scabra_.

Fig. 177. _Rostellaria macroptera_, Sow. One-third of nat. size.

Fig. 178. _Cra.s.satella sulcata._]

The marine sh.e.l.ls of the London clay confirm the inference derivable from the plants and reptiles of a high temperature. Thus, many species of _Conus_, _Mitra_, and _Voluta_ occur, a large _Cypraea_, a very large _Rostellaria_, and sh.e.l.ls of the genera _Terebellum_, _Cancellaria_, _Cra.s.satella_, and others, with four or more species of _Nautilus_ (see fig. 182.) and other cephalopoda of extinct genera, one of the most remarkable of which is the _Belosepia_.[202-A] (See fig. 183.)

[Ill.u.s.tration: Fig. 179. _Nautilus centralis._]

[Ill.u.s.tration: Fig. 180. _Voluta athleta._]