Part 29 (1/2)

_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 Hamps.h.i.+re 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.