Part 26 (2/2)

Upper Eocene. < calcaire=”” silicieux=”” or=”” bembridge=”” beds.=””>Fluvio-marine< calcaire=”” lacustre=”” series.=”” moyen.=”” gypseous=”” series=”” of=””>

/ Osborne beds.

/ Gres de Beauchamp

Headon beds. / and Calcaire Marin.

Upper Bagshot sand. Upper Sables Moyens.

Middle Eocene.< lower=”” sables=”” moyens,=”” barton=”” clay.=”” middle=””>< lower=”” calcaire=”” bracklesham=”” beds.=”” bagshot.=”” grossier,=”” and=”” glauconie=””>

Lower Bagshot / Lits coquillieres.

beds. Glauconie Moyenne.

/ London clay. Wanting.

Woolwich and / Argile Plastique.

Reading beds, or > Glauconie Inferieure.

Lower Eocene. < plastic=”” clay.=”” oldhaven=””>

Thanet sands. Sables Inferieurs.

The Woolwich and Reading Beds, or the Plastic Clay of older writers, consists of extensive beds of sand with occasional beds of potter's clay, which lie at the base of the Tertiary formation in both England and France. Generally variegated, sometimes grey or white, it is employed as a potter's earth in the manufacture of delf-ware.

In England the red-mottled clay of the Woolwich and Reading Beds in Hamps.h.i.+re and the Isle of Wight is often seen in contact with the chalk; but in the south-eastern part of the London basin, Mr. Prestwich shows that the Thanet Sand (consisting of a base of fine, light-coloured sand, mixed with more or less argillaceous matter) intervenes between the Chalk and the Oldhaven Beds, or in their absence the Woolwich and Reading beds, which lie below the London Clay. The Thanet Sands derive their name from their occurrence in the Isle of Thanet, in Kent, in the eastern part of which county they attain their greatest development.

Under London and its southern suburbs the Thanet sand is from thirteen to forty-four feet thick, but it becomes thinner in a westerly direction, and does not occur beyond Ealing.[85]

[85] ”Memoir of the Geological Survey of Great Britain. The Geology of Middles.e.x, &c.;” by W. Whitaker, p. 9.

The Woolwich and Reading beds in the Hamps.h.i.+re basin rest immediately on the Chalk, and separate it from the overlying London Clay, as may be seen in the fine exposure of the Tertiary strata in Alum Bay, at the western extremity of the Isle of Wight, and in Studland Bay, on the western side of the Isle of Purbeck, in Dorsets.h.i.+re.

In the London basin the Woolwich and Reading beds also rest on the Chalk, where the Thanet Sands are absent, as is the case, for the most part, over the area west of Ealing and Leatherhead.

The beds in question are very variable in character, but may be generally described as irregular alternations of clays and sands--the former mostly red, mottled with white, and from their plastic nature suitable for the purposes of the potter; the latter also of various colours, but sometimes pure white, and sometimes containing pebbles of flint.

The Woolwich and Reading beds are called after the localities of the same names; they are fifty feet thick at Woolwich, and from sixty to seventy feet at Reading.

The Oldhaven beds (so termed by Mr. W. Whitaker from their development at the place of the same name in Kent) are a local deposit, occurring beneath the London Clay on the south side of the London basin, from Croydon eastward, at the most eastern part of Surrey, and through Kent--in the north-western corner of which county they form some comparatively broad tracts. The beds consist of rounded flint pebbles, in a fine sandy base, or of fine light-coloured sand, and are from eighty to ninety feet thick under London.

The London Clay, which has a breadth of twenty miles or more about London, consists of tenacious brown and bluish-grey clay, with layers of the nodular concretions, called Septaria, which are well known on the Ess.e.x and Hamps.h.i.+re coasts, where they are collected for making Roman cement. The London Clay has a maximum thickness of nearly 500 feet. The fossils of the London Clay are of marine genera, and very plentiful in some districts. Taken altogether they seem to indicate a moderate, rather than a tropical climate, although the Flora is, as far as can be judged, certainly tropical in its affinities.[86] The number of species of extinct Turtles obtained from the Isle of Sheppey alone, is stated by Prof. Aga.s.siz to exceed that of all the species of Chelone now known to exist throughout the globe. Above this great bed lie the Bracklesham and Bagshot beds, which consist of light-yellow sand with an intermediate layer of dark-green and brown clay, over which lie the Barton Clay (in the Hamps.h.i.+re basin) and the white Upper Bagshot Sands, which are succeeded by the Fluvio-marine series comprising the Headon, Bembridge, and Hempstead series, and consisting of limestones, clays, and marls, of marine, brackish, and fresh-water origin.[87] For fuller accounts of the Tertiary strata of England, the reader is recommended to the numerous excellent memoirs of Mr. Prestwich, to the memoir ”On the Tertiary Fluvio-marine Formations of the Isle of Wight,” by Professor Edward Forbes, and to the memoir ”On the Geology of the London Basin,” by Mr.

W. Whitaker.

[86] Prestwich. _Quart. Jour. Geol. Soc._, vol. x., p. 448.

[87] Detailed sections of the whole of the Tertiary strata of the Isle of Wight have been constructed by Mr. H. W. Bristow from actual measurement of the beds in their regular order of succession, as displayed at Hempstead, Whitecliff Bay, Colwell and Tolland's Bays, Headon Hill, and Alum Bay. These sections, published by the Geological Survey of Great Britain, show the thickness, mineral character, and organic remains found in each stratum, and are accompanied by a pamphlet in explanation.

At the base of the _Argile Plastique_ of France is a conglomerate of chalk and of divers calcareous substances, in which have been found at Bas-Meudon some remains of Reptiles, Turtles, Crocodiles, Mammals, and, more lately, those of a large Bird, exceeding the Ostrich in size, the _Gastornis_, which Professor Owen cla.s.ses among the wading rather than among aquatic birds. In the Soissonnais there is found, at the same horizon, a great ma.s.s of lignite, enclosing some sh.e.l.ls and bones of the most ancient Pachyderm yet discovered, the _Coryphodon_, which resembles at once both the Anoplotherium and the Pig. The _Sables Inferieurs_, or Bracheux Sands, form a marine bed of great thickness near Beauvais; they are princ.i.p.ally sands, but include beds of calciferous clay and banks of sh.e.l.ly sandstone, and are considered to be older than the plastic clay and lignite, and to correspond with the Thanet Sands of England. They are rich in sh.e.l.ls, including many Nummulites. At La Fere, in the Department of the Aisne, a fossil skull of _Arctocyon primaevus_, supposed to be related both to the Bear and to the Kinkajou, and to be the oldest known Tertiary Mammal, was found in a deposit of this age.

This series seems to have been formed chiefly in fresh water.

The _Calcaire grossier_, consisting of marine limestones of various kinds, and with a coa.r.s.e, sometimes compact, grain, is suitable for mason-work. These deposits, which form the most characteristic member of the Paris basin, naturally divide themselves into three groups of strata, characterised, the first, by _Nummulites_; the second by _Miliolites_; and the third or upper beds by _Cerithia_. The beds are also sometimes named Nummulite limestone, Miliolite limestone, and Cerithium limestone. Above these a great ma.s.s, generally sandy, is developed. It is marine at the base, and there are indications of brackish water in its upper parts; it is called Beauchamp Sandstone, or Sables Moyens (_Gres de Beauchamp_). These sands are very rich in sh.e.l.ls. The _siliceous limestone_, or lower travertin, is a compact siliceous limestone extending over a wide area, and resembles a precipitate from mineral waters. The _gypseous_ formation consists of a long series of marly and argillaceous beds, of a greyish, green, or white colour, in the intervals between which a thick deposit of gypsum, or sulphate of lime, is intercalated. This gypsum bed is found in its greatest thickness in France at Montmartre and Pantin near Paris. The formation of this gypsum is probably due to the action of free sulphuric acid upon the carbonate of lime of the formation; the sulphuric acid itself being produced by the transformation of the gaseous ma.s.ses of sulphuretted hydrogen emanating from volcanic vents, into that acid, by the action of air and water. It was, as we have already said, in the gypsum-quarries of Montmartre that the numerous bones of Palaeotherium and Anoplotherium were found. It is exclusively at this horizon that we find the remains of these animals, which seem to have been preceded by the _Coryphodon_, and afterwards by the _Lophiodon_; the order of succession in the appearance of these animals is now perfectly established. It may be added that round Paris the Eocene formation, from its lowest beds to the highest, is composed of beds of plastic clay, of the _Calcaire grossier_ with its _Nummulites_, _Miliolites_, and _Alveolites_, followed by the gypseous formation; the series terminating in the Fontainebleau Sandstone, remarkable for its thickness and also for its fine scenery, as well as for its usefulness in furnis.h.i.+ng paving-stone for the capital. In Provence the same series of rocks are continued, and attain an enormous thickness. This upper part of the Eocene deposit is entirely of lacustrine formation. Grignon has procured from a single spot, where they were embedded in a calcareous sand, no less than 400 fossils, chiefly formed of comminuted sh.e.l.ls, in which, however, were well-preserved species both of marine, terrestrial, and fresh-water sh.e.l.ls. Of the Paris basin, Sir Charles Lyell says: ”Nothing is more striking in this a.s.semblage of fossil testacea than the great proportion of species referable to the genus _Cerithium_. There occur no less than 137 species of this genus in the Paris basin, and almost all of them in the _Calcaire grossier_. Most of the living _Cerithia_ (Figs.

157 and 168) inhabit 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.”[88]

[88] ”Elements of Geology,” p. 300.

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