Part 30 (1/2)

[Ill.u.s.tration: Fig. 181. _Terebellum fusiforme._]

[Ill.u.s.tration: Fig. 182. _Aturia zigzag._ Bronn. Syn. _Nautilus zigzag._ Sow. London clay. Sheppey.]

[Ill.u.s.tration: Fig. 183. _Belosepia sepiodea_, De Blainv.

London clay. Sheppey.]

The above sh.e.l.ls are accompanied by a sword-fish (_Tetrapterus priscus_, Aga.s.siz), about 8 feet long, and a saw-fish (_Pristis bisulcatus_, Ag.), about 10 feet in length; genera now foreign to the British seas. On the whole, no less than 50 species of fish have been described by M. Aga.s.siz from these beds in Sheppey, and they indicate, in his opinion, a warm climate.

[Ill.u.s.tration: Fig. 184. Molar of monkey (_Macacus_).]

_Strata of Kyson in Suffolk._--At Kyson, a few miles east of Woodbridge, a bed of Eocene clay, 12 feet thick, underlies the red crag. Beneath it is a deposit of yellow and white sand, of considerable interest, in consequence of many peculiar fossils contained in it. Its geological position is probably the lowest part of the London clay proper. In this sand has been found the first example of a fossil quadrumanous animal discovered in Great Britain, namely, the teeth and part of a jaw, shown by Mr. Owen to belong to a monkey of the genus _Macacus_ (see fig. 184.). The mammiferous fossils, first met with in the same bed, were those of an opossum (_Didelphys_) (see fig. 185.), and an insectivorous bat (fig. 186.), together with many teeth of fishes of the shark family. Mr. Colchester in 1840 obtained other mammalian relics from Kyson, among which Mr. Owen has recognized several teeth of the genus _Hyracotherium_, and the vertebrae of a large serpent, probably a _Palaeophis_. As the remains both of the _Hyracotherium_ and _Palaeophis_ were afterwards met with in the London clay, as before remarked, these fossils confirmed the opinion previously entertained, that the Kyson sand belongs to the Eocene period. The _Macacus_, therefore, const.i.tutes the first example of any quadrumanous animal found in strata as old as the Eocene, or so far from the equator as lat. 52 N. It was not until after the year 1836 that the existence of any fossil quadrumana was brought to light. Since that period they have been found in France, India, and Brazil.

[Ill.u.s.tration: Fig. 185. Molar tooth and part of jaw of opossum.

From Kyson.[203-A]]

[Ill.u.s.tration: Fig. 186. Molars of insectivorous bats, twice nat. size.

From Kyson, Suffolk.]

_Mottled or Plastic Clays_, _&c._ (3. _b_, Table, p. 197.).--No formations can be more dissimilar on the whole in mineral character than the Eocene deposits of England and Paris; those of our own island being almost exclusively of mechanical origin,--acc.u.mulations of mud, sand, and pebbles; while in the neighbourhood of Paris we find a great succession of strata composed of a coa.r.s.e white limestone, and compact siliceous limestone with beds of crystalline gypsum and siliceous sandstone, and sometimes pure flint used for millstones. Hence it is by no means an easy task to inst.i.tute an exact comparison between the various members of the English and French series, and to settle their respective ages. It is clear that a continual change was going on in the fauna and flora by the coming in of new species and the dying out of others; and contemporaneous changes of geographical conditions were also in progress in consequence of the rising and sinking of the land and bottom of the sea. A particular subdivision, therefore, of time was occasionally represented in one area by land, in another by an estuary, in a third by the sea, and even where the conditions were in both areas of a marine character, there was often shallow water in one, and deep sea in another, producing a want of agreement in the state of animal life.

At the commencement, however, of the Eocene formations in France and England, we find an exception to this rule, for a marked similarity of mineral character reigns in the lowest division, whether in the basins of Paris, Hamps.h.i.+re, or London. This uniformity of aspect must be seen in order to be fully appreciated, since the beds consist simply of sand, mottled clays, and well-rolled flint pebbles, derived from the chalk, and varying in size from that of a pea to an egg. These strata may be seen at Reading, at Blackheath, near London, and at Woolwich. In some of the lowest of them, banks of oysters are observed, consisting of _Ostrea bellovicina_, so common in France in the same relative position, and _Ostrea edulina_, scarcely distinguishable from the living eatable species. In this formation at Bromley, Dr. Buckland found one large pebble to which five full-grown oysters were affixed, in such a manner as to show that they had commenced their first growth upon it, and remained attached to it through life.

In several places, as at Woolwich on the Thames, at Newhaven in Suss.e.x, and elsewhere, a mixture of marine and freshwater testacea distinguishes this member of the series. Among the latter, _Melania inquinata_ (see fig. 188.) and _Cyrena cuneiformis_ are very common. They probably indicate points where rivers entered the Eocene sea.

[Ill.u.s.tration: Fig. 187. _Cyrena cuneiformis_, Min. Con. Natural size.]

[Ill.u.s.tration: Fig. 188. _Melania inquinata_, Des. Nat. size.

Syn. _Cerithium melanoides_, Min. Con.]

With us as in France, clay of this formation is used in some places, as near Poole in Dorsets.h.i.+re, for pottery; and hence the name of plastic clay was adopted for the group by Mr. T. Webster. Lignite also is a.s.sociated with it in some spots, as in the Paris basin.

Before the minds of geologists had become familiar with the theory of the gradual sinking of the land, and its conversion into sea at different periods, and the consequent change from shallow to deep water, the freshwater and littoral character of this inferior group appeared strange and anomalous. After pa.s.sing through many hundred feet of London clay, proved by its fossils to have been deposited in salt water of considerable depth, we arrive at beds of fluviatile origin. Thick ma.s.ses, also, of s.h.i.+ngle indicate the proximity of land, where the flints of the chalk were rolled into sand and pebbles, and spread continuously over wide s.p.a.ces, as in the Isle of Wight, in the south of Hamps.h.i.+re, and near London, always appearing at the bottom of the Eocene series. It may be asked why they did not const.i.tute simply a narrow littoral zone, such as we might look for in strata formed at a moderate distance from the sh.o.r.e. In answer to this inquiry, the student must be reminded, that wherever a gently-sloping land is gradually sinking and becoming submerged, s.h.i.+ngle may be heaped up successively over a wide area, although marine currents have no power of dispersing it simultaneously over a large s.p.a.ce. In such cases it is not the s.h.i.+ngle which recedes from the coast, but the coast which recedes from the s.h.i.+ngle, which is formed one ma.s.s after another as often as successive portions of the land are converted into sea and others into a sea beach.

The London area appears to have been upraised before that of Hamps.h.i.+re, so that it never became the receptacle of the Barton clays, nor of the overlying fluvio-marine and freshwater beds of Hordwell and the north part of the Isle of Wight. On the other hand, the Hamps.h.i.+re Eocene area seems to have emerged before that of Paris, so that no marine beds of the Upper Eocene era were ever thrown down in Hamps.h.i.+re.

_Nummulitic formation of the Alps and Pyrenees._--It has long been matter of controversy, whether the nummulitic rocks of the Alps and Pyrenees should be regarded as Eocene or Cretaceous; but the number of geologists of high authority who regard this important group as belonging to the lowest tertiary system of Europe has for many years been steadily increasing. The late M. Alex. Brongniart first declared the specific ident.i.ty of many of the sh.e.l.ls of this formation with those of the marine strata near Paris, although he obtained them from the summit of the Diablerets, one of the loftiest of the Swiss Alps, which rises more than 10,000 feet above the level of the sea.

Deposits of the same age, found on the flanks of the Pyrenees, contain also a great number of sh.e.l.ls common to the Paris and London areas, and three or four species only which are common to the cretaceous formation.

The calcareous division consists often of a compact crystalline marble, full of nummulites (see fig. 189.), sh.e.l.ls of the cla.s.s _Foraminifera_.

[Ill.u.s.tration: Fig. 189. _Nummulites_. Peyrehorade, Pyrenees.

_a._ external surface of one of the nummulites, of which longitudinal sections are seen in the limestone.

_b._ transverse section of same.]

The nummulitic limestone of the Alps is often of great thickness, and is immediately covered by another series of strata of dark-coloured slates, marls, and fucoidal sandstones, to the whole of which the provincial name of ”flysch” has been given in parts of Switzerland. The researches of Sir Roderick Murchison in the Alps in 1847 enable us to refer the whole of these beds to the Eocene period, and it seems probable that they most nearly coincide in age with the Lower Eocene. They enter into the disturbed and loftiest portions of the Alpine chain, to the elevation of which they enable us therefore to a.s.sign a comparatively modern date.