A Manual of Elementary Geology Part 30

[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.

The nummulitic formation, with its characteristic fossils, plays a far more conspicuous part than any other tertiary group in the solid framework of the earth's crust, whether in Europe, Asia, or Africa. It often attains a thickness of many thousand feet, and extends from the Alps to the Apennines. It is found in the Carpathians, and in full force in the north of Africa, as, for example, in Algeria and Morocco. It has also been traced from Egypt into Asia Minor, and across Persia by Bagdad to the mouths of the Indus. It occurs not only in Cutch, but in the mountain ranges which separate Scinde from Persia, and which form the pa.s.ses leading to Caboul; and it has been followed still farther eastward into India.

Some members of this lower tertiary formation in the central Alps, including even the superior strata called _flysch_, have been converted into crystalline rocks, and changed into saccharoid marble, quartz, rock, and mica-schist.[206-A]

EOCENE STRATA IN THE UNITED STATES.

In North America the Eocene formations occupy a large area bordering the Atlantic, which increases in breadth and importance as it is traced southwards from Delaware and Maryland to Georgia and Alabama. They also occur in Louisiana and other states both east and west of the valley of the Mississippi. At Claiborne in Alabama no less than four hundred species of marine sh.e.l.ls, with many echinoderms and teeth of fish, characterize one member of this system. Among the sh.e.l.ls the _Cardita planicosta_, before mentioned (fig. 171. p. 199.), is in abundance; and this fossil, and some others identical with European species, or very nearly allied to them, make it highly probable that the Claiborne beds agree in age with the central or Bracklesham group of England, and the calcaire grossier of Paris.[206-B]

Higher in the series is a remarkable calcareous rock, formerly called "the nummulite limestone," from the great number of discoid bodies resembling nummulites which it contains, fossils now referred by A. d'Orbigny to the genus _Orbitoides_, which has been demonstrated by Dr. Carpenter to belong to the Foraminifera.[206-C] The following section will enable the reader to understand the position of the three subdivisions of the series, Nos. 1, 2, and 3., the relations of which I ascertained in Clarke County, between the rivers Alabama and Tombeckbee.

[Ill.u.s.tration: Fig. 190. Cross section.

1. Sand, marl, &c., with numerous fossils. } 2. White or rotten limestone, with _Zeuglodon_. } Eocene.

3. Orbitoidal, or so called nummulitic limestone. } 4. Overlying formation of sand and clay without fossils. Age unknown.]

The lowest set of strata, No. 1., having a thickness of more than 100 feet, comprise marly beds, in which the _Ostrea sellaeformis_ occurs, a sh.e.l.l ranging from Alabama to Virginia, and being a representative form of the _Ostrea flabellula_ of the Eocene group of Europe. In others beds of No. 1., two European sh.e.l.ls, _Cardita planicosta_, before mentioned, and _Solarium ca.n.a.liculatum_ are found, with a great many other species peculiar to America. Numerous corals, also, and the remains of placoid fish and of rays occur, and the "swords," as they are called, of sword fishes, all bearing a great generic likeness to those of the Eocene strata of England and France.

No. 2. (fig. 190.) is a white limestone, sometimes soft and argillaceous, but in parts very compact and calcareous. It contains several peculiar corals, and a large Nautilus allied to _N. zigzag_, also in its upper bed a gigantic cetacean, called _Zeuglodon_ by Owen.[207-A]

[2 Ill.u.s.trations: _Zeuglodon cetoides_, Owen. _Basilosaurus_, Harlan.]

Fig. 191. Molar tooth, natural size.]

Fig. 192. Vertebra, reduced.]

The colossal bones of this cetacean are so plentiful in the interior of Clarke County as to be characteristic of the formation. The vertebral column of one skeleton found by Dr. Buckley at a spot visited by me, extended to the length of nearly 70 feet, and not far off part of another backbone nearly 50 feet long was dug up. I obtained evidence, during a short excursion, of so many localities of this fossil animal within a distance of 10 miles, as to lead me to conclude that they must have belonged to at least forty distinct individuals.

Mr. Owen first pointed out that the huge animal was not reptilian, since each tooth was furnished with double roots (see fig. 191.), implanted in corresponding double sockets; and his opinion of the cetacean nature of the fossil was afterwards confirmed by Dr. Wyman and Professor R. W.

Gibbes. That it was an extinct species of the whale tribe has since been placed beyond all doubt by the discovery of the entire skull of another fossil of the same family, found to have the double occipital condyles only met with in mammals, and the convoluted tympanic bones which are characteristic of cetaceans.

Near the junction of No. 2. and the inc.u.mbent limestone, No. 3., next to be mentioned, are strata characterized by the following sh.e.l.ls: Spondylus dumosus (_Plagiostoma dumosum_, Morton), _Pecten Poulsoni_, _Pecten perpla.n.u.s_, and _Ostrea cretacea_.

No. 3. (fig. 190.) is a white limestone, for the most part made up of the _Orbitoides_ of d'Orbigny before mentioned (p. 206.), formerly supposed to be a nummulite, and called _N. Mantelli_, mixed with a few lunulites and small corals and sh.e.l.ls.[208-A] The origin of this cream-coloured soft stone, like that of our white chalk, which it much resembles, is, I believe, due to the decomposition of the orbitoides. The surface of the country where it prevails is sometimes marked by the absence of wood, like our chalk downs, or is covered exclusively by the _Juniperus Virginiana_, as certain chalk districts in England by yew trees and juniper.

Some of the sh.e.l.ls of this limestone are common to the Claiborne beds, but many of them are peculiar.

It will be seen in the section (fig. 190. p. 155.) that the strata, Nos. 1, 2, 3., are, for the most part, overlaid by a dense formation of sand or clay without fossils. In some points of the bluff or cliff of the Alabama river, at Claiborne, the beds Nos. 1, 2., are exposed nearly from top to bottom, whereas at other points the newer formation, No. 4., occupies the face of nearly the whole cliff. The age of this overlying ma.s.s has not yet been determined, as it has. .h.i.therto proved dest.i.tute of organic remains.

The burr-stone strata of the Southern States contain so many fossils agreeing with those of Claiborne, that it doubtless belongs to the same part of the Eocene group, though I was not fortunate enough to see the relations of the two deposits in a continuous section. Mr. Tuomey considers it as the lower portion of the series. It may, perhaps, be a form of the Claiborne beds in places where lime was wanting, and where silex, derived from the decomposition of felspar, predominated. It consists chiefly of slaty clays, quartzose sands, and loam, of a brick red colour, with layers of chert or burr-stone, used in some places for millstones.

FOOTNOTES:

[191-A] Leyde Magaz. voor Wetensch Konst en Lett., partie v. cahier i. p.

71. Cited by Rozet, Journ. de Geologie, tom. i. p. 43.

[191-B] M. C. Prevost, Submersions Iteratives, &c. Note 23.