The Ancient Life History of the Earth Part 22

(2.) MIDDLE EOCENE.--The inferior portion of the Middle Eocene of Britain consists of marine beds, chiefly consisting of sand, clays, and gravels, and attaining a very considerable thickness (_Bag-shot and Bracklesham beds_). The superior portion of the Middle Eocene of Britain, on the other hand, consists of deposits which are almost exclusively fresh-water or brackish-water in origin (_Headon and Osborne series_).

The chief Continental formations of Middle Eocene age are the "Calcaire grossier" of the Paris basin, and the "Nummulitic Limestone" of the Alps.

(3.) UPPER EOCENE.--If the Headon and Osborne beds of the Isle of Wight be placed in the Middle Eocene, the only British representatives of the Upper Eocene are the _Bembridge beds_.

These strata consist of limestones, clays, and marls, which have for the most part been deposited in fresh or brackish water.

II. EOCENE BEDS OF THE PARIS BASIN.--The Eocene strata are very well developed in the neighbourhood of Paris, where they occupy a large area or basin scooped out of the Chalk. The beds of this area are partly marine, partly freshwater in origin; and the following table (after Sir Charles Lyell) shows their subdivisions and their parallelism with the English series:--

GENERAL TABLE OF FRENCH EOCENE STRATA.

UPPER EOCENE.

_French Subdivisions._ _English Equivalents._ A. 1. Gypseous series of Mont 1. Bembridge series.

Montmartre.

A. 2. Calcaire silicieux, or 2. Osborne and Headon series.

Travertin Inferieur.

A. 3. Gres de Beauchamp, or 3. White sand and clay of Sables Moyens. Barton Cliff, Hants.

MIDDLE EOCENE.

B. 1. Calcaire Grossier. 1. Bagshot and Bracklesham beds.

B. 2. Soissonnais Sands, or 2. Wanting.

Lits Coquilliers.

LOWER EOCENE.

C. 1. Argile de Londres at base 1. London clay.

of Hill of Ca.s.sel, near Dunkirk.

C. 2. Argile plastique and 2. Plastic clay and sand with lignite. lignite (Woolwich and Reading series).

C. 3. Stables de Bracheux. 3. Thanet sands.

III. EOCENE STRATA OF THE UNITED STATES.--The lowest member of the Eocene deposits of North America is the so-called "_Lignitic Formation_," which is largely developed in Mississippi, Tennessee, Arkansas, Wyoming, Utah, Colorado, and California, and sometimes attains a thickness of several thousand feet. Stratigraphically, this formation exhibits the interesting point that it graduates downwards insensibly and conformably into the Cretaceous, whilst it is succeeded _uncomformably_ by strata of Middle Eocene age.

Lithologically, the series consists princ.i.p.ally of sands and clays, with beds of lignite and coal, and its organic remains show that it is princ.i.p.ally of fresh-water origin with a partial intermixture of marine beds. These marine strata of the "Lignitic formation" are of special interest, as showing such a commingling of Cretaceous and Tertiary types of life, that it is impossible to draw any rigid line in this region between the Mesozoic and Kainozoic systems. Thus the marine beds of the Lignitic series contain such characteristic Cretaceous forms as _Inoceramus_ and _Ammonites_, along with a great number of Univalves of a distinctly Tertiary type (Cones, Cowries, &c.) Upon the whole, therefore, we must regard this series of deposits as affording a kind of transition between the Cretaceous and the Eocene, holding in some respects a position which may be compared with that held by the Purbeck beds in Britain as regards the Jura.s.sic and Cretaceous.

The Middle Eocene of the United States is represented by the _Claiborne_ and _Jackson_ beds. The _Claiborne series_ is extensively developed at Claiborne, Alabama, and consists of sands, clays, lignites, marls, and impure limestones, containing marine fossils along with numerous plant-remains. The _Jackson series_ is represented by lignitic clays and marls which occur at Jackson, Mississippi. Amongst the more remarkable fossils of this series are the teeth and bones of Cetaceans of the genus _Zeuglodon_.

Strata of Upper Eocene age occur in North America at Vicksburg, Mississippi, and are known as the _Vicksburg series_. They consist of lignites, clays, marls, and limestones. Freshwater deposits of Eocene age are also largely developed in parts of the Rocky Mountain region. The most remarkable fossils of these beds are Mammals, of which a large number of species have been already determined.

LIFE OF THE EOCENE PERIOD.

The fossils of the Eocene deposits are so numerous that nothing more can be attempted here than to give a brief and general sketch of the life of the period, special attention being directed to some of the more prominent and interesting types, amongst which--as throughout the Tertiary series--the Mammals hold the first place.

It is not uncommon, indeed, to speak of the Tertiary period as a whole under the name of the "Age of Mammals," a t.i.tle at least as well deserved as that of "Age of Reptiles" applied to the Mesozoic, or "Age of Molluscs" applied to the Palaeozoic epoch.

As regards the _plants_ of the Eocene, the chief point to be noticed is, that the conditions which had already set in with the commencement of the Upper Cretaceous, are here continued, and still further enforced. The _Cycads_ of the Secondary period, if they have not totally disappeared, are exceedingly rare; and the _Conifers_, losing the predominance which they enjoyed in the Mesozoic, are now relegated to a subordinate though well-defined place in the terrestrial vegetation. The great majority of the Eocene plants are referable to the groups of the Angiospermous Exogens and the Monocotyledons; and the vegetation of the period, upon the whole, approximates closely to that now existing upon the earth. The plants of the European Eocene are, however, in the main most closely allied to forms which are now characteristic of tropical or sub-tropical regions. Thus, in the London Clay are found numerous fruits of Palms (_Napdites_, fig. 213), along with various other plants, most of which indicate a warm climate as prevailing in the south of England at the commencement of the Eocene period. In the Eocene strata of North America occur numerous plants belonging to existing types--such as Palms, Conifers, the Magnolia, Cinnamon, Fig. Dog-wood, Maple, Hickory, Poplar, Plane, &c. Taken as a whole, the Eocene flora of North America is nearly related to that of the Miocene strata of Europe, as well as to that now existing in the American area. We conclude, therefore, that "the forests of the American Eocene resembled those of the European Miocene, and even of modern America" (Dana).

[Ill.u.s.tration: Fig. 213.--_Napadites ellipticus_, the fruit of a fossil Palm. London Clay, Isle of Sheppey.]

As regards the _animals_ of the Eocene period, the _Protozoans_ are represented by numerous _Foraminifera_, which reach here their maximum of development, both as regards the size of individuals and the number of generic types. Many of the Eocene Foraminifers are of small size; but even these not uncommonly form whole rock-ma.s.ses.

Thus, the so-called "Miliolite Limestone" of the Paris basin, largely used as a building-stone, is almost wholly composed of the sh.e.l.ls of a small species of _Miliola_. The most remarkable, however, of the many members of this group of animals which flourished in Eocene times, are the "Nummulites" (_Nummulina_), so called from their resemblance in shape to coins (Lat. _nummus_, a coin). The Nummulites are amongst the largest of all known _Foraminifera_, sometimes attaining a size of three inches in circ.u.mference; and their internal structure is very complex (fig. 214). Many species are known, and they are particularly characteristic of the Middle and Upper of these periods--their place being sometimes taken by _Orbitoides_, a form very similar to the Nummulite in external appearance, but differing in its internal details. In the Middle Eocene, the remains of Nummulites are found in vast numbers in a very widely-spread and easily-recognised formation known as the "Nummulitic Limestone" (fig. 10). According to Sir Charles Lyell, "the Nummulitic Limestone of the Swiss Alps rises to more than 10,000 feet above the level of the sea, and attains here and in other mountain-chains a thickness of several thousand feet. It may be said to play 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 occurs in Algeria and Morocco, and has been traced from Egypt, where it was largely quarried of old for the building of the Pyramids, into Asia Minor, and across Persia by Bagdad to the mouths of the Indus. It has been observed not only in Cutch, but in the mountain-ranges which separate Scinde from Persia, and which form the pa.s.ses leading to Cabul; and it has been followed still further eastward into India, as far as Eastern Bengal and the frontiers of China."

The sh.e.l.ls of Nummulites have been found at an elevation of 16,500 feet above the level of the sea in Western Thibet; and the distinguished and philosophical geologist just quoted, further remarks, that "when we have once arrived at the conviction that the Nummulitic formation occupies a middle and upper place in the Eocene series, we are struck with the comparatively modern date to which some of the greatest revolutions in the physical geography of Europe, Asia, and Northern Africa must be referred. All the mountain-chains--such as the Alps, Pyrenees, Carpathians, and Himalayas--into the composition of whose central and loftiest parts the Nummulitic strata enter bodily, could have had no existence till after the Middle Eocene period. During that period, the sea prevailed where these chains now rise; for Nummulites and their accompanying Testacea were unquestionably inhabitants of salt water."

[Ill.u.s.tration: Fig. 214.--_Nummulina loevigata_. Middle Eocene.]

The _Coelenterates_ of the Eocene are represented princ.i.p.ally by _Corals_, mostly of types identical with or nearly allied to those now in existence. Perhaps the most characteristic group of these is that of the _Turbinolidoe_, comprising a number of simple "cup-corals," which probably lived in moderately deep water. One of the forms belonging to this family is here figured (fig. 215). Besides true Corals, the Eocene deposits have yielded the remains of the "Sea-pens" (_Pennatulidoe_) and the branched skeletons of the "Sea-shrubs" (_Gorgontidoe_).

The _Echinoderms_ are represented princ.i.p.ally by Sea-urchins, and demand nothing more than mention. It is to be observed, however, that the great group of the Sea-lilies (_Crinoids_) is now verging on extinction, and is but very feebly represented.

Amongst the _Mollusca_, the _Polyzoans_ and _Brachiopods_ also require no special mention, beyond the fact that the latter are greatly reduced in numbers, and belong princ.i.p.ally to the existing genera _Terebratula_ and _Rhynchonella_. The Bivalves (_Lamellibranchs_) and the Univalves (_Gasteropods_) are exceedingly numerous, and almost all the princ.i.p.al existing genera are now represented; though less than five percent of the Eocene _species_ are identical with those now living. It is difficult to make any selection from the many Bivalves which are known in deposits of this age; but species of _Cardita, Cra.s.satella, Leda, Cyrena, Mactra, Cardium, Psammobia_, &c., may be mentioned as very characteristic.

The _Caradita planicosta_ here figured (fig. 216) is not only very abundant in the Middle Eocene, but is very widely distributed, ranging from Europe to the Pacific coast of North America. The _Univalves_ of the Eocene are extremely numerous, and generally beautifully preserved. The majority of them belong to that great section of the _Gasteropods_ in which the mouth of the sh.e.l.l is notched or produced into a ca.n.a.l (when the sh.e.l.l is said to be "siphonostomatous")--this section including the carnivorous and most highly-organized groups of the cla.s.s. Not only is this the case, but a large number of the Eocene Univalves belong to types which now attain their maximum of development in the warmer regions of the globe. Thus we find numerous species of Cones (_Conus_), Volutes (_Voluta_), Cowries (_Cyproea_, fig. 218), Olives and Rice-sh.e.l.ls (_Oliva_), Mitre-sh.e.l.ls (_Mitra_), Trumpet-sh.e.l.ls (_Triton_), Auger-sh.e.l.ls (_Terebra_), and Fig-sh.e.l.ls (_Pyrula_).

Along with these are many forms of _Pleurotoma, Rostellaria_, Spindle-sh.e.l.ls (_Fusus_), Dog-whelks (_Na.s.sa_), _Murices_, and many round-mouthed ("holostomatous") species, belonging to such genera as _Turritella, Nerita, Natica, Scalaria_, &c. The genus _Cerithium_ (fig. 219), most of the living forms of which are found in warm regions, inhabiting fresh or brackish waters, undergoes a vast development in the Eocene period, where it is represented by an immense number of specific forms, some of which attain very large dimensions. In the Eocene strata of the Paris basin alone, nearly one hundred and fifty species of this genus have been detected. The more strictly fresh-water deposits of the Eocene period have also yielded numerous remains of Univalves such as are now proper to rivers and lakes, together with the sh.e.l.ls of true Land-snails. Amongst these may be mentioned numerous species of _Limnoea_ (fig. 220), _Physa_ (fig. 221), _Melania, Paludina, Planorbis, Helix, Bulimus_, and _Cyclostoma_ (fig. 222).

[Ill.u.s.tration: Fig. 215.--_Turbinolia sulcata_, viewed from one side, and from above. Eocene.]

[Ill.u.s.tration: Fig. 216.--_Cardita planicosta_. Middle Eocene.]

[Ill.u.s.tration: Fig. 217.--_Typhis tubifer_, a "siphonostomatous"

Univalve. Eocene.]

[Ill.u.s.tration: Fig. 218.--Cyproea elegans. Eocene.]

[Ill.u.s.tration: Fig. 219.--_Cerithium hexagonum_. Eocene.]

With regard to the _Cephalopods_, the chief point to be noticed is, that all the beautiful and complex forms which peculiarly characterised the Cretaceous period have here disappeared. We no longer meet with a single example of the Turrilite, the Baculite, the Hamite, the Scaphite, or the Ammonite. The only exception to this statement is the occurrence of one species of Ammonite in the so-called "Lignitic Formation" of North America; but the beds containing this may possibly be rather referable to the Cretaceous--and this exception does not affect the fact that the _Ammonitidoe_, as a family, had become extinct before the Eocene strata were deposited. The ancient genus _Nautilus_ still survives, the sole representative of the once mighty order of the Tetrabranchiate Cephalopods. In the order of the _Dibranchiates_, we have a like phenomenon to observe in the total extinction of the great family of the "Belemnites." No form referable to this group has. .h.i.therto been found in any Tertiary stratum; but the internal skeletons of Cuttle-fishes (such as _Belosepia_) are not unknown.

[Ill.u.s.tration: Fig. 220.--_Limnoea pyramidalis_. Eocene.]

[Ill.u.s.tration: Fig. 221.--_Physa columnaris_. Eocene.]

[Ill.u.s.tration: Fig. 222.--_Cyclostoma Arnoudii_. Eocene.]

Remains of _Fishes_ are very abundant in strata of Eocene age, especially in certain localities. The most famous depot for the fossil fishes of this period is the limestone of Monte Bolca, near Verona, which is interstratified with beds of volcanic ashes, the whole being referable to the Middle Eocene. The fishes here seem to have been suddenly destroyed by a volcanic eruption, and are found in vast numbers. Aga.s.siz has described over one hundred and thirty species of Fishes from this locality, belonging to seventy-seven genera. All the _species_ are extinct; but about one-half of the _genera_ are represented by living forms. The great majority of the Eocene Fishes belong to the order of the "Bony Fishes" (_Teleosteans_), so that in the main the forms of Fishes characterising the Eocene are similar to those which predominate in existing seas. In addition to the above, a few _Ganoids_ and a large number of _Placoids_ are known to occur in the Eocene rocks. Amongst the latter are found numerous teeth of true Sharks, such as _Otodus_ (fig. 224) and _Carcharodon_.

The pointed and serrated teeth of the latter sometimes attain a length of over half a foot, indicating that these predaceous fishes attained gigantic dimensions; and it is interesting to note that teeth, in external appearance very similar to those of the early Tertiary genus _Carcharodon_, have been dredged from great depths during the recent expedition of the Challenger.

There also occur not uncommonly the flattened teeth of Rays (fig.

225), consisting of flat bony pieces placed close together, and forming "a kind of mosaic pavement on both the upper and lower jaws" (Owen).

[Ill.u.s.tration: Fig. 223.--_Rhombus minimus_, a small fossil Turbot from the Eocene Tertiary, Monte Bolca.]

[Ill.u.s.tration: Fig. 224.--Tooth of _Otodus obliquus_. Eocene.]

[Ill.u.s.tration: Fig. 225.--Flattened dental plates of a Ray (_Myliobatis Edwardsii_). Eocene.]

In the cla.s.s of the _Reptiles_, the disappearance of the characteristic Mesozoic types is as marked a phenomenon as the introduction of new forms. The Ichthyosaurs, the Plesiosaurs, the Pterosaurs, and the Mosasaurs of the Mesozoic, find no representatives in the Eocene Tertiary; and the same is true of the Deinosaurs, if we except a few remains from the doubtfully-situated "Lignitic formation" of the United States, On the other hand, all the modern orders of Reptiles are known to have existed during the Eocene period. The _Chelonians_ are represented by true marine Turtles, by "Terrapins" (_Emydidoe_), and by "Soft Tortoises"

(_Trionycidoe_). The order of the Snakes and Serpents (_Ophidia_) makes its appearance here, for the first time under several forms--all of which, however, are referable to the non-venomous group of the "Constricting Serpents" (_Boidoe_). The oldest of these is the _Paloeophis toliapicus_ of the London Clay of Sheppey, first made known to science by the researches of Professor Owen.

The nearly-allied _Paloeophis typhoeus_ of the Eocene beds of Bracklesham appears to have been a Boa-constrictor-like Snake of about twenty feet in length. Similar Python-like Snakes (_Paloeophis, Dinophis_, &c.) have been described from the Eocene deposits of the United States. True Lizards (_Lacertilians_) are found in some abundance in the Eocene deposits,--some being small terrestrial forms, like the common European lizards of the present day; whilst others equal or exceed the living Monitors in size. Lastly, the modern order of the _Crocodilia_ is largely represented in Eocene times, by species belonging to all the existing genera, together with others referable to extinct types.

As pointed out by Owen, it is an interesting fact that in the Eocene rocks of the south-west of England, there occur fossil remains of all the three living types of Crocodilians--namely, the Gavials, the true Crocodiles, and the Alligators (fig. 226)--though at the present day these forms are all geographically restricted in their range, and are never a.s.sociated together.

[Ill.u.s.tration: Fig. 226.--Upper jaw of Alligator. Eocene Tertiary, Isle of Wight.]