EOCENE, in geology, the name suggested by Sir Charles Lyell in 1833 for the earliest period of the Tertiary era. The term was intended to convey the idea that this was the period which saw the dawn of the recent or existing forms of life, be cause it was estimated that among the fossils of this period only 31% of the species are still living. Since Lyell's time much has been learned about the fauna and flora of the period, and many palaeontologists doubt if any of the Eocene species are still ex tant, unless it be some of the lowest forms of life. Nevertheless, the name is a convenient one and is in general use. The Eocene as originally defined was not long left intact, for E. Beyrich in 1854 proposed the term "Oligocene" for the upper portion, and later, in 1874, K. Schimper suggested "Palaeocene" as a separate appel lation for the lower portion. The Oligocene division has been gen erally accepted as a distinct period, but "Palaeocene" is not so widely used. In the present article we shall include also the "Palaeocene" deposits. Although in Germany the Oligocene de posits are transgressive and cover wide areas from which the Eocene is absent, in France and southern England there is little or no break between the Eocene and Oligocene and it is much more logical to include the whole of the lower Tertiary in a single period. This is the tendency among modern stratigraphers and the names "Palaeogene," "Nummulitic" or "Older Tertiary Period" (in opposition to Neogene or Newer Tertiary Period) are widely used to embrace the Palaeocene, Eocene and Oligocene. As a whole, the Palaeogene System is characterized by the almost exclusive occurrence of Nummulites (a few early forms have been recorded from the highest Cretaceous) ; further, its stratigraphic unity is ensured since the upper and lower limits are marked each by periods of marine regression whose effects are traceable over practically the whole of Europe. There are, in fact, very few areas indeed where sedimentation proceeded uninterruptedly on the one hand from the Cretaceous to the Palaeogene and on the other hand from the Palaeogene to the Neogene.
The stratigraphy of the Eocene was first worked out in detail in the basins of Paris, Belgium, Hampshire and London with the result that the Anglo-Franco-Belgian basin has become the type area. This is unfortunate, since the deposits there, both in the Eocene and Oligocene periods, were laid down in a large but shallow gulf affected by a succession of secondary oscillations of level which have resulted in a complex series of alternating marine and continental deposits. A continuous succession of marine deposits is found in the Mediterranean area—laid down in the Tethys or Mediterranean sea of Eocene times which stretched from Italy to India and in which were evolved the series of Nummulites by which the subdivisions of the period are dated.
It is towards the centres of the larger Tertiary seas that Eocene sedimentation succeeds without a break that of the Cretaceous and where the boundary line is difficult to draw. This is the case in parts of Egypt, Denmark, some areas in northern India, as well as in the continental region of central North America. In the classical localities of England and France a great gap in time separates the Tertiary from the Mesozoic deposits, though the angular discordance between the Cretaceous and Eocene is usually but slight.
Associated, no doubt, with the slight crustal movements which closed the Cretaceous and inaugurated the Eocene period there were local and intermittent manifestations of volcanic activity throughout the period. Intrusive rocks are found in the Eocene of the central and northern Apennines. Tuffs, basalts and other igneous rocks appear also in many of the Rocky Mountain States, in Central America, the West Indies and South America.
It has been very generally assumed by geologists, mainly upon the evidence of plant remains, that the Eocene period opened with a temperate climate in the northern latitudes, later becoming almost subtropical. The frequent admixture of temperate forms with what are now tropical genera makes it difficult to speak with certainty as to the degree of warmth experienced. The meteoro logical researches of Dr. C. E. P. Brooks (Climate through the Ages, London, 1926) have tended to confirm the belief in the existence of a warmer climate over Europe in early or middle Eocene times, becoming cooler towards the close of the period. The difference in the character of the faunas of the Tethys and the Northern sea points fairly conclusively to the differentiation of climate-zones in Europe comparable to those of the present day.
Upper Eocene—small nummulites and orthophragmines.
Middle Eocene—large and small nummulites, orthophragmines and assilines.
Lower Eocene—small nummulites and orthophragmines.
It was only at certain periods that a temporary connection was established between the Eocene Mediterranean and such areas of the Northern sea as the Paris basin, hence it is only isolated species and not phylogenetic series which are found in the latter areas. Thus whilst sedimentation was continuous in the Anglo Franco-Belgian basin there are marked faunal breaks with a sudden influx of new forms.
In the northern province only isolated corals occur but in the Mediterranean reef-building forms of modern genera flourished. This is strong evidence of the differentiation into climatic zones as at present with a warm Mediterranean and a cold Northern sea.
In the marine mollusca the most noteworthy change is the entire absence of ammonoids, the group which throughout the Mesozoic era had taken so prominent a place, but disappeared completely with the close of the Cretaceous. Nautiloids were more abundant than they are at present, but as a whole the Cephalopods took a more subordinate part than they had done in previous periods. On the other hand, gastropods and pelecypods found in the numerous shallow seas a very suitable environment, flourished exceedingly, and their shells are often perfectly pre served and in enormous numbers. Of the gastropod genera Cerithium with its estuarine and lagoonal forms is very charac teristic and the series of mutations have been of great service in studying the detailed stratigraphy of the Paris basin. Rostellaria, V oluta, Fusus, Pleurotoma, Conus may also be cited. Cardium, V enericardia, Crassatella, Cytherea, Lucina, Anomia, Ostrea are a few of the many genera of pelecypods abundantly represented. Echinoderms are especially important in the Tethys and the warmer seas and include the sea urchins Lint/iia, Conoclypeus, etc.
Crustacea were fairly abundant and most of the orders as well as numerous families of modern insects were repre sented.
When we turn to the higher forms of life, the reptiles and mam mals, we find a remarkable contrast between the fauna of the Eocene and those periods which preceded and succeeded it. The great groups of reptiles,' whose members had held dominion on land and sea during most of the Mesozoic time, had almost en tirely disappeared by the beginning of the Eocene; in their place placental mammals made their appearance and rapidly became the dominant group. Among the early Eocene mammals no trace can be found of the numerous and clearly-marked orders with which we are familiar to-day; instead we find obscurely differentiated forms, which cannot be fitted without violence into any of the modern orders. The early placental mammals were generalized types (with certain non-placental characters) with potentialities for rapid divergence and development in the direction of the more specialized modern orders. Thus, the Creodonta foreshadowed the Carnivora, the Condylarthra presaged the herbivorous groups; but before the close of this period, so favourable were the condi tions of life to a rapid evolution of types, that most of the great orders had been clearly defined, though none of the Eocene genera are still extant. Among the early carnivores were Arctocyon, Palaeorictis, Amblyctonus, Hyaenodon, Cynodon, Provivera, Pa trio f elis. The primitive dog-like forms did not appear until late in the period, in Europe ; and true cats did not arrive until later, though they were represented by Eusmilus in the Upper Eocene of France. The primitive ungulates (Condylarths) were generalized forms with five effective toes, exemplified in Phenacodus. The gross Amblypoda, with five-toed stumpy feet (Coryphodon), were prominent in the early Eocene ; par=ticularly striking forms were the Dinoceratidae, Dinoceras, with three pairs of horns or pro tuberances on its massive skull and a pair of huge canine teeth projecting downwards, Tinoceras, Uintatherium, Loxolopliodon etc. These enormous creatures, whose remains are so abundant in the Eocene deposits of western America, died out before the close of the period. The divergence of the hoofed mammals into the two prominent divisions, the odd-toed and even-toed, began in this period, but the former did not get beyond the three-toed stage. The least differentiated of the odd-toed group were the Lophiodonts : tapirs were foreshadowed by Systemodon and simi lar forms (Palaeotherium, Palaplotherium); the peccary-like Hy racotherium was a forerunner of the horse, Hyrachyus was a primi tive rhinoceros. The evolution of the horse through such forms as Hyracotherium, Pacflynolophus, Eohippus, etc., appears to have proceeded along parallel lines in Eurasia and America, but the true horse did not arrive until later. Ancestral deer were repre sented by Dicfiobune, Amphitragulus and others, while many small hog-like forms existed (Diplopus, Eohyus, Hyopotamus, Homa codon). The primitive stock of the camel group developed in North America in late Eocene time and sent branches into South America and Eurasia. Certain very generalized forms (Gan odonta) are commonly considered to be ancestral edentates, while the Tillodonta, whose affinities are still uncertain, are often grouped with the rodents, though far larger than any modern members of this group. The Insectivores had Eocene forerunners, and the Lemuroids—probable ancestors of the apes—were forms of great interest. The Cetaceans were well represented by Zeug lodon and others.
The non-placental mammals although still numerous were tak ing a secondary place; Didelphys, the primitive opossum, is note worthy on account of its wide geographical range.
Among the birds, the large flightless forms, Eupterornis, Gas tornis, were prominent, and many others were present, such as the ancestral forms of our modern gulls, albatrosses, herons, buzzards, eagles, owls, quails, plovers. Reptiles are represented solely by still surviving orders ; they are not very common, though the remains of crocodilians, tortoises, turtles and some large snakes are locally plentiful.
The flora of the Eocene period, although full of interest, does not convey the impression of newness that is afforded by the fauna. The reason for this difference is this : the newer flora had been introduced and had developed to a considerable extent in the Cretaceous period, and there is no sharp break between the flora of the earlier and that of the later period. The reason for this difference is that the flora of modern aspect, characterized by the dominance of the flowering plants, had already come into being in the Cretaceous period; hence in general facies there is no sharp line of demarcation between Cretaceous and Eocene floras. Early Eocene floras are tropical or sub-tropical in character but in post Ypresian times there is evidence of cooling climatic con ditions. (See PALAEOBOTANY.) Eocene Stratigraphy.—The lower limit of the Eocene (in cluding the Paleocene) is, generally speaking, well defined. There is general agreement that the Danian stage forms the highest Cretaceous and that the Eocene commences with the Montian. In the Montian there are no longer Rudistes or Ammonites but, on the other hand, a series of Cerithium, decidedly Tertiary in aspect, appears. The upper limit of the Eocene is far from clearly defined; it is usually taken at the summit of the Bartonian stage (including the Ludian).
The type area for Eocene stratigraphy is the Anglo-Franco Belgian basin. At the opening of the Eocene period a broad gulf advanced from the Northern sea (roughly from the area occupied by the North sea of to-day) and covered most of south-eastern England, north-eastern France, Belgium and Holland. The de posits laid down in that gulf are now found confined mainly to four tectonic basins—the basins of London, Hampshire, Paris and Belgium. Occasional outliers or relict blocks serve to connect these areas and to indicate the original extent of the beds. The Eocene sea advanced intermittently, more or less towards the south in the Paris basin, more or less towards the west in Eng land. In the intervals between the marine phases and on the borders of the gulf, lacustrine, fluviatile and estuarine deposits were laid down. Sediment was poured into the basin by rivers flowing from the west in England and from the south in France.
The multiple variations in facies have led to the distinction of a crowd of stages and a very complex nomenclature. Many of the beds and stages separated have a purely local significance. The marine faunas reflect the variations in facies, it is very difficult to separate faunal diff er"nces due simply to changes in facies from those due to evolution and immigration. It was only at certain periods that the gulf was in direct communication via the Atlantic coast with the Mediterranean area, so that only at times is there a marked influx of species from the southern seas.
The very varied deposits are most simply grouped into six or seven stages. Each stage commenced with a marine invasion, fol lowed by a shallowing of the sea and then by continental condi tions, the whole abruptly terminated by the next marine invasion. The deposits of each stage correspond to a "cycle of sedimenta tion." It will be clear also that the deposits are predominantly marine towards the centre of the old gulf ; predominantly con tinental towards its margins. The arrangement of the wedges of strata is illustrated in the annexed diagrams.
The Montian Stage (Mons, Belgium) marks the first episode in the invasion of the Tertiary sea. During the long period of emergence which separated the Cretaceous and Tertiary periods in this part of Europe, the Chalk of the Parisian region and Bel gium had been carved into hills and valleys and the earliest Ter tiary deposits occupy valleys invaded by the sea and consist of limestones very localized in their occurrence—in a trough near Mons, at Meudon near Paris, Laversines near Beauvais and at Vertus near Epernay. The Montian is absent in England. The Landenian Stage (Landen in Belgium) is marked by a widespread marine invasion, the deposits no longer confined to old valleys. Generally the lower Landenian deposits consist of glauconitic sands resting on a plane of marine denudation on the Chalk sur face. At the base there is generally a layer of green coated flints in a clayey, glauconitic matrix (the Bullhead Bed of English geologists). This is succeeded by a series of sands (Thanet Sands of the London basin, Sables de Bracheux of the Paris basin, Lower Landenian of Belgium) . In some parts of Belgium the lowest beds are pure white chalky marls (Heersian) with plant remains.
In the London basin a thin bed of rounded pebbles separates the Thanet sands from the glauconitic sands of the Woolwich Bottom Bed (the oldest Tertiary found in the Hampshire basin). Over nearly the whole of the old gulf the marine sands are suc ceeded by a series of freshwater sands and mottled clays (the Reading Beds of England, Upper Landenian of Belgium, Argile plastique of France) or by a series of estuarine clays and sands often crowded with brackish-water fossils such as Cyrena and Ceritlaium. These are the Woolwich Beds of England, the Lignites du Soissonnais of France. The lower marine base of the Lan denian is often referred to as the Thanetian Stage ; the upper continental stage as the Sparnacian. Only in East Kent—towards the centre of the old gulf—is there Landenian marine throughout. In the sandy beds of the Upper Landenian masses of sand consoli dated by a siliceous cement are common which, when weathered out and left on the surface form "sarsens." The Ypresian Stage is marked by a still more widespread marine invasion. At the base are often beds of well rounded black flint pebbles with or without a sandy matrix (Blackheath Beds of England, Sinceny Beds of France) . The fauna of the pebble beds consists of sur vivors from the Woolwich phase mixed with new marine invaders common to the overlying deposits. In East Kent purely marine, sandy equivalents of these beds are known as the Oldhaven Beds. In England and Belgium the principal member of the Ypresian Stage is a thick mass of dark blue clay—reaching nearly 50o feet in thickness in parts of the London basin. This is the famous London Clay—the Argile des Flandres or Argile Ypresien of Bel gium—which is the most characteristic Tertiary formation of the London basin and in addition occupies large areas in the Hamp shire basin. It is so well contrasted with all the underlying sandy beds in the London basin that the latter are often grouped to gether as the "Lower London Tertiaries." In the Paris basin the place of the London Clay is taken by a group of sands, richly fossiliferous—the Sands of Cuise from which the name Cuisian is derived and often used instead of Ypresian. The characteristic fossil of the sandy phase of the Ypresian is Nummulites planula tus-elegans. Towards the south-east in Belgium the clay passes upwards into some extremely fine sands—the Upper Ypresian or Sands of Mons-en-Pevele. In England the sandy upper part of the London Clay is known as the Claygate Beds which pass up into the continental phase, the Bagshot Beds. On the hills of Essex the Bagshot Beds are represented by beds of shingle; south west of London by a series of coarse, false-bedded sands which give rise to a delightful stretch of heathland for which the county of Surrey is well known. In the Hampshire basin the Bagshot Beds are very important and include the well known Alum Bay Leaf Beds, Bournemouth Leaf Beds and the Pipe Clays of Poole. They are there a succession of false-bedded sands with lenticular clay and pebble beds, all often highly charged with vegetable matter. In the Paris basin the continental Ypresian is represented by the "Sables a Unios" and at Ay near Epernay have furnished important mammalian remains.
The Lutetian Stage (Lutetia =Paris) in general marks the wid est extent of the Eocene marine invasion. The Paris basin was un doubtedly thrown into communication with the Mediterranean area and enormous numbers of Nummulites laevigatus-Lamarcki swarmed into the basin and formed the most characteristic fossil of the stage. The Lutetian Stage is particularly well developed in the Paris basin and there, apart from the basal sandy beds, con sists mainly of beds of limestone, the famous Calcaire grossier. Many beds, especially of the middle division, afford excellent building stone which has been widely quarried throughout the Paris basin. It is the presence of the Calcaire grossier which has given rise to the characteristic scenery of the Paris basin—wooded limestone plateaux intersected by steep sided valleys where the limestone has been cut through. The upper part of the Calcaire grossier is estuarine (caillasses) and represents the continental phase. In Belgium the Flanders Clay, in the Plain of Flanders, passes up into a group of glauconitic clays and sands—the "Pan iselian" of Belgian geologists (from Mont Panisel near Mons). The contemporaneity of this series with the Lutetian is generally agreed, though some geologists regard the Paniselian as equivalent rather with the upper part of the London Clay. It does not yield Nummulites laevigatus. In the heart of Belgium the Ypresian sands are succeeded by white marine sands (Bruxellian) with this fossil and it is difficult to prove the contemporaneity of the Bruxel lian and the Paniselian which are, however, probably two facies of the same deposit. The Bruxellian is well seen round Brussels. In England the Lutetian is represented by the lower part of the Bracklesham Beds, a group of sands, clays and carbonaceous sands well developed in the Hampshire basin and there yielding Num mulites laevigatus. Eocene beds, younger than the Bagshot Sands, are found in the London basin only over a small area in Surrey and fossils are scarce.
The Ledian or Auversian Stage (Lede in Belgium; Auvers in the Paris basin) is represented in the Paris basin by a series of sands (Sables de Beauchamp, or the Sables moyens of the older geologists) in which the characteristic fossil is Nummulites variolarius-Herberti. A similar but thin group of sands overlies the Bruxellian in Belgium, whilst in the Hampshire basin this stage is represented by the upper part of the Bracklesham Beds with the same fossil. Fresh-water limestones occur on the borders of the Paris basin belonging to the upper part of the Auversian.
The Bartonian Stage (Barton, Hampshire in England) is not quite so clearly defined in its upper limits as the preceding stages and there is still more than one interpretation commonly accepted. In England the richly fossiliferous Barton clay of the Hampshire basin has near its lower limits a bed with Nummulites prestwicliii. The clays pass up into marine sands (Barton Sands and Headon Hill Sands) and then into brackish and freshwater clays and sands fading gradually into the Lower Headon Beds. The limit of the Eocene in Hampshire is drawn sometimes at the base of the Lower Headon Beds—a line difficult to define—but more naturally at the base of the succeeding marine Middle Headon Beds. In the Paris basin, the marine Bartonian is thin (Sables de Marines or Ma rinesian) but the continental phase is well represented by the freshwater St. Ouen Limestone with Planorbis, Lirnnaea, etc.
Succeeding this, however, are the famous Montmartre gypsum deposits and the thin band of marl with stunted marine fossils (Ludian). The Paris basin seems to have been cut off from the sea at this period and to have formed a salt water lagoon in which the Ludian fauna was evolved in situ and the gypsum de posited. On the evidence of mammalian remains, the limit of Eocene and Oligocene must be drawn in or at the top of the main mass of gypsum. In Belgium the Ludian is followed by fine sands with Nummulites wemmelensis (=N. prestwichii) known as Wemmelian, but a higher thin stage of sandy clays, rarely seen, constitutes the Asschian of the Ludian of the Paris basin.
In England the famous Eocene sections are at either end of the Isle of Wight, the cliffs of the mainland from the Isle of Pur beck to the Solent and in the London basin in very numerous in land sections. Reference should be made especially to Geology in the Field (Iwo) and the Proceedings of the Geologists' Association.
In Denmark, Scandinavia and northern Germany the Eocene is for the most part poor in fossils and is usually hidden under the Quaternary. The interest of the first two regions lies in the continuous sequence which exists from the Cretaceous upwards. Over the large area in central Europe which separated the North ern sea of Eocene times from the Mediterranean, the Eocene de posits are very localized and consist of fluviatile sands and clays, red earths and lacustrine limestones.
As already mentioned the Atlantic of Eocene times extended over parts of south-west France and north-east Spain as broad gulfs. There are the small gulfs of Basse Loire with Lutetian deposits and the large gulf of Aquitaine. The latter had its great est extent in Lutetian times ; the oldest recognizable beds along the northern borders of the old gulf are Ypresian, along the south border in the beautiful cliff sections of Biarritz they are Lute tian. Elsewhere in the basin there is a complete series of deposits from the Cretaceous upwards. On the borders, as in the Paris basin, are continental deposits. There is no doubt that the chain of the Pyrenees existed in a rudimentary form even in the Eocene and separated the Aquitaine gulf from the Eocene area of the Ebro valley.
The deposits which were laid down in the Eocene geosyncline now represented by the Alps are very different from those of the northern basins. Thicknesses are measured by thousands of feet rather than by dozens; the beds are intensely folded and even metamorphosed; the principal beds are nummulitic limestones, often hard and crystalline, with thin marls and nummulitic sand stones. Such rocks are not only found in southern Europe, includ ing all the Alpine tracts, the Balkans and southern Russia, but they are well developed in northern Africa, Asia Minor, Palestine and may be followed through Persia, Baluchistan and India to China and Tibet. Locally, as in Italy between Vicenzia and Verona (the Vicentin), one finds a littoral phase of the Alpine geosyncline with a complete series of Eocene deposits, except the Landenian, richly fossiliferous. In Italy the term "Pria bonian" is often used instead of Bartonian and Ludian.
While the f oraminif eral limestones were being formed over most of southern Europe, a series of elastic beds were in course of formation in the Carpathians and the northern Alpine region— viz. the Flysch (q.v.) and the Vienna sandstone. Some portions of this Alpine Eocene are coarsely conglomeratic, and in places there are boulders of non-local rocks of enormous dimensions in cluded in the argillaceous or sandy matrix. The Flysch is spar ingly fossiliferous and difficult to correlate. In Egypt one finds the Eocene deposits transgressive over a continent which lay to the south of the old ocean; the "Libyan" of older authors includes the highest Cretaceous and lower Eocene; the Mokattam—includ ing the famous limestones with huge nummulites so well known near Cairo—represents the middle and upper Eocene. In the north of India, the Eocene has been divided into the Ranikot (Landen ian-Ypresian), Laki (Lutetian) and Kirthar (Lutetian and Bar tonian). Exact correlation with Europe is extremely difficult. The first of the three stages in the Himalayan uplift took place, in the middle Eocene and even in early Eocene times there existed round the fringes of Asia a series of basins in which sediments were deposited in much the same way as in the Anglo-Franco Belgian basin. Two excellent examples exist in the Gulf of Assam and the twin gulf of Burma. Unlike the northern European de posits the beds in the Burma gulf are upwards of to,000 feet in thickness; the deposits towards the landward end of the gulf are predominantly continental—sands and fresh-water to estuarine clays; towards the seaward end of the gulf they are predominantly marine.
Broadly it may be said that the same is true of the Tertiary gulfs which fringe the North and South American continents. The deposits of the Atlantic and Gulf Coast tracts belong definitely to the epicontinental non-folded type; the deposits of the West Indies and the Pacific coastal chains are rather of the geosynclinal type, whilst the deposits in the interior of the continent are of continental type. On the Atlantic coast there are no nummulites and correlation with Europe is difficult. Alabama is typical of the Gulf Coast Eocene; the succession is : Upper—Jacksonian—Limestone with Orthophragmina.
Middle Claibornian with a rich fauna of mollusca.
Chickasawan Sands and Lignites.
Lower—Midwayan or Clayton Limestones.
On the Pacific coast marine formations are found in California and Oregon; such are the Tejon series with lignite and coal; the Escondido series of southern California (7,000f t.) , part of the Pascadero series of the Santa Cruz mountains; the Pulaski, Tyee, Arago and Coaledo beds—with coals in Oregon. In the Puget formation of Washington we have a great series of sediments, largely of brackish water origin, and in parts coal-bearing. The total thickness of this formation has been estimated at 2o,000ft. (it may prove to be less than this), but it is probable that only the lower portion is of Eocene age. The most interesting of the North American deposits are those of the Rocky mountains and the ad jacent western plains, in Wyoming, Nevada, Nebraska, Colorado, etc.; they are of terrestrial, lacustrine or aeolian origin, and on this account and because they are not strictly synchronous, there is considerable difficulty in placing them in their true position in the time-scale. The main divisions or groups are generally recognized as follows :— See A. de Lapparent, Traite de geologic vol. iii. (5th ed., 1906) which contains a good general account of the period with numerous references to original papers. Also M. Gignoux Geologie strati graphique (1926) for a more modern general account; L. D. Stamp, Stratigraphy (1923), for the British Isles, with references; P. Lemoine, Geologic du Bassin de Paris; H. F. Osborn, The Age of Mammals (19io) for the American mammalian horizons. (L. D. S.)