Chapter 47

THE PLIOCENE PERIOD.

The highest division of the Tertiary deposits is termed the _Pliocene_ formation, in accordance with the classification proposed by Sir Charles Lyell. The Pliocene formations contain from 40 to 95 per cent of existing species of _Mollusca_, the remainders belonging to extinct species. They are divided by Sir Charles Lyell into two divisions, the Older Pliocene and Newer Pliocene.

The Pliocene deposits of Britain occur in Suffolk, and are known by the name of "Crags," this being a local term used for certain shelly sands, which are employed in agriculture. Two of these Crags are referable to the Older Pliocene, viz., the White and Red Crags,--and one belongs to the Newer Pliocene, viz., the Norwich Crag.

The _White or Coralline Crag_ of Suffolk is the oldest of the Pliocene deposits of Britain, and is an exceedingly local formation, occurring in but a single small area, and having a maximum thickness of not more than 50 feet. It consists of soft sands, with occasional intercalations of flaggy limestone. Though of small extent and thickness, the Coralline Crag is of importance from the number of fossils which it contains. The name "Coralline" is a misnomer; since there are few true Corals, and the so-called "Corals" of the formation are really _Polyzoa_, often of very singular forms. The shells of the Coralline Crag are mostly such as inhabit the seas of temperate regions; but there occur some forms usually looked upon as indicating a warm climate.

The _Upper_ or _Red Crag_ of Suffolk--like the Coralline Crag--has a limited geographical extent and a small thickness, rarely exceeding 40 feet. It consists of quartzose sands, usually deep red or brown in colour, and charged with numerous fossils.

Altogether more than 200 species of shells are known from the Red Crag, of which 60 per cent are referable to existing species. The shells indicate, upon the whole, a temperate or even cold climate, decidedly less warm than that indicated by the organic remains of the Coralline Crag. It appears, therefore, that a gradual refrigeration was going on during the Pliocene period, commencing in the Coralline Crag, becoming intensified in the Red Crag, being still more severe in the Norwich Crag, and finally culminating in the Arctic cold of the Glacial period.

Besides the _Mollusca_, the Red Crag contains the ear-bones of Whales, the teeth of Sharks and Rays, and remains of the Mastodon, Rhinoceros, and Tapir.

The _Newer Pliocene_ deposits are represented in Britain by the _Norwich Crag_, a local formation occurring near Norwich. It consists of incoherent sands, loams, and gravels, resting in detached patches, from 2 to 20 feet in thickness, upon an eroded surface of Chalk. The Norwich Crag contains a mixture of marine, land, and fresh-water shells, with remains of fishes and bones of mammals; so that it must have been deposited as a local sea-deposit near the mouth of an ancient river. It contains altogether more than 100 marine shells, of which 89 per cent belong to existing species. Of the Mammals, the two most important are an Elephant (_Elephas meridionalis_), and the characteristic Pliocene Mastodon (_M. Arvernensis_), which is hitherto the only Mastodon found in Britain.

According to the most recent views of high authorities, certain deposits--such as the so-called "Bridlington Crag" of Yorkshire, and the "Chillesford beds" of Suffolk--are to be also included in the Newer Pliocene, upon the ground that they contain a small proportion of extinct shells. Our knowledge, however, of the existing Molluscan fauna, is still so far incomplete, that it may reasonably be doubted if these supposed extinct forms have actually made their final disappearance, whilst the strata in question have a strong natural connection with the "Glacial deposits," as shown by the number of Arctic Mollusca which they contain. Here, therefore, these beds will be included in the Post-Pliocene series, in spite of the fact that some of their species of shells are not known to exist at the present day.

The following are the more important Pliocene deposits which have been hitherto recognised out of Britain:--

1. In the neighbourhood of Antwerp occur certain "crags," which are the equivalent of the White and Red Crag in part. The lowest of these contains less than 50 per cent, and the highest 60 per cent, of existing species of shells, the remainder being extinct.

2. Bordering the chain of the Apennines, in Italy, on both sides is a series of low hills made up of Tertiary strata, which are known as the Sub-Apennine beds. Part of these is of Miocene age, part is Older Pliocene, and a portion is Newer Pliocene. The Older Pliocene portion of the Sub-Apennines consists of blue or brown marls, which sometimes attain a thickness of 2000 feet.

3. In the valley of the Arno, above Florence, are both Older and Newer Pliocene strata. The former consist of blue clays and lignites, with an abundance of plants. The latter consist of sands and conglomerates, with remains of large Carnivorous Mammals, Mastodon, Elephant, Rhinoceros, Hippopotamus, &c.

4. In Sicily, Newer Pliocene strata are probably more largely developed than anywhere else in the world, rising sometimes to a height of 3000 feet above the sea. The series consists of clays, marls, sands, and conglomerates, capped by a compact limestone, which attains a thickness of from 700 to 800 feet. The fossils of these beds belong almost entirely to living species, one of the commonest being the Great Scallop of the Mediterranean (_Pecten Jacoboeus_).

5. Occupying an extensive area round the Caspian, Aral, and Azof Seas, are Pliocene deposits known as the "Aralo-Caspian" beds. The fossils in these beds are partly freshwater, partly marine, and partly intermediate in character, and they are in great part identical with species now inhabiting the Caspian. The entire formation appears to indicate the former existence of a great sheet of brackish water, forming an inland sea, like the Caspian, but as large as, or larger than, the Mediterranean.

6. In the United States, strata of Pliocene age are found in North and South Carolina. They consist of sands and clays, with numerous fossils, chiefly _Molluscs_ and _Echinoderms_. From 40 to 60 per cent of the fossils belong to existing species. On the Loup Fork of the river Platte, in the Upper Missouri region, are strata which are also believed to be referable to the Pliocene period, and probably to its upper division. They are from 300 to 400 feet thick, and contain land-shells, with the bones of numerous Mammals, such as Camels, Rhinoceroses, Mastodons, Elephants, the Horse, Stag, &c.

As regards the _life_ of the Pliocene period, there are only two classes of organisms to which our attention need be directed--namely, the Shell-fish and the Mammals. So far as the former are concerned, we have to note in the first place that the introduction of new species of animals upon the globe went on rapidly during this period. In the Older Pliocene deposits, the number of shells of existing species is only from 40 to 60 per cent; but in the Newer Pliocene the proportion of living forms rises to as much as from 80 to 95 per cent. Whilst the Molluscs thus become rapidly modernised, the Mammals still all belong to extinct species, though modern generic types gradually supersede the more antiquated forms of the Miocene. In the second place, there is good evidence to show that the Pliocene period was one in which the climate of the northern hemisphere underwent a gradual refrigeration. In the Miocene period, there is evidence to show that Europe possessed a climate very similar to that now enjoyed by the Southern United States, and certainly very much warmer than it is at present. The presence of Palm-trees upon the land, and of numerous large Cowries, Cones, and other shells of warm regions in the sea, sufficiently proves this. In the Older Pliocene deposits, on the other hand, northern forms predominate amongst the Shells, though some of the types of hotter regions still survive. In the Newer Pliocene, again, the Molluscs are such as almost exclusively inhabit the seas of temperate or even cold regions; whilst if we regard deposits like the "Bridlington Crag" and "Chillesford beds" as truly referable to this period, we meet at the close of this period with shells such as nowadays are distinctively characteristic of high latitudes. It might be thought that the occurrence of Quadrupeds such as the Elephant, Rhinoceros, and Hippopotamus, would militate against this generalisation, and would rather support the view that the climate of Europe and the United States must have been a hot one during the later portion of the Pliocene period. We have, however, reason to believe that many of these extinct Mammals were more abundantly furnished with hair, and more adapted to withstand a cool temperature, than any of their living congeners. We have also to recollect that many of these large herbivorous quadrupeds may have been, and indeed probably were, more or less migratory in their habits; and that whilst the winters of the later portion of the Pliocene period were cold, the summers might have been very hot. This would allow of a northward migration of such terrestrial animals during the summer-time, when there would be an ample supply of food and a suitably high temperature, and a southward recession towards the approach of winter.

The chief palæontological interests of the Pliocene deposits, as of the succeeding Post-Pliocene, centre round the Mammals of the period; and amongst the many forms of these we may restrict our attention to the orders of the Hoofed Quadrupeds (_Ungulates_), the _Proboscideans_, the _Carnivora_, and the _Quadrumana_. Almost all the other Mammalian orders are more or less fully represented in Pliocene times, but none of them attains any special interest till we enter upon the Post-Pliocene.

Amongst the Odd-toed Ungulates, in addition to the remains of true Tapirs (_Tapirus Arvernensis_), we meet with the bones of several species of Rhinoceros, of which the _Rhinoceros Etruscus_ and _R. Megarhinus_ (fig. 249) are the most important. The former of these (fig. 249, A) derives its specific name from its abundance in the Pliocene deposits of the Val d'Arno, near Florence, and though principally Pliocene in its distribution, it survived into the earlier portion of the Post-Pliocene period. _Rhinoceros Etruscus_ agreed with the existing African forms in having two horns placed one behind the other, the front one being the longest; but it was comparatively slight and slender in its build, whilst the nostrils were separated by an incomplete bony partition. In the _Rhinoceros megarhinus_ (fig. 249, B), on the other hand, no such partition exists between the nostrils, and the nasal bones are greatly developed in size. It was a two-horned form, and is found associated with _Elephas meridionalis_ and _E. Antiquus_ in the Pliocene deposits of the Val d'Arno, near Florence. Like the preceding, it survived, in diminished numbers, into the earlier portion of the Post-Pliocene period.

The Horses (_Equidoe_) are represented, both in Europe and America, by the three-toed Hipparions, which survive from the Miocene, but are now verging upon extinction. For the first time, also, we meet with genuine Horses (_Equus_), in which each foot is provided with a single complete toe only, encased in a single broad hoof. One of the American species of this period (the _Equus excelsus_) quite equalled the modern Horse in stature; and it is interesting to note the occurrence of indigenous horses in America at such a comparatively late geological epoch, seeing that this continent certainly possessed none of these animals when first discovered by the Spaniards.

Amongst the Even-toed Ungulates, we may note the occurrence of Swine (_Suida_), of forms allied to the Camels (_Camelidoe_), and of various kinds of Deer (_Cervidoe_); but the most interesting Pliocene Mammal belonging to this section is the great _Hippopotamus major_ of Britain and Europe. This well-known species is very closely allied to the living _Hippopotamus amphibius_ of Africa, from which it is separated only by its larger dimensions, and by certain points connected with the conformation of the skeleton. It is found very abundantly in the Pliocene deposits of Italy and France, associated with the remains of the Elephant, Mastodon, and Rhinoceros, and it survived into the earlier portion of the Post-Pliocene period. During this last-mentioned period, it extended its range northwards, and is found associated with the Reindeer, the Bison, and other northern animals. From this fact it has been inferred, with great probability, that the _Hippotamus major_ was furnished with a long coat of hair and fur, thus differing from its nearly hairless modern representative, and resembling its associates, the Mammoth and the Woolly Rhinoceros.

Passing on to the Pliocene Proboscideans, we find that the great _Deinotheria_ of the Miocene have now wholly disappeared, and the sole representatives of the order are Mastodons and Elephants. The most important member of the former group is the _Mastodon Arvernensis_ (fig. 250), which ranged widely over Southern Europe and England, being generally associated with remains of the _Elephas meridionalis, E. antiquus, Rhinoceros megarhinus_, and _Hippopotamus major_. The lower jaw seems to have been destitute of incisor teeth; but the upper incisors are developed into great tusks, which sometimes reach a length of nine feet, and which have the simple curvature of the tusks of the existing Elephants. Amongst the Pliocene Elephants the two most important are the _Elephas meridionalis_ and the _Elephas antiquus_. Of these, the _Elephas meridionalis_ (fig. 251) is found abundantly in the Pliocene deposits of Southern Europe and England, and also survived into the earlier portion of the Post-Pliocene period. Its molar teeth are of the type of those of the existing African Elephant, the spaces enclosed by the transverse enamel-plates being more or less lozenge-shaped, whilst the curvature of the tusks is simple. The _Elephas antiquus_ (fig. 252) is very generally associated with the preceding, and it survived to an even later stage of the Post-Pliocene period. The molar teeth are of the type of the existing Indian Elephant, with comparatively thin enamel-ridges, placed closer together than in the African type; whilst the tusks were nearly straight.

Amongst the Pliocene _Carnivores_, we meet with true Bears (_Ursus Arvernensis_), Hyænas (such as _Hyoena Hipparionum_), and genuine Lions (such as the _Felis angustus_ of North America); but the most remarkable of the beasts of prey of this period is the great "Sabre-toothed Tiger" (_Machairodus_), species of which existed in the earlier Miocene, and survived to the later Post-Pliocene. In this remarkable form we are presented with perhaps the most highly carnivorous type of all known beasts of prey. Not only are the jaws shorter in proportion even than those of the great Cats of the present day, but the canine teeth (fig. 253) are of enormous size, greatly flattened so as to assume the form of a poignard, and having their margins finely serrated. A part from the characters of the skull, the remainder of the skeleton, so far as known, exhibits proofs that the Sabre-toothed Tiger was extraordinarily muscular and powerful, and in the highest degree adapted for a life of rapine. Species of _Machairodus_ must have been as large as the existing Lion; and the genus is not only European, but is represented both in South America and in India, so that the geographical range of these predaceous beasts must have been very extensive.

Lastly, we may note that the Pliocene deposits of Europe have yielded the remains of Monkeys (_Quadrumana_), allied to the existing _Semnopitheci_ and Macaques.

LITERATURE.

The following list comprises a small selection of some of the more important and readily accessible works and memoirs relating to the Tertiary rocks and their fossils. With few exceptions, foreign works relating to the Tertiary strata of the continent of Europe or their organic remains have been omitted:--

(1) 'Elements of Geology.' Lyell. (2) 'Students' Elements of Geology.' Lyell. (3) 'Manual of Palæontology.' Owen. (4) 'British Fossil Mammals and Birds.' Owen. (5) 'Traité de Paléontologie.' Pictet. (6) 'Cours Elémentaire de Paléontologie.' D'Orbigny. (7) "Probable Age of the London Clay," &c.--'Quart. Journ. Geol. Soc.,' vol. iii. Prestwich. (8) 'Structure and Probable Age of the Bagshot Sands'--Ibid., vol. iii. Prestwich. (9) 'Tertiary Formations of the Isle of Wight'--Ibid., vol. ii. Prestwich. (10) 'Structure of the Strata between the London Clay and the Chalk,' &c.--Ibid., vols. vi., viii., and x. Prestwich. (11) 'Correlation of the Eocene Tertiaries of England, France, and Belgium'--Ibid., vol. xxvii. Prestwich. (12) 'On the Fluvio-marine Formations of the Isle of Wight'--Ibid., vol. ix. Edward Forbes. (13) 'Newer Tertiary Deposits of the Sussex Coast'--Ibid., vol. xiii. Godwin-Austen. (14) 'Kainozoic Formations of Belgium'--Ibid., vol. xxii. Godwin-Austen. (15) 'Tertiary Strata of Belgium and French Flanders'--Ibid., vol. viii. Lyell. (16) 'On Tertiary Leaf-beds in the Isle of Mull'--Ibid., vol. vii. The Duke of Argyll. (17) 'Newer Tertiaries of Suffolk and their Fauna'--Ibid., vol. xxvi. Ray Lankester. (18) 'Lower London Tertiaries of Kent'--Ibid., vol. xxii. Whitaker. (19) "Guide to the Geology of London"--'Mem. Geol. Survey.' Whitaker. (20) 'Memoirs of the Geological Survey of Great Britain.' (21) 'Introductory Outline of the Geology of the Crag District' (Supplement to Crag Mollusca, Palæontographical Society). S. V. Wood, jun., and F. w. Harmer. (22) "Tertiary Fluvio-marine Deposits of the Isle of Wight." Edward Forbes. Edited by Godwin-Austen; with Descriptions of the Fossils by Morris, Salter, and Rupert Jones--'Memoirs of the Geological Survey.' (23) 'Geological Excursions round the Isle of Wight.' Mantell. (24) 'Catalogue of British Fossils.' Morris. (25) 'Catalogue of Fossils in the Museum of Practical Geology.' Etheridge. (26) 'Monograph of the Crag Polyzoa' (Palæontographical Society). Busk. (27) 'Monograph of the Tertiary Brachiopoda' (Ibid.) Davidson. (28) 'Monograph of the Tertiary Malacostracous Crustacea' (Ibid.) Bell. (29) 'Monograph of the Tertiary Corals' (Ibid.) Milne-Edwards and Haime. (30) 'Supplement to the Tertiary Corals' (Ibid.) Martin Duncan. (31) 'Monograph of the Eocene Mollusca' (Ibid.) Fred. E. Edwards. (32) 'Monograph of the Eocene Mollusca' (Ibid.) Searles V. Wood. (33) 'Monograph of the Crag Mollusca' (Ibid.) Searles V. Wood. (34) 'Monograph of the Tertiary Entomostraca' (Ibid.) Rupert Jones. (35) 'Monograph of the Foraminifera of the Crag' (Ibid.) Rupert Jones, Parker, and H. B. Brady. (36) 'Monograph of the Radiaria of the London Clay' (Ibid.) Edward Forbes. (37) 'Monograph of the Cetacea of the Red Crag' (Ibid.) Owen. (38) 'Monograph of the Fossil Reptiles of the London Clay' (Ibid.) Owen and Bell. (39) "On the Skull of a Dentigerous Bird from the London Clay of Sheppey"--'Quart. Journ. Geol. Soc.,' vol. xxix. Owen. (40) 'Ossemens Fossiles.' Cuvier. (41) 'Fauna Antiqua Sivalensis.' Falconer and Sir Proby Cautley. (42) 'Palæontological Memoirs.' Falconer. (43) 'Animaux Fossiles et Géologie de l'Attique.' Gaudry. (44) "Principal Characters of the Dinocerata"--'American Journ. of Science and Arts,' vol. xi. Marsh. (45) 'Principal Characters of the Brontotheridæ' (Ibid.) Marsh. (46) 'Principal Characters of the Tillodontia' (Ibid.) Marsh. (47) "Extinct Vertebrata of the Eocene of Wyoming"--'Geological Survey of Montana,' &c., 1872. Cope. (48) "Ancient Fauna of Nebraska"--'Smithsonian Contributions to Knowledge,' vol. vi. Leidy. (49) 'Manual of Geology.' Dana. (50) "Palæontology and Evolution" (Presidential Address to the Geological Society of London, 1870)--'Quart. Journ. Geol. Soc.,' vol. xxvi. Huxley.' (51) 'Mineral Conchology.' Sowerby. (52) 'Description des Coquilles Fossiles,' &c. Deshayes. (53) 'Description des Coquilles Tertiaires de Belgique.' Nyst. (54) 'Fossilen Polypen des Wiener Tertiär-beckens.' Reuss. (55) 'Palæontologische Studien über die älteren Tertiär-schichten der Alpen.' Reuss. (56) 'Land und Süss-wasser Conchylien der Vorwelt.' Sandberger. (57) 'Flora Tertiaria Helvetica.' Heer. (58) 'Flora Fossilis Arctica.' Heer. (59) 'Recherches sur le Climat et la Végétation du Pays Tertiaire.' Heer. (60) 'Fossil Flora of Great Britain.' Lindley and Hutton. (61) 'Fossil Fruits and Seeds of the London Clay.' Bowerbank. (62) "Tertiary Leaf-beds of the Isle of Mull"--'Quart. Journ. Geol. Soc.,' vol. vii. Edward Forbes. (63) 'The Geology of England and Wales.' Horace B. Woodward.[25]

[Footnote 25: This work--published whilst these sheets were going through the press--gives to the student a detailed view of all the strata of England and Wales, with their various sub-divisions, from the base of the Palæozoic to the top of the Tertiary.]