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.
[Illustration: Fig. 249.--A. Under surface of the skull of
_Rhinoceros Etruscus_, one-seventh of the natural size--Pliocene,
Italy.; B, Crowns of the three true molars of the upper jaw, left
side, of _Rhinoceros megarhinus_ (_R. Leptorhinus_, Falconer),
one-half of the natural size--Pliocene, France. (After Falconer.)]
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.
[Illustration: Fig. 250.--Third milk-molar of the left side of
the upper jaw of _Mastodon Arvernensis_, showing the grinding
surface. Pliocene.]
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.
[Illustration: Fig. 251.--Molar tooth of _Elephas meridionalis_,
one-third of the natural size. Pliocene and Post-Pliocene.]
[Illustration: Fig. 252.--Molar tooth of _Elephas antiquus_,
one-third of the natural size. Pliocene and Post-Pliocene.]
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.
[Illustration: Fig. 253.--A, Skull of _Machairodus cultridens_,
without the lower jaw, reduced in size; B, Canine tooth of the
same, one-half the natural size. Pliocene, France.]
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.]