It will be seen that the greatest variations in lithological character occur in the Bathonian and Bajocian beds, and it will be of interest to give some account of the principal variations and to attempt to account for them. In so doing it will be convenient to consider the four major divisions of the Jurassic rocks separately, and to enter into particulars concerning the local classification applied to the rocks of these divisions.

The Lias. The British Lias deposits are divided into the Lower Lias, the Marlstone, and the Upper Lias corresponding in general terms only with the Sinemurian, Liassian, and Toarcian. The Marlstone is separated from the Upper and Lower Lias on account of the greater percentage of carbonate of lime which it contains, so that the bands of argillaceous limestone are much more marked in the Marlstone than in the upper and lower divisions, which consist chiefly of clay. The three divisions possess very much the same characters throughout the country, though the presence of the Mendip ridge and its continuation beneath London is marked by the attenuation of this and succeeding strata, and by the conglomeratic character of some of the Liassic strata where they abut against it. The British Lias, as a whole, seems to have been deposited in a fairly shallow sea at no great distance from the land. It passes down conformably into the Rhætic beds, indeed the zone of Ammonites (Aegoceras) planorbis, referred by British geologists to the Lower Lias is included by some continental writers with the Rhætic beds, and the plane of demarcation here as in other cases is conventional.

The Lower Oolites. Of all the British strata, these perhaps cause most trouble to the learner, on account of the different nomenclature applied to the rocks in different parts of England, and the rapid variations in lithological character, when the beds are traced laterally. The following divisions are usually adopted for the beds of the south-western counties where the most marked marine development occurs:—

Cornbrash,
Forest Marble,
Great Oolite (with Bradford Clay),
Fuller's Earth,
Inferior Oolite.

Of these divisions, the uppermost one, the Cornbrash, though thin, retains its characters with great constancy across the island. Of the others the Forest Marble may be looked upon as a local development of the upper portion of the Great Oolite, and the Fuller's Earth is a local deposit, so that the Inferior Oolite and Great Oolite constitute the important divisions of the Lower Oolites. The variations in the characters of the rocks may be best shown in tabular form.

The changes may be explained very simply if we leave out of account for the moment the development of Lincolnshire Limestone, with its equivalent the Scarbro' Limestone, and the Millepore series. The beds in Gloucestershire and other south-western counties are essentially marine; whilst in Northamptonshire and Lincolnshire estuarine conditions set in after the deposition of the Upper Lias, and continued throughout the deposition of the Bajocian and Lower Bathonian beds, being replaced by marine conditions during the formation of the Upper Bathonian strata, and still further north in Yorkshire the estuarine conditions generally prevailed throughout Bajocian and Bathonian times. These changes point to the existence of land towards the north. The general simplicity is modified by temporary prevalence of marine conditions twice over (during the deposition of the Millepore Oolite and the Scarbro' Limestone) in Yorkshire, and once (during the deposition of the Lincolnshire Limestone) in Lincolnshire.

Certain local deposits have not been noticed, but two of them merit brief reference. At the base of the Great Oolite of Oxfordshire is an estuarine deposit of finely laminated mechanical sediment mixed with calcareous matter known as the Stonesfield Slate, especially interesting on account of its fossils, while a bed with similar lithological characters but with a different fauna occurring at the base of the Lincolnshire Limestone (of Bajocian age) is termed the Collyweston Slate. Neither of these deposits is a slate in the true sense of the word, as they have not been affected by cleavage subsequently to their accumulation, but each has been somewhat extensively used for roofing purposes.

The Middle Oolites are much less complicated though considerable variations arise with respect to the Corallian Rocks. The Oxfordian with Callovian consist chiefly of clay, though the Callovian of the south of England is represented by calcareous sandstone, with a peculiar fauna which seems to be represented in the lower part of the Oxford Clay further north, though this Callovian fauna has not been everywhere recognised.

The Corallian of the southern counties consists of limestones with calcareous grits, the limestones being often largely composed of the remains of reef-building corals, and a similar development of the rocks of this series is found in Yorkshire, while a local development of the same character is found at Upware in Cambridgeshire, though in the other parts of the Fenland counties the Corallian is represented by an argillaceous deposit with Corallian fossils known as the Ampthill Clay.

The Upper Oolites have a tolerably constant base, the Kimmeridge Clay, usually consisting of laminated bituminous argillaceous material, but the Portlandian and Purbeckian divisions vary greatly, and are only locally developed, though their absence in some parts of central England is no doubt due to unconformity.

The Portlandian rocks of the south of England consist of limestones and sandstones which pass further northward into shallower water mechanical deposits often charged with iron hydrate, and the beds disappear in Oxfordshire. The Purbeckian rocks of the south are also limited as regards area of exposure: they consist of estuarine deposits with some terrestrial accumulations of the nature of old surface soils. Representations of the Portlandian and Purbeckian beds are found in Lincolnshire and Yorkshire, as arenaceous deposits in the former county and argillaceous ones in the latter. Both are marine deposits of a northern type, developed elsewhere in northern European and circumpolar regions, and in these counties we find a complete passage from the Jurassic rocks through the Cretaceous rocks, but the exact lines of demarcation between the different series of the passage beds are difficult to define.

The foreign Jurassic rocks of Europe and of some parts of Asia strongly resemble in general characters those which have been described above as occurring in Britain. One of the most remarkable features of the Jurassic rocks as a whole, is the absence of the Lias over wide areas, the continental period which in Britain existed in Permo-Triassic times is elsewhere frequently replaced by one of Liassic age.

The Jurassic and Cretaceous rocks are of interest on account of the evidence which they supply as to the existence of climatic zones in these periods, which run fairly parallel with those at present existing. The late Dr Neumayr in a paper already cited divides the world during later Mesozoic times into four distinct climatic zones, equatorial, north and south temperate and boreal zones (the corresponding austral zone is not known owing no doubt to the extensive sea of South Polar regions and our general ignorance of its lands). In Europe the Mediterranean Province belongs to the equatorial zone, the Middle European to the North temperate zone, and the Russian or Boreal to the Boreal zone. The last-named is marked partly by negative characters, the absence of certain Ammonite-genera and of coral reefs being noticeable, whilst the lamellibranch Aucella is very frequent. In the North temperate zone, certain Ammonite genera as Aspidoceras and Oppelia are abundant and there are also extensive coral-reefs. The Equatorial zone is marked by the Ammonite-genera Phylloceras and Lytoceras and by the Diphya group of Terebratulæ. It is of special interest to note that the fauna of the South temperate bears closer relationship to that of the North temperate than to that of the intermediate Equatorial zone.

Jurassic floras and faunas. The Jurassic flora is very similar in its characters to that of the Lower Cretaceous rocks, and the two taken together afford a decided contrast with that of later Palæozoic times, and also with that which succeeds them in the Upper Cretaceous rocks, which bears a marked resemblance to the existing flora. Cycads predominate, accompanied by conifers, and a fair number of ferns and Equisetaceæ.

The Jurassic fauna is specially noteworthy on account of the character of the vertebrata, but some notice of the invertebrates must also be taken. The abundance of corals in the Temperate zones has already been pointed out, but the mollusca form the bulk of the invertebrate fauna, lamellibranchs, gastropods and cephalopods being all abundant; of the last-named the ammonites and belemnites contribute most largely. The vertebrates include remains of fishes, amphibia, reptiles, birds and mammals. The Jurassic reptilia furnish representatives of some modern orders as the Chelonia and Crocodilia, but the most important orders are essentially characteristic of later Mesozoic times and their representatives abound in the Jurassic strata. These are the Sauropterygia (including the Plesiosaurs), the Ichthyopterygia (including the Ichthyosaurs), the Dinosauria, and the Pterosauria commonly known as Pterodactyls. No birds have hitherto been discovered in the British Jurassic rocks, but the Solenhofen Slate of Bavaria (of Kimmeridgian age) has furnished the celebrated Archæopteryx macrura, which is not only placed in a family but also in an order by itself, the order Saururæ. Many remains of mammals have been extracted from the estuarine deposits of Stonesfield, and the old surface soils of the Purbeckian beds; representatives of the Monotremata are furnished by the Plagiaulacidæ and Tritylodontidæ, the former family containing the genus Plagiaulax of the Purbeck Beds and the latter, Stereognathus of the Stonesfield slate. The Marsupialia are represented by the Amphitheridæ, Spalacotheridæ and Triconodontidæ. Some forms have been referred to the Insectivora, but there is still disagreement concerning the correctness of this reference.

Before dismissing the subject of the Jurassic fossils, attention may be called to a feature which has been frequently commented upon, namely, the general resemblance of the flora and fauna of Jurassic times to the modern Australian fauna and flora. The explanation which has been offered to account for this resemblance has been given in a preceding chapter, where it was stated that Mr A. R. Wallace considers, after review of the geological and biological evidence, that Australia was severed from the adjoining continental lands in Mesozoic times, and that the higher forms of life which on the larger continents have replaced the earlier and lower forms have not succeeded in obtaining a footing in Australia, which therefore furnishes us with a local survival of a once widespread fauna. In connection with this matter the actual existence of the genus Trigonia (a form peculiarly abundant in Jurassic strata and characteristic of Mesozoic strata in Britain) in the Australian sea is of considerable interest. [100]

CHAPTER XXIII.

THE CRETACEOUS SYSTEM.

Classification. The rocks of the Cretaceous system are conveniently divided into Upper and Lower Cretaceous. The following classification has been widely used for the British deposits, and is founded on lithological characters:

As the result of examination of the faunas, a more generally applicable classification has been established and is now largely adopted. It is as follows:

In this classification the Neocomian practically represents the Wealden and Hastings beds, the Aptian the Lower Greensand and the Albian the Gault, placed according to this classification in the Lower Cretaceous, while the Upper divisions represent the strata above the Gault, consisting essentially of Chalk in England.

Description of the Strata.

(i) The Neocomian and Aptian Beds. In the south of England the Lower Cretaceous beds succeed the Jurassic rocks with little or no break, and the type of the lower beds is similar to that of the beds deposited during the Purbeck age, consisting of estuarine deposits of variable characters, chiefly arenaceous below (the Hastings sands) and argillaceous above (the Wealden series), though impure limestones are found, largely composed of the shells of the freshwater Paludina, and much ironstone is developed in places. At the close of Neocomian times, the freshwater conditions in southern England were replaced by marine conditions and the Lower Greensand strata with their marine fauna were deposited in the Aptian sea. The Neocomian and Aptian beds thin out westward, and much more rapidly to the northward, so that both divisions disappear against the now buried ridge which forms a continuation of the Mendip axis. North of this they appear in another form. At first the highest Aptian beds alone are developed as shore deposits. Passing into Norfolk lower beds come in until in Lincolnshire we get a complete development of the Neocomian and Aptian beds with a marine facies, though of fairly shallow water character, whilst in Yorkshire the two divisions are represented by a deeper water clay, forming the Upper portion of the Speeton series. There is a consensus of opinion in favour of the Neocomian beds of southern Britain having been laid down in an estuary of a river flowing from the west over a continent now destroyed. To the north of this river stood the London ridge of the Palæozoic rocks, the northern borders of which formed the coast line off which were deposited the sediments of Neocomian and Aptian ages which occur in northern England. Before the deposition of the Albian beds a considerable upheaval of some parts of Britain occurred, and an unconformity separates the higher Cretaceous beds from older strata of Cretaceous and Jurassic ages, thus complicating the major phases by local changes in the characters of the strata.

(ii) The Albian and higher Cretaceous Beds. The commencement of the deep-water phase of the third marine period may be said to occur in Albian times in Britain, reaching its maximum during the deposition of the chalk. The existence of a deeper sea towards the north of England is indicated by the characters of the Albian and newer strata. The Albian beds of gault consist of a stiff clay in southern England, replaced by coarser mechanical sediments towards the west. As one passes north from the London ridge (which exerted its influence in Albian times, after which it was finally buried in sediment) the gault thins out, and becomes gradually replaced by calcareous deposit when it is known as the red chalk which replaces the gault in northern Norfolk, Lincolnshire and Yorkshire.

A local unconformity separating the chalk and gault in parts of East Anglia points to another local uplift with its accompanying complications in the characters of the strata. After the uplift had ceased, general depression must have occurred, and the various divisions of the chalk were accumulated in a fairly open sea, though, for reasons to be given presently, this was probably of no great lateral extent, save when united with the open ocean, probably in a manner similar to the connexion between the Gulf of Mexico and the Atlantic.

The general variations in the lithological characters of the various members of the Cretaceous system will probably be rendered clearer by reference to the accompanying diagram (Fig. 24) representing the variations when traced across England from south to north[101].

The clue to the physical geography of Britain during Cretaceous times is furnished to a considerable extent by study of the foreign deposits. In Northern Europe the Cretaceous beds of England are met with in Northern France, and there the characters are generally speaking similar to those of our British deposits. In Germany shallower water conditions prevailed, the lower beds gradually disappear, and the upper beds are replaced by mechanical sediments of various degrees of coarseness, becoming on the whole coarser, as one travels eastward, so that in Saxony the chalk is partly replaced by arenaceous deposits (the 'Quader' sandstones) which are responsible for the remarkable scenery of the Elbe district above Dresden. In passing northwards, indications of similar change are noted in the deposits of Denmark and Scania, whilst to the south, we get a complete change in the character of the rocks, after crossing the Loire in France, and a similar change is observable in districts lying further east. Furthermore, as will be noted more fully in a subsequent paragraph, the character of the Upper Cretaceous flora indicates the existence of a large tract of land lying to the north and north-west of Europe, so that it would appear that the Cretaceous rocks of Northern Europe were deposited in a gulf-like expansion of a western ocean, bounded on the north by Scandinavia, on the west by eastern Germany, and on the south by a ridge running eastward from the mouth of the Loire[102]. We may speak of this gulf as the Chalk gulf. To the south of the presumed ridge the character of the strata alters, and also that of the included organisms. This southern type of Cretaceous rocks is one which is very widely spread, being found in Europe south of the Loire, and of the Alps, and in Greece and Turkey, while it also occurs in the northern parts of Africa. The beds of this type are traceable through Asia Minor into India and to the shores of the Indian Ocean, indicating the existence of a widespread Cretaceous ocean, which is sometimes spoken of as the Hippurite-limestone sea, for reasons which will eventually appear. The deposits are largely formed of hard limestone which is very different in its character from the soft chalk of the northern gulf.

The climatic conditions which prevailed during Cretaceous times were apparently similar in most respects to those of the preceding Jurassic period, and as already stated the climatic zones which Neumayr defined for Jurassic times are also maintained by him to have existed during the Cretaceous period. The existence of cold has sometimes been inferred from the presence of large foreign blocks in the chalk, especially at its base, but if these are due to the transport, they might well be caused by masses of floating ice, which are often found at considerable distances from the coast in temperate regions after the break-up of the frost which succeeds an unusually hard winter. The flora and fauna are not suggestive of severe conditions.

The Cretaceous flora and fauna. It has been noted in the last chapter that the gymnospermous flora of the Jurassic period, in which cycads form a considerable percentage of the whole flora, was prevalent in Lower Cretaceous times. In the Upper Cretaceous rocks this flora is replaced by one which consists to a large extent of dicotyledonous angiosperms. These are found in the Upper Cretaceous rocks of Europe and North America, and as the researches of botanists indicate their origin in circumpolar regions, their arrival in Europe is an additional argument in favour of the existence of an extensive northern continent, sending a prolongation to the southward in eastern Europe.

The invertebrate fauna bears considerable resemblance to that of Jurassic times, and many of the dominant Jurassic genera are also found in Cretaceous rocks. A most interesting feature is connected with the character and geographical distribution of the Ammonites. In Europe they are almost exclusively confined to the deposits of the northern gulf, and before their final disappearance they undergo many changes of form. We find the discoid spiral shells of earlier times, but these are accompanied by shells which are straight, curved, boat-shaped, and coiled into various helicoid spirals, sometimes having the whorls in contact, while at other times they are separate.

In the chalk of Britain gastropods are on the whole rare, and this fact serves to emphasize the palæontological break which occurs between the Cretaceous and Tertiary rocks; but when conditions were favourable, as during the deposition of some of the strata of the Middle Chalk, gastropods are abundant, and some are related to Tertiary genera, so that we may assume that the palæontological break alluded to is exaggerated by the difference of conditions which prevailed during the deposition of the earliest Tertiary and latest Cretaceous sediments.

In the Cretaceous deposits of the southern sea, where the Ammonite tribe is almost unknown, the remarkable family of the lamellibranchs known as the Hippuritidæ furnish the dominant invertebrates of the period, and the representatives of this family are exceedingly scarce amongst the Cretaceous strata of the northern gulf, though they are found on two or three horizons.

Of vertebrates, the most interesting are the reptiles. The families which predominate in Jurassic times have many representatives amongst the Cretaceous strata also, but the order Squamata is represented by the sub-order Pythonomorpha, which is characteristic of the Cretaceous rocks. The best known representative is the gigantic Mosasaurus. Lastly, we have the remarkable toothed birds or Odontornithes, now placed in different orders, the genus Hesperornis being the only representative of the sub-order Odontolcæ of the Ratitæ, whilst Ichthyornis and allied forms are placed in the sub-order Odontormæ of the Carinatæ.

CHAPTER XXIV.

THE EOCENE ROCKS.

Classification. The Eocene Beds of the south of England have been subdivided according to the variations in their lithological characters, and the subdivisions have received local names. The following classification is generally adopted, though the different subdivisions are by no means of equal value:

The deposits vary greatly when traced abroad, and the exact equivalents of the minor subdivisions of the British rocks can seldom be ascertained at any distance from England, though the division into Upper, Middle, and Lower Eocene can be made over wide areas.

Description of the strata. The character of the strata of Europe and Asia indicates the persistence of the northern gulf and southern ocean of Cretaceous times in Eocene times also, though the area of each had shrunk in the meantime, owing to the physiographical changes which occurred at the end of Cretaceous times, giving rise to more extended land areas, and producing a shallow water phase over wide extents of ocean,—the final shallow water phase of the third and last great marine period of the British area. It is difficult to ascertain the exact importance of the physical break between Cretaceous and Eocene rocks in the south-east of England, owing to the subterranean solution of the upper part of the chalk, subsequently to the deposition of the Eocene strata, but the contraction of the Cretaceous gulf is shown in several ways, one of the most significant being the distribution of Cretaceous and Eocene rocks in the south-west of England. The existence of an outlier of Cretaceous rock at Buckland Brewer in North Devon, only three miles from the Atlantic Ocean, indicates the former extension westward of the Upper Cretaceous beds, while the occurrence of an outlier of Eocene rocks at Bovey Tracey in South Devon, resting not on Cretaceous but on Palæozoic rocks, shows that there was an uplift after the deposition of the Cretaceous rocks and before the Eocene rocks were deposited there, and that during the period of uplift the Cretaceous rocks were removed.

Owing to these physical changes, the Eocene rocks of Britain are mainly mechanical sediments, some, as the Oldhaven beds, being composed of coarse pebbles over a fairly wide district, while some of the earlier Eocene rocks are estuarine or fluvio-marine.

The Eocene rocks of Britain occur in four areas, namely, the London Basin, the Hampshire Basin, the Bovey Tracey outlier, and the north-east of Ireland and western Isles of Scotland. The deposits of the three southern areas may be considered together, and give general indications of an approach to land when passing westward. The Lower London Tertiary strata are fluvio-marine at the east end of the London Basin; they become shallower water deposits when traced westward, and begin to disappear. The London Clay is an estuarine deposit, which is generally supposed to have been laid down at the mouth of a large river flowing from the west. It is absent in the Bovey Tracey outlier.

Local disturbances caused the existence of a shallow water region in the east during the deposition of the Middle and Upper Eocene deposits, and accordingly the well-marked marine deposits which form the representatives of these divisions in Hampshire are replaced by the Bagshot beds of the London Basin, consisting chiefly of coarse mechanical sediments with a poor marine fauna, but even in the west shallow water prevailed at times during the accumulation of various plant-bearing strata. The Middle Eocene beds only are found in the Bovey Tracey outlier, though the Upper Eocene beds may originally have been laid down in that area, and subsequently denuded.

The fourth area displays a very different succession of Eocene strata, and one of extreme interest. Mechanical sediments and plant-bearing clays and lignites alternate with a vast accumulation of basaltic lavas, indicating the outbreak of the volcanic forces in the British area, after a period of quiescence which lasted through the greater part of Mesozoic times. The region in which these lavas were poured out was probably a land area during the greater part of the period of volcanic activity, but the horizontal lie of the lava flows and their wide extent indicate the existence of a flat tract of country, gradually raised into a plateau by the accumulation of sheet over sheet of basalt. How far this plateau extended it is impossible to say. The distribution of the lavas at the present day is somewhat limited in our isles, but there is no sign of dying out at the present margins of the accumulations, and they have probably escaped denudation in these regions, as maintained by Professor Judd, on account of the faults which have depressed them, while the portions which were not depressed have been removed by denudation. Two views as to the origin of the lavas have been put forward: according to Prof. Judd, they were poured forth from gigantic volcanoes, while Sir A. Geikie maintains that they represent portions of massive or fissure eruptions, the molten rock having welled out from great cracks in the earth, which are now filled by once molten rock in the form of dykes. As these dykes extend far away from the present volcanic plateau, one actually extending to the Yorkshire coast, we may well believe, whatever was the origin of the sheets of lava, that they were formerly spread far away from their present terminations[104]. Without entering here into a discussion of the exact nature of extrusion of these igneous sheets, it will suffice to say that all the evidence points to the formation of extensive plateaux, which must have presented a fairly uniform surface, similar to that which is still found characterising the volcanic districts of the western territories of North America.

The Eocene rocks of the north-west of Europe possess characters very similar to those of the south of England, and there are indications that the northern gulf had diminished in extent towards the east as well as towards the west.

Passing to southern Europe, Central Asia and northern Africa, we find the conditions of Cretaceous times reproduced, and an extensive series of marine deposits extends very widely over these regions, the most persistent deposit being a mass of limestone of Middle Eocene age, which is almost entirely composed of the tests of Nummulites, whence the development is known as the Nummulitic Limestone facies, and we may speak of the ocean as the Nummulitic Limestone Sea. The incoming of shallow water conditions marked by accumulation of coarse mechanical sediments towards the end of the Eocene period in some parts of the southern European area indicates the setting in, even then, of those continental conditions which culminated during the Miocene period.

In North America we get similar evidence of the contractions of the oceans which in Mesozoic times occupied large expanses of our present continents.

The climatic conditions of Eocene times have been noticed in passing in chapter IX., and evidence was given to prove the prevalence of a warmer climate over the British area than that which now exists. A study of the floras of various parts of the northern hemisphere suggests that climatic zones, whose lines of demarcation ran practically parallel with the Equator, existed in Eocene times also, though further information upon this subject is desirable.

The Eocene flora and fauna. The flora of prevalent dicotyledonous angiosperms, which appeared in Upper Cretaceous times, also marks the Eocene and later deposits, but a study of the floras indicates that the differentiation which now marks off the floras of different areas from one another had not occurred to so great an extent as at the present time. The existence of a rich flora in the Eocene beds of circumpolar regions in the northern hemisphere should be noted, though perhaps its importance has been somewhat exaggerated.

The invertebrate fauna shows an approximation to that of the present day. The remarkable ammonite fauna of Mesozoic times has disappeared, and gastropods and lamellibranchs predominate, many of the forms belonging to existing genera, though very rarely to existing species. The Nummulites are the most characteristic Eocene fossils, and the period may be spoken of as the Nummulitic Period, though it is now known that Nummulites are not confined to the Eocene strata.

The vertebrate fauna is very noteworthy. The fishes and reptiles are closely related to existing forms, and the orders of reptiles which predominated in Mesozoic times have completely disappeared. But the mammals are the most interesting vertebrates of the Eocene period. Instead of the lowly organised forms of Mesozoic times, we find representatives of many orders, including the highest, the Primates. The generalised forms which serve as links between groups which are now separated to a considerable extent are of particular importance. They have been detected in Eocene rocks of various regions, though the most complete series have been obtained from the Eocene rocks of North America and made known to us through the numerous memoirs of Professors Cope and Marsh[105].

CHAPTER XXV.

THE OLIGOCENE AND MIOCENE PERIODS.

(i) The Oligocene Beds.

Classification. The Oligocene Beds of Britain are classified as follows:—

Description of the strata. Little need be said of the deposits of this period, either in Britain or abroad, except to remark that they show the further spread of continental conditions over the regions now occupied by land. The British deposits are now seen in the Hampshire Basin only, and have been spoken of as the fluvio-marine series, as many of the strata were laid down in continental sheets of water, while the true marine sediments are thin and infrequent.

The lithological characters of deposits formed under these conditions naturally vary greatly, consisting of different kinds of mechanical sediments occasionally mixed with thin freshwater marls and limestones. On the Continent similar conditions prevailed, though the occurrence of fairly wide tracts of level surface is indicated by the widespread distribution of beds of brown coal or lignite, and the coarse and thick Oligocene 'nagelfluh' of Switzerland points to the elevation of mountain ranges in the neighbourhood.

The flora and fauna. The remarks made concerning the Eocene flora and fauna are generally applicable to those of Oligocene times, except that the Oligocene fossils bear a still closer resemblance to living forms, and the Nummulites are no longer dominant.

(ii) The Miocene Period. Beds of Miocene age are wanting in Britain, and on the Continent they occur in isolated basins deposited in gulf-like prolongations of the ocean, never very far from land. A description of the strata and their fossil contents would be of little use for our present purposes, and the remarks made concerning the Oligocene beds will apply to the Miocene strata also.

The period was mainly remarkable on account of the important physical changes which occurred, to which we must devote some consideration. Commencing with the British area, we find in the south evidence of the separation of the London and Hampshire Basins at this time, for the Oligocene beds of Hampshire are tilted up on the south side of an anticline, which separates the Hampshire Basin from that of London, indicating that the movement was post-Miocene, while in Kent, beds of Pliocene age rest on the denuded top of the chalk, showing that the elevation and denudation which accompanied it were pre-Pliocene; the great Wealden anticline is thus seen to be of Miocene age. On the north side of the London Basin the line of demarcation between Eocene and Mesozoic beds runs approximately parallel to the strike of the latter in that part of Britain, and this points to the elevation of the Mesozoic strata which gave them their present south-easterly dip about the same period, though in the absence of Oligocene rocks it cannot be definitely stated that the movement was altogether post-Oligocene. The present physical geography of considerable parts of Britain must date from Miocene times.

Important as the changes were in Britain, they were slight as compared with those which affected Europe and many parts of Asia. The great mountain chains of the Old World received their maximum uplift during this great period of earth-movement, and orogenic structures were impressed upon the rocks of many regions, for the Tertiary Mountain Chains of the Old World have an Alpine structure impressed upon them as the result of intense lateral pressure, accordingly we find the Eocene strata lifted far above their original level to heights of 8,000 feet in the Alps and over 12,000 feet in the Himalayas. Away from these marked uplifts epeirogenic movements caused the disappearance of the seas of earlier Eocene times, so that towards the close of the Miocene Period, the main features of the Eurasian continent were much as they are now. The present drainage-systems must have originated at the same time, and the sculpture of our continent has been carried on more or less continuously by subaerial agents from Miocene times to the present day. That any addition to the total area of land was made is doubtful. The land which appears to have existed to the west of Britain during Cretaceous and Eocene times finally disappeared beneath the waters of the Atlantic Ocean, and the movement probably gave rise to the prominent submarine feature which now exists at some distance from the coast of Ireland. A great marine period is now existent in our ocean areas, but so far as the existing continents are concerned, we are living on the fourth continental period which practically came into existence in Miocene times.

The strike of the uplifted strata naturally coincides on the whole with the axes of the major uplifts, and accordingly we find the Mesozoic and early Tertiary strata folded around axes which have a prevalent east and west direction, with others which have a trend at right angles to this. The strike of the British Mesozoic rocks seems to have been determined by each of these sets of movements, so that although it is east and west in the south of England, it runs north and south in the eastern counties north of the Thames.

In America, although epeirogenic movements had occurred before Miocene times, with the formation of wide continental tracts, these appear to have been of the nature of plains, diversified by extensive inland sheets of water, and uplift of orogenic character converted these plains into uneven tracts in Miocene times. Many of the movements in America, which like those of Europe are still progressing with enfeebled power, differ from those of Eurasia, giving rise to raised monoclinal blocks rather than to violent folds of Alpine character, as seen in the western territories of North America, and as proved also by the differential movements which are now known to affect the Atlantic coast of that continent.

Accompanying these changes in the earth's crust were others which affected the climate, at any rate locally. The warm climate of Eocene times gradually gave way to a cooler climate in Oligocene times, and this lowering of temperature was still further advanced in Miocene times, though there is evidence that the temperature of those parts of Europe which have strata representative of the Miocene period was higher than it is at the present day.

Owing to the changes which occurred in Miocene times, the area of sedimentation was extensively shifted to our present oceans, and accordingly we find that the times subsequent to those of the Miocene uplifts are marked by scattered accumulations of continental character, with a few insignificant marine strata seldom found far inland from the present coast-lines.

CHAPTER XXVI.

THE PLIOCENE BEDS.

Classification. The Italian Pliocene Beds which have long been known have been divided into three stages, to which names have been applied which are somewhat widely used, though the division of the British deposits into the same three stages has not been made. The stages are:—

Astian.
Plaisancean.
Zanclean.

The classification of the British deposits may be made as follows:—

Cromer "Forest" Series.
Weybourne Crag and Bure Valley Beds.
Chillesford Crag.
Norwich Crag and Red Crag.
Upper Coralline Crag.
Lower Coralline Crag.

As the English deposits are somewhat scattered it is difficult to make out the exact order of succession, but the above shows the classification which is adopted by the best authorities, the Norwich Crag (or Fluvio-marine Crag as it is sometimes termed) being now supposed to represent the upper portion of the Red Crag.

Description of the strata. The British deposits are chiefly found in the counties of Norfolk and Suffolk, but isolated patches have been detected in Kent and at St Erth in Cornwall; while the inclusion of Pliocene fossils in the glacial deposits of Aberdeenshire and on the west coasts and islands of Great Britain suggests the occurrence of Pliocene beds beneath sea-level, around the British coasts, at no great distance from the land.

The term 'Crag' has been applied to shelly sands of which the British Pliocene beds are largely composed. The oldest British Pliocene strata are supposed to be the Lenham Beds, occurring in 'pipes' on the Chalk of the North Downs, which are referred to the Lower Coralline Crag, and some writers believe that the St Erth beds of Cornwall are of similar age[106]. The former are ferruginous sands, and the latter shelly sands and clays. The higher beds of the Coralline Crag are found in Suffolk, and are largely calcareous, being made of remains of polyzoa, molluscs, and other invertebrates. They were probably deposited in deeper water than the rest of the British Pliocene strata, and contain a far larger percentage of carbonate of lime. The Red Crag consists of ferruginous shelly sands, of the nature of sand-banks, formed near land; while the Norwich Crag is of a still more littoral character, and contains remains of land shells and the bones of mammalia mingled with the marine shells of the coast. The higher Pliocene deposits are also coastal accumulations, even the so-called Forest bed being a deposit and not a true surface soil, as proved by the observations of Mr Clement Reid. At the summit of the Cromer 'Forest' Series, however, is a true freshwater bed. These British deposits appear to have been laid down on a coast line which formed one side of the estuary of a large river, of which the present Rhine is the 'betrunked' portion (to use a term introduced by Prof. W. M. Davis)[107].