sheets of basalt forming the terraced hills of the north of Ireland and Western Islands of Scotland were thrown out.

An epoch of frequent change in the relative positions of sea and land is one in which there may be exceptional facilities for the preservation of a record of the plants and animals of the time. Oligocene strata have accordingly a peculiar interest from the abundant remains they contain of the contemporaneous terrestrial plants and animals. The land flora of that period is probably better known than that of any other section of the Geological Record, chiefly from the extraordinary abundance of its remains which have been preserved in the sediments of the ancient Swiss lake. Judging of it from these remains, we learn that it was in great measure made up of evergreens, and in various ways resembled the existing vegetation of tropical India and Australia and that of sub-tropical America. Its fan-palms, feather-palms, conifers, evergreen oaks, laurels, and other evergreen trees, gave a peculiarly verdant umbrageous character to the landscape in all seasons of the year, while numerous proteaceous shrubs glowed with their bright blooms on the lower grounds.

Of the terrestrial fauna numerous remains have been found in the lacustrine deposits of the time. We know that the borders of

FIG. 197.-Oligocene Molluscs. (a) Ostrea ventilabrum (±); (b) Corbula subpisum (f); (c) Paludina lenta (natural size).

the lakes in Central France were frequented by many different kinds of birds-paroquets, trogons, flamingoes, ibises, pelicans, maraboots, cranes, secretary birds, eagles, grouse, and other forms. This association of birds recalls that around the lakes of Southern Africa at the present time. The mammals appeared in still more numerous and abundant types. Pachyderms abounded, including the Anoplotherium-a slender, long-tailed animal, about the size of an ass, with two toes on each foot; certain transitional types of ungulates, with affinities to the pigs, peccaries, and chevrotains (Anthracotherium, Charopotamus, Hyopotamus, etc.); various forms of the tapir family, and of dogs, civets, martens, marmots, bats, moles, and shrews. The carnivora still presented marsupial characters, and in not a few of the animal types features of structure were combined which are now only found in distinct genera. The Eocene palæotheres and the Oligocene anoplotheres appear to have died out before the end of the Oligocene period. The fresh water teemed with molluscs, belonging chiefly to genera that still live in our rivers and lakes, such as Unio, Cyrena, Paludina, Planorbis, Limnæa, Helix, and others (Fig. 197).

In the Isle of Wight the highest Eocene strata were followed by a group of fresh-water, estuarine, and marine deposits, formerly classed as Upper Eocene, but placed here in the Oligocene division. They are arranged in the following manner in descending

order :

Hamstead series-clays, marls, and shelly layers, with fresh-water and estuarine shells and land plants. About 260 feet. Bembridge series-marls and limestone, with fresh-water shells below, and estuarine shells above. About 110 feet.

Osborne series--clays, marls, sands, and limestones, with abundant freshwater shells. About 100 feet.

Headon series-consisting of an upper and lower division, containing fresh and brackish water fossils and a middle group in which marine shells and corals occur. About 150 feet.

These Isle of Wight strata, having a total depth of more than 600 feet, were for many years the only known examples in Britain referable to this portion of the Geological Record, and they are still the only beds in this country which in their abundant molluscs allow a comparison to be made between them and corresponding rocks on the Continent. But at Bovey Tracey in Devonshire a small lake-basin has been discovered, the deposits of which have yielded a large number of terrestrial plants comparable with those found in the Oligocene strata of Switzerland and Germany. Between the great sheets of basalt, also, that form the plateaux of Antrim and the Inner Hebrides, numerous remains of a similar vegetation have been discovered. There can be no doubt that these volcanic rocks were poured out over the surface of the land, and that the plants, whose remains have been disinterred from the intercalated layers of tuff and hardened clay, grew upon that land. The basalts and other lavas, even after

Z

the great denudation which they have undergone, are still in some places more than 3000 feet thick. They were poured out in widespreading sheets that completely buried the previous topography and extended as vast lava-plains, like those of younger date, which form so impressive a feature in the scenery of Montana, Idaho, and Oregon, in Western North America.

In the Paris basin, the Oligocene strata follow immediately upon the Eocene group described on p. 334. They consist of (1) a lower division of gypsum (65 feet) and marls, with terrestrial shells, and remains of palæotheres and anoplotheres; (2) a middle band of marl, limestone, and sand, with lacustrine and estuarine shells; and (3) an upper division, in which the most conspicuous members are the sands and hard siliceous sandstone of Fontainebleau.

In Northern Germany the subjoined succession of strata in descending order has been noted.

Marine marls, clays, and sands.

Brown coal of the Lower Rhine, with abundant terrestrial vegetation and some marine bands.

Sands and Septaria-clay, with abundant marine fauna; occasionally a brown-coal group occurs.

Upper

Middle

Marine beds of Egeln, with marine shells and corals.

Lower

Amber beds of Königsberg, containing 4 or 5 feet of glauconitic sand, with abundant pieces of amber, which is the fossil resin of different species of coniferous trees. A large number of species of insects has been enclosed and preserved in the amber.

Lower brown coal-sands, sandstones, clays, and conglomer ates, with interstratified seams of brown coal and an abundant terrestrial flora, in which coniferæ are prominent.

CHAPTER XXVI

MIOCENE-PLIOCENE

THE geological period at which we are now arrived, one of the most important in the history of the configuration of the existing continents, embraced that portion of geological time during which the great mountain-chains of the globe were uplifted into their present commanding positions. There is good reason to believe that these lines of elevation are of great geological antiquity, and that they have again and again been pushed upward during great terrestrial disturbances. But the intervals between these successive upthrusts were probably often of immense duration, so that the mountains, being exposed to continuous and prolonged denudation, were worn down, sometimes perhaps almost to the very roots. In all probability the nucleus of the line of the Alps, for example, dates back to a remote geological period. But only in Tertiary time did it attain its present dimensions. We have seen that, during the Eocene period, the sea of the nummulitic limestone extended over at least a considerable part of the Alpine region, and that, as the limestone now forms crumpled and dislocated mountainous masses, the great upheaval of the chain must have taken place after Eocene time. Not improbably the process was a prolonged one, advancing in successive uplifts with intervals of rest. The final upheaval that gave the Alps their colossal bulk did not take place until the Miocene period or later, for the Miocene strata have been involved in the earth-movements, and have been thrust up, bent, and broken. Nor were the terrestrial convulsions confined to Central Europe, all over the globe there seem to have been extensive disturbances. The Eocene sea-bed with its thick accumulations of nummulite-limestone was ridged up into land, and portions of it were carried upward on the flanks of the mountains, in the Himalayas to a height of 16,500 feet above the sea.

While these revolutions were taking place in its topography, Europe continued to enjoy a climate which, to judge from the remains of plants and animals preserved in the Miocene rocks, must still have been of a somewhat tropical character. The flora that clothed the slopes of the Alps was not unlike that of the forests of India and Australia at the present time. Palms of various kinds still flourished all over Central and Western Europe, mingled with conifers, laurels, evergreen oaks, magnolias, myrtles, mimosas, acacias, sumachs, figs, oaks, and various still living

FIG. 198. Miocene Plants; (a) Magnolia Inglefieldi(); (b) Rhus Meriani (natural size); (c) Ficus decandolleana (); (d) Quercus ilicoides (3).

genera of proteaceous shrubs (Fig. 198). But there is evidence of the incoming of a more temperate climate, for, in the higher parts of the Miocene series of strata, the vegetation was characterised by the abundance of its beeches, poplars, hornbeams, elms, laurels, pondweeds, etc.

Remains of the terrestrial fauna have been well preserved in the deposits that gathered over the floors of the lakes. We know, for instance, that in the woodlands surrounding the large Miocene lake of Switzerland insect life was remarkably abundant. From the proportions of the different kinds that have been exhumed, it has been inferred that the total insect population was then more