the sub-Apennine beds, have been uplifted into a range of low hills. These deposits swell out southwards, reaching their greatest thickness (2000 feet or more) in Sicily, which was probably the region of maximum subsidence during Pliocene time. Here and there, in the Italian strata of this period, remains of terrestrial vegetation and land-animals are abundantly preserved. One of the most noted localities for these fossils is the upper part of the valley of the Arno.

Perhaps the most curious and interesting assemblage of the land-fauna of Europe during Pliocene time has been found in some hard red clays, alternating with gravels at Pikermi in Attica. Thirty-one genera of mammals have there been obtained, of which twenty-two are extinct. The ruminants, specially well represented among these remains, include species of giraffe, helladotherium (Fig. 203), antelopes,

FIG. 203.-Helladotherium Duvernoyi (g)-a gigantic animal intermediate in structure between the giraffe and the antelope, Pikermi, Attica.

gazelles, and other forms allied to, but distinct from, any living genera. There are, likewise, the bones of gigantic wild boar, several species of rhinoceros, mastodon, deinotherium, porcupine, hyæna, various extinct carnivores, and a monkey.

In India, a somewhat similar fauna has been obtained from a massive series of fresh-water sandstones, known as the Siwalik group. A large proportion of the remains belong to existing genera of animals, such as macaque, bear, elephant, horse, hippopotamus, giraffe, ox, porcupine, goat, sheep, and camel. Various extinct types were contemporary with these animals, two of the most extraordinary of them being the Sivatherium and Bramatherium-colossal, fourhorned creatures allied to our living antelopes and prongbucks.

CHAPTER XXVI.

POST-TERTIARY OR QUATERNARY PERIODS-PLEISTOCENE OR POST-PLIOCENE-RECENT.

We have now arrived at the last main division of the Geological Record, that which is named Post-tertiary or Quaternary, and which includes all the formations accumulated from the close of the Tertiary periods down to the present day. But no sharp line can be drawn at the top of the Tertiary groups of strata. On the contrary, it is often difficult, or indeed impossible, satisfactorily to decide whether a particular deposit should be classed among the younger Tertiary or among the Post-tertiary groups. In the latter, all the molluscs are believed to belong to still living species, and the mammals, although also mostly still of existing species, include some which have become extinct. These extinct forms are numerous in proportion to the antiquity of the deposits in which they have been preserved. Accordingly, a classification of the Quaternary strata has been adopted, in which the older beds, containing a good many extinct mammals, have been formed into what is termed the Pleistocene, Post-pliocene, or Glacial group, while the younger beds, containing few or no extinct mammals, are termed Recent.

The gradual refrigeration of climate which is revealed to us by the shells of the crag was prolonged and intensified in Post-tertiary time. Ultimately the northern part of the northern hemisphere was covered with snow and ice, which extended into the heart of Europe and descended far southward in North America. The previous denizens of land and sea were in large measure driven out or even in many cases wholly extirpated by the cold, while northern forms advanced southward to take their places. The reindeer, for instance, roamed in great numbers across Southern France, and Arctic vegetation spread all over Northern and Central Europe, even as far as the Pyrenees. After the cold had reached its climax, the ice-fields began to retreat, and the northern flora and fauna to retire before the advance of the plants and animals which had been banished by the increasingly severe temperature. And at last the present conditions of climate were reached. The story of this Ice Age is told by the Pleistocene or Post-pliocene formations, while that of the changes which immediately led to the establishment of the present order of things is made known in the Recent deposits.

PLEISTOCENE, POST-PLIOCENE, or Glacial.

The evidence from which geologists have unravelled the history of the Ice Age or cold episode which came after the Tertiary periods in the northern hemisphere may here be briefly given. All over Northern Europe and the northern part of North America the solid rocks, where of hardness sufficient to retain it, are found to present a characteristic smoothed, polished, and striated surface. Even on crags and rocky bosses that have remained for long periods exposed to the action of the weather, this peculiar worn surface may be traced; but where they have been protected by a covering of clay, these markings are often as fresh as when they were first made. The groovings and fine striæ do not occur at random, but in every district run in one or more determinate directions. The faces of rock that look one way are rounded off, smoothed, and polished; those that face to the opposite quarter are more or less rough and angular. The quarter to which the worn faces are directed corresponds with that to which the striæ and grooves on the rock-surfaces point. There can be no doubt that all this smoothing, polishing, grooving, and striation has been done by land-ice; that the trend of the striæ marks the direction in which the ice moved, those faces of rock which looked towards the ice being ground away, while those that looked away from it escaped. By following out the directions of the rock-striæ we can still trace the march of the ice across the land (see chapter vi.)

more or less Some of this

As the ice travelled, it carried with it detritus, as a glacier does at the present day. material may have lain on the surface, but probably most of it was pushed along at the bottom of the ice. Accordingly, above the ice-worn surfaces of rock, there lies a great deposit of clay and boulders, evidently the debris that accumulated under the ice-sheet and was left on the surface of the ground when the ice retired. This deposit, called boulder-clay or till, bears distinct corroborative testimony to the movement of the ice. It is always more or less local in origin, but contains a variable proportion of stones which have travelled for a greater or less distance, sometimes for several hundred miles. When these stones are traced to their places of origin, which are often not hard to seek, they are found to have come from the same quarter indicated