silting up of the estuary and an increase in the area of the delta, but also of a gradual lowering of the temperature; till at length, in the time of the Hempstead Beds, the vegetation ceased to have any specially tropical characters, and the former estuary was converted into a swampy tract covered with a growth of the rushes and water-plants that belong to temperate climates.'

With respect to the highest member of the Hempstead group, Mr. Gardner remarks that the assemblage of marine mollusca which it contains presents such a paucity of species, and these so stunted in growth, as to suggest that the sea which they inhabited had contracted to the dimensions of a mere salt or brackish-water lake, without any communication with the open sea then lying over Belgium; at the same time, it is clear that at this period the depth of the water in the Isle of Wight area was increased, not diminished, and this must have been due either to subsidence or to an influx of river-water into the lake.

When we consider that the whole of the British Oligocene series consists of shallow-water deposits, it is evident that a certain amount of subsidence must have taken place during the period in order to allow of the accumulation of such a thickness of beds (600 feet). That such subsidence took place is proved also by the Belgian succession, and by the comparatively deep-water character of the Argile de Boom, which is generally regarded as the equivalent of our Hempstead Beds. It is probable, however, that this subsidence was more or less localized to the basins of deposition, and it may have been contemporaneous with and complementary to a rise of the Wealden axis.

The records of the Hampshire delta are here abruptly broken off, and we have no means of ascertaining the last phases of its history; we can only guess from the analogy

1 "Proc. Geol. Assoc.," vol. vi. p. 98.

of the French deposits that there was a gradual upheaval of the whole area, and that the depressions of this terrestrial surface were occupied by large lakes. Whether any such lakes existed on British ground we do not know, but many were scattered through the centre of Europe, in France, Switzerland, Germany, and Austria.

THE

HE reasons for grouping the Miocene, Pliocene, and Pleistocene deposits into one system have been given elsewhere. Their geographical extension is entirely different from that of the older (Hantonian) Tertiaries; the later Miocene and the older Pliocene deposits of the continent are so intimately connected that it is often difficult to separate one from the other, while the close relations of the Pliocene and Pleistocene are universally admitted. As, however, the treatment of the Pleistocene epoch involves a consideration of the Ice Age and the many debatable questions connected with the Glacial deposits, it will be more convenient to consider the geographical changes which took place during the Miocene and Pliocene epochs first, and to discuss the complicated phenomena of the Pleistocene in a separate chapter.

§ 1. Stratigraphical Evidence.

Miocene Epoch.-Throughout both the Oligocene and Miocene epochs the greater part of Britain remained in the condition of dry land, and no actual Miocene deposits have been found either in Britain or in the north-eastern part of France. They occur in southern and western France, and patches of them exist as far north as the Cotentin in Normandy. Lately, also, it has been decided that certain

"Historical Geology," by the author, pp. 36 and 486.