THE OLIGOCENE AND MIOCENE PERIODS.
(i) The Oligocene Beds.
Classification. The Oligocene Beds of Britain are classified as follows:—
| Upper | Wanting | |
| Middle | Hempstead Beds | |
| Lower | Bembridge Beds Osborne Beds Headon Beds |
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.