The tertiary strata described in the preceding chapters are all of them characterized by fossil shells, of which a considerable proportion are specifically identical with the living mollusca; and the greater the number, the more nearly does the entire fauna approach in species and genera to that now inhabiting the adjoining seas. But in the Eocene formations next to be considered, the proportion of recent species is very small, and sometimes scarcely appreciable, and those agreeing with the fossil testacea often belong to remote parts of the globe, and to various zoological provinces. This difference in conchological character implies a considerable interval of time between the Eocene and Miocene periods, during which the whole fauna and flora underwent other changes as great, and often greater, than those exhibited by the mollusca. In the accompanying map, the position of several Eocene areas is pointed out, such as the basin of the Thames, part of Hampshire, part of the Netherlands, and the country round Paris. The deposits, however, occupying these spaces comprise a great succession of marine and freshwater formations, which, although they may all be termed Eocene, as being newer than the chalk, and older than the faluns, are nevertheless divisible into separate groups, of high geological importance. The newest of these, like the Faluns of the Loire, have no true representatives, or exact chronological equivalents, in the British Isles. Their place in the series will best be understood by referring to the order of superposition of the successive deposits found in the neighbourhood of Paris. The area which has been called the Paris basin is about 180 miles in its greatest length from north-east to south-west, and about 90 miles from east to west. This space may be described as a depression in the chalk, which has been filled up by alternating groups of marine and freshwater strata. MM. Cuvier and Brongniart attempted, in 1810, to distinguish five different formations, comprising three freshwater and two marine, which alternated with each other. It was imagined that the waters of the ocean had been by turns admitted and excluded from the same region; but the subsequent investigations of several geologists, especially of M. Constant Prevost,[175-A] have led to great modifications in these theoretical views; and now that the true order of succession is better understood, it appears that several of the deposits, which were supposed to have originated one after the other, were, in fact, in progress at the same time by the joint action of the sea and rivers. The whole series of strata may be divided into three groups, as expressed in the following table:-- { _a._ Upper freshwater limestone, marls, and siliceous 1. Upper Eocene { millstone. { _b._ Upper marine sands, or Fontainebleau sandstone { and sand.