formation is deposited or takes place during a certain time; and that time is the period of the formation ; but a geological period may include several formations, and is defined by the preponderance of certain orders, families, or genera, according to the extent of the period spoken of ; and the passage of some of the forms into the next geological series does not carry the period with them, any more than would any particular historical epoch be delayed until the survivors of the preceding one had died out. Period is an arbitrary time-division. The Chalk or the " London Clay " formations mark definite stratigraphical divisions. We may speak of the period of the London Clay, or we may speak of the Tertiary period. It merely refers to the " time when " either were in course of construction. The occurrence of Triassic forms in the Jurassic series, of Oolitic forms in the Cretaceous series, and of Cretaceous forms in the Eocene, in no way lessens the independence of each series, although it may sometimes render it difficult to say where one series ceases and the other commences. The land and littoral faunas are necessarily more liable to change than a deep-sea fauna, because an island or part of a continent may be submerged and all on it destroyed, while the fauna of the adjacent oceans would survive ; and as we cannot suppose the elevation of entire ocean-beds at the same time, the marine fauna of one period must be in part almost necessarily transmitted to the next.
Thus while continental Europe and the sea-bed, as far as from 200 to 300 miles west of the British Islands, was subject to successive changes of level, giving rise to a series of Eocene, Miocene, and Pliocene strata with their diversified and varying faunas, the adjacent depths of the Atlantic may have continued with little variation, except that produced by currents and relatively small differences of depth. Of the nature of that deep-sea fauna we were until lately entirely ignorant. At the same time it may be observed that geologists held to the opinion of deep-sea deposits ; and the views of E. Forbes, with regard to the bathymetrical limits of life in the sea, were by no means generally accepted. The Chalk, attaining as it does a thickness of 1000 to 1500 feet, and having a special fauna, was always looked upon by geologists as the deposit of a very deep sea. Even supposing the conclusions of E. Forbes to have been accepted, no geologist could have safely inferred, from a rock being non-fossiliferous, that it had been deposited in a sea the depth of which exceeded the limits he assigned to marine life. In the first place, the sediment of which the rocks are formed may have been
