IXX PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



formation is deposited or takes place during a certain time; and 

 that time is the period of ilie 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 arbi- 

 trary 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 con- 

 struction. 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 sometiraes 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 de- 

 stroyed, 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 neces- 

 sarily 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 adja- 

 cent depths of the Atlantic may have continued with little variation, 

 except that produced by currents and relatively small difierences 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. Eorbes, 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 



