280 CONTEMPORANEITY OF STRATA AND THE 



that "some considerable portion of the deep sea bed of the mid- 

 Atlantic has contiaued submerged since the period of our chalk, and 

 although the more adaptable forms of life may have been transmitted 

 in unbroken succession through this channel, the immigration of 

 other and more recent faunas may have so modified the old popula- 

 tion, that the original chalk element is of no more importance than 

 is the original British element in our own English people. As well 

 might it have been said in the last century that we were living in 

 the period of the early Britons, because their descendants and 

 language still lingered in Cornwall, as, that we are living in the 

 Cretaceous period, because a few Cretaceous forms still linger in the 

 deep Atlantic. Period in geology must not be confounded with 

 'system' or 'formation.' The one is only relative, the other definite. 

 A 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 on 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 the 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 ocean would survive ; and as we cannot sup- 

 pose the elevation of entire ocean beds at the same time, the 

 maritime fauna of one period must be in part almost necessarily 

 transmitted to the next." 



In accordance, therefore, with the principles here laid down, we 

 may conclude that it is not correct to say that we are " living in the 



