52 INTRODUCTION. 



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 Creta- 

 ceous forms still linger in the deep Atlantic. Period in 

 Geology must not be confounded with ' system ' or ' forma- 

 tion/ The one is only relative, the other definite. A for- 

 mation 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 par- 

 ticular 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 arid littoral faunas are 

 necessarily more liable to change than a deep-sea fauna, be- 

 cause 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 eleva- 

 tion of entire ocean-beds at the same time, the maritime 

 fauna of one period must be in part almost necessarily trans- 

 mitted 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 Cretaceous period," in any other sense 

 than one might say that we are living in the Silurian period, 

 with this difference, that the Cretaceous period is much nearer 



