DIFFERENT AGES OF THE AQUEOUS ROCKS. 29 



thickness, which are wholly destitute of any traces of life. 

 Many fossils, again, range vertically through many groups 

 of strata, and in some cases even through several formations. 

 Such, fossils, therefore, if occurring by themselves, or con- 

 sidered apart from other associated organisms, are not con- 

 clusive as to the age of any particular set of beds. As the 

 result, however, of combined palseontological and geological 

 researches, it is now possible for us to divide the entire 

 series of stratified deposits into a number of definite rock- 

 groups or formations, each of which is characterised by 

 possessing an assemblage of organic remains which do not 

 occur in association in any other formation. Such an 

 assemblage of fossils, characteristic of any given formation, 

 represents the life of the particular period in which the 

 formation was deposited. It follows from this, that when- 

 ever we can get a group or collection of fossils from any 

 particular bed or set of beds, there is rarely any difficulty 

 in determining the precise geological horizon of the beds 

 in which the fossils occur. 



With certain limitations, however, we may go much further 

 than this. Not only are the great formations characterised 

 by special and characteristic assemblages of animals and 

 plants ; but, in a general way, each subdivision of each for- 

 mation has its own peculiar fossils, by which it may be re- 

 cognised by a skilled worker in palaeontology. Whenever, 

 for instance, we meet in Britain with the fossils known as 

 Graptolites, we may be sure that we are dealing with Silu- 

 rian Eocks. We may, however, go much further than this. 

 If the Graptolites belong to certain genera, we may be sure 

 that we are dealing with Lower Silurian Eocks. Further- 

 more, if certain special forms are present, we may be even 

 able to say to what exact part or subdivision of the Lower 

 Silurian series they belong. 



All these conclusions, however, would have to be accom- 

 panied by a tacit but well-understood reservation. No Grap- 

 tolites have ever been found in Britain out of rocks known 

 upon other grounds to be Silurian ; but there is no reason 

 why they might not at any time be found in younger de- 

 posits. In the same way, the species and genera which we 



