DIFFERENT AGES OF THE AQUEOUS ROCKS. 37 



of feet of little altered sediments ; and even amongst beds which do 

 contain fossils, we often meet with strata of a few feet or yards in 

 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, there- 

 fore, if occurring by themselves, or considered apart from other 

 associated organisms, are not conclusive as to the age of any par- 

 ticular set of beds. As the result, however, of combined palseonto- 

 logical and geological researches, it is now possible for us to divide 

 the entire series of stratified deposits in any given region 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, repre- 

 sents the life of the particular period in which the formation was 

 deposited. It follows from this, that whenever we can get a group 

 or collection of fossils from any particular bed or set of beds, there 

 is rarely any difficulty in determining, as regards the particular region 

 under examination, 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 formation has its own peculiar fossils, 

 by which it may be recognised 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 Cambrian, 

 Ordovician, or Silurian rocks. We may, however, go much further 

 than this. If the Graptolites belong to certain genera, we may be 

 sure that we are dealing with Ordovician rocks. Furthermore, if 

 certain special forms are present, we may be even able to say to 

 what exact part or subdivision of the Ordovician series they belong. 



All these conclusions, however, would have to be accompanied 

 by a tacit but well-understood reservation. No Graptolites have 

 ever been found in Britain out of rocks known upon other grounds 

 to belong to one or other of the three formations above mentioned ; 

 but there is no reason why they might not at any time be found in 

 younger deposits. In the same way, the species and genera which 

 we now regard as characteristic of the Ordovician, might at any 

 time be found to have survived into the Silurian period. We 

 should never forget, therefore, in determining the age of a rock by 

 palaeontological evidence only, that we are always reasoning upon 

 generalisations which are the result of experience alone, and which 

 may at any time be overthrown by fresh discoveries. 



There is, moreover, another important principle to take into 



