38 PRINCIPLES OF PALEONTOLOGY. 



are confined to particular rocks ^ and particular groips of fossils 

 are confined to particular groups of rocks. Fossils, then, are 

 distinctive of the rocks in which they are found — much more 

 distinctive, in fact, than the mere mineral character of the rock 

 can be, for that commonly changes as a formation is traced 

 from one region to another, whilst .the fossils remain unaltered. 

 It would therefore be quite possible for the palaeontologist, 

 by an appeal to the fossils alone, to arrange the series of sedi- 

 mentary deposits into a pile of strata having a certain definite 

 order. Not only would this be possible, but it would be found 

 — if sufficient knowledge had been brought to bear on both 

 sides — that the palaeontological arrangement of the strata would 

 coincide in its details with the stratigraphical or physical 

 arrangement. 



Happily for science, there is no such division between the 

 palaeontologist and the physical geologist as here supposed; 

 but by the combined researches of the two, it has been found 

 possible to divide the entire series of stratified deposits into a 

 number of definite rock-gi'oups ox formations, which have a 

 recognised order of succession, and each of which is charac- 

 terised 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, re- 

 presents the life of the psnticulsiT period in which the formation 

 was deposited. In this way the past history of the earth 

 becomes divided into a series of successive life-periods, each of 

 which corresponds with the deposition of a particular forma- 

 tion or group of strata. 



Whilst particular assemblages of organic forms characterise 

 particular groups of rocks, it may be further said that, 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 Palaeontolog}^ Whenever, for instance, we meet 

 with examples of the fossils which are known as Graptolites, we 

 may be sure that we are dealing with Silurian rocks (leaving 

 out of sight one or two forms doubtfully referred to this family). 

 We may, however, go much farther than this with perfect 

 safety. If the Graptolites belong to certain genera, we may 

 be quite certain that we are dealing with Lower Silurian rocks. 

 Furthermore, if certain special forms are present, we may be 

 even able to say to what exact subdivision of the Lower Silu- 

 rian series they belong. 



As regards particular fossils, however, or even particular 

 classes of fossils, conclusions of this nature require to be accom- 

 panied by a tacit but well-understood reservation. So far as 



