40 PRINCIPLES OF PALEONTOLOGY. 



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 characterize 

 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 recognized by a skilled 

 worker in Palaeontology. 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 further 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 Silurian 

 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 

 our present observation goes, none of the undoubted Grapto- 

 lites have ever been discovered in rocks later than those known 

 upon other grounds to be Silurian; but it is possible that they 

 might at any time be detected in younger deposits. Similarly, 

 the species and genera which we now regard as characteristic 

 of the Lower Silurian, may at some future time be found to 

 have survived into the Upper Silurian period. We should not 

 forget, therefore, in determining the age of strata by palseonto- 

 logical evidence, that we are always reasoning upon generaliza- 

 tions which are the result of experience alone, and which are 

 liable to be vitiated by further and additional discoveries. 



When the palseontological evidence as to the age of any 

 given set of strata is corroborated by the physical evidence, our 

 conclusions may be regarded as almost certain; but there are 

 certain limitations and fallacies in the palaeontological method 

 of inquiry which deserve a passing mention. In the first 

 place, fossils are not always present in the stratified rocks ; 

 many aqueous rocks are unfossiliferous, through a thickness of 

 hundreds or even thousands of feet of little-altered sediments; 

 and even amongst beds which do contain fossils, we often meet 

 with strata of many feet or yards in thickness which are wholly 

 destitute of any traces of fossils. There are, therefore, to 



