40 INTRODUCTION. 



CHAPTER V. 

 CONCLUSIONS TO BE DRAWN FROM FOSSILS. 



WE have already seen that geologists have been led by the 

 study of fossils to the all-important generalisation that the vast 

 series of the Fossiliferous or Sedimentary Rocks maybe divided 

 into a number of definite groups or " formations," each of which 

 is characterised by its organic remains. It may simply be re- 

 peated here that these formations are not properly and strictly 

 characterised by the occurrence in them of any one particular 

 fossil. It may be that a formation contains some particular 

 fossil, or fossils, not occurring out of that formation, and that 

 in this way an observer may identify a given group with toler- 

 able certainty. It very often happens, indeed, that some parti- 

 cular stratum, or sub-group of a series, contains peculiar fossils, 

 by which its existence may be determined in various localities. 

 As before remarked, however, the great formations are charac- 

 terised properly by the association of certain fossils, by the 

 predominance of certain families or orders, or by an assemblage 

 of fossil remains representing the " life" of the period in which 

 the formation was deposited. 



Fossils, then, enable us to determine the age of the deposits 

 in which they occur. Fossils further enable us to come to very 

 important conclusions as to the mode in which the fossiliferous 

 bed was deposited, and thus as to the condition of the parti- 

 cular district or region occupied by the fossiliferous bed at the 

 time of the formation of the latter. If, in the first place, the 

 bed contain the remains of animals such as now inhabit rivers, 



/. we know that it is " fluviatile " in its origin, and that it must at 

 one time have either formed an actual river-bed, or been de- 

 posited by the overflowing of an ancient stream. Secondly, if 

 the bed contain the remains of shell-fish, minute crustaceans, 

 or fish, such as now inhabit lakes, we know that it is " lacus- 

 trine," and was deposited beneath the waters of a former lake. 

 Thirdly, if the bed contain the remains of animals such as now 



j people the ocean, we know that it is "marine" in its origin, 

 and that it is a fragment of an old sea-bottom. 



We can, however, often determine the conditions under 

 which a bed was deposited with greater accuracy than this. 

 If, for example, the fossils are of kinds resembling the marine 

 animals now inhabiting shallow waters, if they are accompanied 

 by the detached relics of terrestrial organisms, or if they are 

 partially rolled and broken, we may conclude that the fossili- 



