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77 



CHAPTER V. 



CONCLUSIONS TO BE BRAWN 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 may be divided into a number of 

 definite groups or " systems," each of which is characterised by its 

 organic remains. It may simply be repeated here that these systems 

 are not properly and strictly characterised by the occurrence in them 

 of any one particular fossil. It very often happens, indeed, that 

 some particular stratum, or sub-group of a series, contains peculiar 

 fossils, by which its existence may be determined in various local- 

 ities. As before remarked, however, the great systems are char- 

 acterised properly by the association of certain fossils, by the pre- 

 dominance of certain families or orders, or by an assemblage of fossil 

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

 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 im- 

 portant conclusions as to the mode in which the fossiliferous bed was 

 deposited, and thus as to the condition of the particular 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 deposited by the overflowing of an ancient 

 stream. Secondly, if the bed contain the remains of shell-fish, min- 

 ute crustaceans, or fish, such as now inhabit lakes, we know that it 

 is " lacustrine," and was deposited beneath the waters of a former 

 lake. Thirdly, if the bed contain the remains of animals such as 

 now 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, 



