52 PRINCIPLES OF PALAEONTOLOGY. 



has not been continuity in any given area, still the geological 

 chain could never have been snapped at one point, and taken 

 up again at a totally different one. Thus we arrive at the 

 conviction that coiitinuity is the fundamental law of geology, 

 as it is of the other sciences, and that the lines of demarca- 

 tion between the great formations are but gaps in our own 

 knowledge. 



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 may be 

 divided into a number of definite groups or " formations," 

 each of which is characterised by its organic remains. It may 

 simply be repeated 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 tolerable certainty. It very often happens, 

 indeed, that some particular stratum, or sub-group of a series, 

 contains peculiar fossils, by which its existence may be deter- 

 mined in various localities. As before remarked, however, the 

 great formations are characterised 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 fossil- 

 iferous 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, minute crustaceans, or fish, such as now inhabit lakes, 



