ciur vii THE NATURE AND USE OF FOSSILS 91 



of ground. He follows this stratum, using it as a datum 

 which to work out the arrangement of the 

 series both above and below it. And in regions where 

 the strata have been so greatly disturbed and dislocated 

 that their structure could not otherwise be disentangled, 

 a clue U often supplied by the occurrence of such fossil- 

 iferous cones. This use of fossils will be more evident 

 when we come to deal in a later chapter with the tracing 

 of geological boundaries, and the working out of geo- 

 logical structure. 



5. Gtolwtal Cknmology. To fix the relative geolo- 

 gical position of rocks, and thus to establish a succession 

 ot chronology, is doubtless the most important s< 

 which fossils render to geology. Mere resemblances or 

 differences in the mineral character of strata are seldom 

 good for great distances. We cannot always be sure, 

 simply on the ground of general petrographkal resem- 

 blance, that a group of strata on one side of a country is 

 identical with a similar set on the opposite side. If they 

 closely resembled each other in that respect, but con- 

 tained totally distinct fossils, we should generally conclude, 

 ::e of their outward similarity, that they could not be 

 identified with each other, but must belong to different 

 periods of geological time. 



Each great stratified formation of the earth's crust 

 is distinguished by its own characteristic fossils. A 

 method is thus obtainable of recognising the relative 

 geological date of fossiliferous rocks. To determine and 

 name fossils is the task of the paleontologist. As a rule 

 the field-geologist can do this only to a limited extent, 

 though the greater his power in this respect the more 



