20 PALEONTOLOGY 



A number of consecutive hemeva (varying in different 

 cases from three to ten) are grouped as an age, correspond- 

 ing to the development of a genus or a family : the 

 corresponding thickness of strata is called a stage. Ages 

 are again grouped into epochs, these into periods, and these 

 into eras, the bounds of these being determined by great 

 and rapid changes of fauna. These terms and their 

 equivalents may be tabulated thus : 



TiME-DlVISIONS. ROCK-DIVISIONS. 



Era. Group. 



Period. System. 



Epoch. Series. 



Age. Stage. 



Hernera. Zone. 



Thus the Cornbrash is, approximately, the zone of 

 Terebratula intermedia. Each of the brachiopods shown in 

 Fig. 5 also gives its name to a zone, and all these zones 

 with some others make up the Bathonian stage, which 

 takes its name from Bath, around which city these zones 

 are well displayed. This stage is part of the Upper 

 Jurassic Series of the Jurassic System, so named from 

 the Jura Mountains. This in turn is part of the Mesozoic 

 Group.* 



The sequence of forms in division after division of 

 geological time often appears casual and meaningless, 

 but that is probably only owing to the imperfection of 

 our knowledge. In a number of cases we can trace a 



* See Appendix I. for tabulated statements of time-divisions. 

 Reference to these tables will frequently be necessary during the 

 reading of what follows. 



