USES OF FOSSILS IN GEOLOGY. 9 



enable us to identify the division. In a similar manner, 

 the smaller divisions of the system, the series and stages 

 (or beds), are each characterised by possessing certain 

 fossils which do not occur above or below. Further, it is 

 found that the fauna of the smallest division (stage or 

 beds) is not of uniform character throughout its thickness; 

 although there may be no change in its lithological nature, 

 some of the species which are abundant at one level will 

 become rare or altogether extinct in passing to higher or 

 lower horizons. Consequently, a set of beds may be 

 divided into belts or zones, the general aspect of the 

 fauna of each zone being somewhat different from that of 

 the others, but between these divisions there will be no 

 break either physical or palseontological. If then we 

 have determined the order of succession of the formations 

 in any one area by means of their relative positions, the 

 newer resting on the older, it is easy in any other district, 

 merely by examining the fossils, to refer any set of beds 

 to their proper position in the geological record. But 

 valuable as this law of the identification of strata by their 

 organic remains is, it must not be used without some 

 caution. For although two formations may have been 

 deposited at exactly the same time, it does not necessarily 

 follow that all the genera and species occurring in the 

 two will be identical. Thus for instance in the seas at 

 the present day the same forms of life do not occur in all 

 parts. There is a distribution in provinces, depending 

 largely on climatic conditions, each province possessing 

 some forms peculiar to itself. So that the organisms 

 now being entombed in deposits formed, say, off the 

 British coasts, will as a whole be different from those off 

 the Australian coasts, but still some of the species and 

 many of the genera will be common to both areas, and 



