CLASSIFICATION OF STRATIFIED ROCKS. 199 



(b.) We must also make due allowance for geographical diversity. 

 This, as we have already stated, becomes greater and greater as we 

 pass up the series of rocks. In the lower or older rocks the geographi- 

 cal diversity is small ; in strata of the same age in different countries 

 the fossils are quite similar, most of the genera and many of the species 

 being undistinguishable. It is therefore comparatively easy, by com- 

 parison of fossils, to synchronize the strata and determine the geologi- 

 cal horizon. In the middle rocks the geographical diversity is greater, 

 but the general similarity is still considerable — the difference between 

 organisms of consecutive epochs (geological faunas and floras) is still 

 much greater than the difference between organisms of the same epoch 

 in different countries (geographical faunas and floras) ; and, therefore, 

 it is still quite possible, by comparison of fossils, to synchronize the 

 strata. In the higher or newer rocks the geographical diversity has 

 become so great that we are compelled to determine age and synchronize 

 strata, no longer entirely by comparison of fossils of the different localities 

 with each other, but also by the comparison of the fossils of each local- 

 ity with the living species in the same locality. In these rocks we may 

 also determine relative age by relative percentage of living species, and 

 similarity of age (geological horizon) by similarity of this percentage. 



Manner of constructing a Geological Chronology. — The manner in 

 which a geological chronology has actually grown up, under the com- 

 bined labors of the geologists of all countries, may be briefly stated as 

 follows : First, the order of superposition, and therefore the relative 

 ages of the strata composing the rock-series of many different countries, 

 were determined independently ; next, by comparison of these, partly 

 by lithological character, if the localities are contiguous, and partly by 

 fossils, the geologist determines those which are synchronous and those 

 which are wanting 



in each locality. A . -= =s^^ == ^=^^= -^ =J== ^-J- 



Thus, out of several 

 local series, by in- ' 

 tercalation, he con- g 

 structs a more com- 

 plete ideal series. 2 

 In case of doubt, he 1 

 strives to find places """ "" 



wlioro flio rlrm"hffnl Fig. 179.— Diagram illustrating the Mode of determining the Chrono- 

 Wliere Xlie UOUOIIU1 logical Order of Strata. 



strata come togeth- 

 er, and observes their relative position. In Fig. 179, A and ^rep- 

 resent two contiguous localities in which by independent study the rela- 

 tive positions and ages of 6 and 7 strata respectively have been deter- 

 mined. By comparison, the rocks of the two series are found to con- 

 sist of eleven strata of different ages, some being wanting in the one 



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