STRATIFIED ROCKS. 69 



basis of their succession in chronological sequence. Although 

 the relative position of the strata is the only infallible mark 

 of time-sequence, it is the fossils in the strata that are the 

 only infallible marks of time-periods. 



Nomenclature of the International Congress of Geologists. — In 

 general usage the time-designations have been applied directly 

 to the formations, as in the nomenclature proposed by the 

 International Geological Congress, where the formation-names 

 stage, scries, system, group, have their corresponding time- 

 names age, epocli, period, era. In a similar way various other 

 terms, w^hich apply to the strata of formations, have their 

 corresponding terms for the fossils of such formations. 



Fauna and Flora — A particular bed, stratum, or forma- 

 tion is said to have its fauna or flora, in the same way as a 

 particular geographical region or province has its fauna or 

 flora. A particular rock stratum marks a particular faunal 

 horizon, as the Tully limestone may be called the horizon of 

 the Cuboides fauna. We find an admirable definition of 

 fauna in the Century Dictionary: '■'■Fauna, the total of the 

 animal life of a given region or period ; the sum of the ani- 

 mals living in a given area or time." Flora is used similarly 

 for the plants of a region or period. 



Horizon.^ — -We find under the word horizon an equally apt 

 definition of that term. A geological Jiorizon is defined as "A 

 stratum, or group of strata, characterized by the presence of 

 a particular fossil, or a peculiar assemblage of fossils, not 

 found in the underlying or overlying beds." 



Zone and Stratum. — The term zone is applied in geology to 

 the stratum or the strata in which a particular fauna or flora is 

 distributed. In some cases authors speak of the zone of a 

 particular species ; but whether a single species, or that one 

 and other associated species, be taken as the distinguishing 

 marks of a geological zone, the difference between a zone and 

 a stratum is found in the distinction that the zone is charac- 

 terized by continuity of the same life and the stratum by 

 continuity of the kind of stratified deposit. 



Facies. — The term fades is used in a particular sense in 

 geology to apply to the particular composition or condition 

 of a formation in a given region ; for instance, the Hamilton 



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