204 STRUCTURAL GEOLOGY. 



kind, but in different continents, we shall often find a 

 great difference of species, for we find the same thing 

 true of living species (page 118). But the geographical 

 diversity of fossil species, as a general fact, is not so great 

 as that of living species. Commencing with the earliest 

 times, . the geographical differences of species have in- 

 creased more and more to the present time. 



The Age. The distribution of fossil species according 

 to the age of the rocks is the main subject of Part III, or 

 Historical Geology ; but some general notions on this 

 subject are necessary as a basis of classification of strati- 

 fied rocks, and must therefore precede that part. 



Successive Geological Faunas and Floras. The 

 fossil species found in rocks, even of the same kind and 

 country, will depend largely on the age of the rocks. 

 The whole earth has been inhabited at different times by 

 entirely different species. All the animals and plants in- 

 habiting the earth at one time are called the fauna and 

 flora of that geological time. Thus we have a fauna and 

 flora of Tertiary times, of Jurassic times, of Devonian 

 times, etc. 



Definition of Formation and Period. When the 

 strata are conformable, the change from one geological 

 fauna to another is gradual, but a line of unconformity 

 usually abruptly separates two faunas. A formation, 

 therefore, is a series of conformable strata, in which the 

 fossil species are either the same or change very gradu- 

 ally ; and a geological period is the period during which 

 such a formation has been laid down. There are two 

 tests, therefore, of the limits of a geological formation 

 and a geological period, viz., unconformity of the rock- 

 system and great change in the species. Of these the 

 latter is the more valuable. 



Law of Gradual Approach to the Present. It is a 

 fundamental and very important fact that in the suc- 

 cessive changes of geological species there is a steady 



