THE PRE-ADAMITE EARTH. 



he calls Eocene, Miocene, and Pliocene. He divides the first into 

 three, and the last into two subordinate groups, thus making the 

 whole formation consist of six stages. M. D'Orbigny finds that 

 a division into five stages is more in accordance with the distri >u- 

 tion of the organic remains. The following are the names which 

 he lias given to the five atrata, with their origin, and their esti- 

 mated average thickness : 



Although these estimates of thickness must he regarded as mere 

 approximations, they will, nevertheless, be useful as exponents of 

 the relative duration of the five periods into which the Tertiary 

 age is divided. The total average thickness of this Tertiary 

 formation would appear to be about 10000 feet, or two miles, the 

 chief parts being occupied by the two lowest stages. 



455. The complete series of stages can scarcely be found in 

 any single locality, but, partially united, their order can be easily 

 determined by comparing sections made in different places. 



456. Fig. 180 represents a section from Vertus to Paris, being 

 the continuation of the general section of the country from the 

 Vosges to Paris, the first three divisions of which, commencing 

 from the Vosges, have been already given in figs. 141, 152, and 

 170. In this section, the first three only of the stages, proceeding 

 upwards, are included, being numbered 24, 25, and 26. 



As in the preceding formations, the single stages are distin- 

 guished one from another, partly by discordance of stratification, 

 but much more by what geologists call discordances of isolation ; 

 that is, by the appearance of certain stages, and the absence of 

 .others. For example, while round the Anglo-Parisian basin 

 the first of the Tertiary stages is deposited immediately upon 

 the last of the Cretaceous ; in other places many intermediate 

 st iges are wanting. Thus, near Orgon, in France, the first Ter- 

 tiary stage is deposited upon the first Cretaceous, all the other 

 six stages of the Cretaceous formation being absent. In like 

 nianner, in the department of the Aude, the first Tertiary stage 

 is deposited upon the Palaeozoic formation ; and in the department 

 of the Var and the Lower Alps upon the Jurassic formation. In 

 118 



