12 Principles of Geology. 



That different strata contain generally different fossils ; but that 

 the same stratum over a very large extent of country, contains generally 

 the same fossils. Hence he deduced the important conclusion, 



That strata may be discriminated and identified by their organic 

 contents. 



Since then, rocks of different antiquity contain different fossils, it 

 is possible to class the organic remains according to their respective 

 periods of existence. They may thus be successively compared 

 with the analogous beings now living, and with one another. This 

 comparison elicits most curious and interesting results. 



First, we perceive that nearly all the immense multitude of buried 

 beings belongs to species different from any that now exist ! but in 

 this difference between fossil and recent specimens, are several de- 

 grees ; some species are allied, others are analogous, and the re- 

 mainder so discrepant, as to bear hardly any mutual resemblance. 



Now, it is an established fact, that the greater number of fossils 

 which nearly resemble living objects, belongs to the most recent of 

 all the strata, viz. those above the chalk ; and that many of the ex- 

 tinct genera are confined to the lowest and oldest part of the series. 

 Place together, for instance, existing species of shells, and the fossils 

 of the least ancient of British strata, as those of Flordwell and the 

 Isle of Wight, — the resemblance is obvious and decided ; but on a 

 similar comparison between recent specimens and the fossil produc- 

 tions of the mountain limestone, one of the oldest of the secondary 

 rocks, the difference is evident and remarkable. Considered in this 

 manner, the living and fossil tribes constitute one mighty series of 

 organic productions, formed upon one general plan, but called suc- 

 cessively into existence, to suit the changing conditions of the earth 

 and the ocean. The striking contrast between the imbedded fossils 

 of different rocks, has given rise to an opinion, that, whilst the strata 

 were successively deposited, many races of organic bodies became 

 extinct, and others were created to supply their place, more and still 

 more nearly assimilated to the present productions of nature. 



We must now attend to certain phenomena, in the relative posi- 

 tions of rocks, which demonstrate that the internal parts of our planet 

 have been shaken by often-repeated convulsions. Rocks appear gen- 

 erally in planes, deviating but little from the horizontal, but sometimes 

 they decline at great angles into the earth, and in several instances 



