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 Hordwell and the 
Isle of Wight,—the resemblance is obvious and decided 3 but ona 
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. 
ener nL aN 
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 
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