In order, therefore, to establish a chronological succession of fossiliferous groups, a geologist must begin with a single section, in which several sets of strata lie one upon the other. He must then trace these formations, by attention to their mineral character and fossils, continuously, as far as possible, from the starting point. As often as he meets with new groups, he must ascertain by superposition their age relatively to those first examined, and thus learn how to intercalate them in a tabular arrangement of the whole.
By this means the German, French, and English geologists have determined the succession of strata throughout a great part of Europe, and have adopted pretty generally the following groups, almost all of which have their representatives in the British Islands.
_Groups of Fossiliferous Strata observed in Western Europe, arranged in what is termed a descending Series, or beginning with the newest._ (_See a more detailed Tabular view_, pp. 360. 365.)
1. Post-Pliocene, including those of the Recent, or human period.
2. Newer Pliocene, or Pleistocene. } 3. Older Pliocene. } Tertiary, Supracretaceous[103-A], 4. Miocene. } or Cainozoic.[103-B] 5. Eocene. }
6. Chalk. } 7. Greensand. } 8. Wealden. } 9. Upper Oolite. } Secondary, or Mesozoic.[103-B] 10. Middle Oolite. } 11. Lower Oolite. } 12. Lias. } 13. Trias. }
14. Permian. } 15. Coal. } 16. Old Red sandstone, or Devonian. } Primary fossiliferous, 17. Upper Silurian. } or paleozoic.[103-B] 18. Lower Silurian. } 19. Cambrian and older fossiliferous strata. }
It is not pretended that the three principal sections in the above table, called primary, secondary, and tertiary, are of equivalent importance, or that the eighteen subordinate groups comprise monuments relating to equal portions of past time, or of the earth's history. But we can assert that they each relate to successive periods, during which certain animals and plants, for the most part peculiar to their respective eras, have flourished, and during which different kinds of sediment were deposited in the space now occupied by Europe.
If we were disposed, on palæontological grounds[103-C], to divide the entire fossiliferous series into a few groups less numerous than those in the above table, and more nearly co-ordinate in value than the sections called primary, secondary, and tertiary, we might, perhaps, adopt the six groups or periods given in the next table (p. 104.).
