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[103-5], 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.]).
At the same time, I may observe, that, in the present state of the science, when we have not yet compared the evidence derivable from all classes of fossils, not even those most generally distributed, such as shells, corals, and fish, such generalizations are premature, and can only be regarded as conjectural or provisional schemes for the founding of large natural groups.
Fossiliferous Strata of Western Europe divided into Six Groups.
| 1. Post Pliocene and Tertiary | } | from the Post-Pliocene to the Eocene inclusive. |
| 2. Cretaceous | { | from the Maestricht Chalk to the Lower Greensand inclusive. |
| 3. Oolitic | } | from the Wealden to the Lias inclusive. |
| 4. Triassic | { | including the Keuper, Muschelkalk, and Bunter Sandstein of the Germans. |
| 5. Permian, Carboniferous, and Devonian | } | including Magnesian Limestone (Zechstein), Coal, Mountain Limestone, and Old Red sandstone. |
| 6. Silurian and Cambrian | { | from the Upper Silurian to the oldest fossiliferous rocks inclusive. |
CHAPTER X.
CLASSIFICATION OF TERTIARY FORMATIONS.—POST-PLIOCENE GROUP.
General principles of classification of tertiary strata — Detached formations scattered over Europe — Strata of Paris and London — More modern groups — Peculiar difficulties in determining the chronology of tertiary formations — Increasing proportion of living species of shells in strata of newer origin — Terms Eocene, Miocene, and Pliocene — Post-Pliocene strata — Recent or human period — Older Post-Pliocene formations of Naples, Uddevalla, and Norway — Ancient upraised delta of the Mississippi — Loess of the Rhine.
Before describing the most modern of the sets of strata enumerated in the tables given at the end of the last chapter, it will be necessary to say something generally of the mode of classifying the formations called tertiary.