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BRITISH.
Arbroath paving-stones, with Cephalaspis and Pterygotus. (Chapter 25.) Lower sandstones of Forfars.h.i.+re, with Pterygotus. (Chapter 25.) Sandstones and slates of the Foreland and Linton. (Chapter 25.)
FOREIGN.
Oriskany sandstone of Western Canada and New York. (Chapter 25.) Sandstones of Gaspe, with Cephalaspis. (Chapter 25.)
SILURIAN.
25. UPPER SILURIAN.
BRITISH.
Upper Ludlow formation, Downton sandstone, with bone-bed. (Chapter 26.) Lower Ludlow formation, with oldest known fish remains. (Chapter 26.) Wenlock limestone and shale. (Chapter 26.) Woolhope limestone and grit. (Chapter 26.) Tarannon shales. (Chapter 26.) Beds of pa.s.sage between Upper and Lower Silurian: Upper Llandovery, or May-hill sandstone, with Pentamerus oblongus, etc. (Chapter 26.) Lower Llandovery slates. (Chapter 26.)
FOREIGN.
Niagara limestone, with Calymene, Homalonotus, etc. (Chapter 26.) Clinton group of America, with Pentamerus oblongus, etc. (Chapter 26.) Silurian strata of Russia, with Pentamerus. (Chapter 26.)
26. LOWER SILURIAN.
BRITISH.
Bala and Caradoc beds. (Chapter 26.) Llandeilo flags. (Chapter 26.) Arenig or Stiper-stones group (Lower Llandeilo of Murchison.) (Chapter 26.)
FOREIGN.
Ungulite or Obolus grit of Russia. (Chapter 26.) Trenton limestone, and other Lower Silurian groups of North America. (Chapter 26.) Lower Silurian of Sweden. (Chapter 26.)
CAMBRIAN.
27. UPPER CAMBRIAN.
BRITISH.
Tremadoc slates. (Chapter 27.) Lingula flags, with Lingula Davisii. (Chapter 27.)
FOREIGN.
"Primordial" zone of Bohemia in part, with trilobites of the genera Paradoxides, etc. (Chapter 27.) Alum schists of Sweden and Norway. (Chapter 27.) Potsdam sandstone, with Dikelocephalus and Obolella. (Chapter 27.)
28. LOWER CAMBRIAN.
BRITISH.
Menevian beds of Wales, with Paradoxides Davidis, etc. (Chapter 27.) Longmynd group, comprising the Harlech grits and Llanberis slates. (Chapter 27.)
FOREIGN.
Lower portion of Barrande's "Primordial" zone in Bohemia. (Chapter 27.) Fucoid sandstones of Sweden. (Chapter 27.) Huronian series of Canada? (Chapter 27.)
LAURENTIAN.
29. UPPER LAURENTIAN.
BRITISH.
Fundamental gneiss of the Hebrides? (Chapter 27.) Hypersthene rocks of Skye? (Chapter 27.)
FOREIGN.
Labradorite series north of the river St. Lawrence in Canada. (Chapter 27.) Adirondack mountains of New York. (Chapter 27.)
30. LOWER LAURENTIAN.
BRITISH.
Wanting?
FOREIGN.
Beds of gneiss and quartzite, with interstratified limestones, in one of which, 1000 feet thick, occurs a foraminifer, Eozoon Canadense, the oldest known fossil. (Chapter 27.)
CHAPTER IX.
CLa.s.sIFICATION OF TERTIARY FORMATIONS.
Order of Succession of Sedimentary Formations.
Frequent Unconformability of Strata.
Imperfection of the Record.
Defectiveness of the Monuments greater in Proportion to their Antiquity.
Reasons for studying the newer Groups first.
Nomenclature of Formations.
Detached Tertiary Formations scattered over Europe.
Value of the Sh.e.l.l-bearing Mollusca in Cla.s.sification.
Cla.s.sification of Tertiary Strata.
Eocene, Miocene, and Pliocene Terms explained.
By reference to the tables given at the end of the last chapter the reader will see that when the fossiliferous rocks are arranged chronologically, we have first to consider the Post-tertiary and then the Tertiary or Cainozoic formations, and afterwards to pa.s.s on to those of older date.
ORDER OF SUPERPOSITION.
(FIGURE 86. Section through Primary (left), Secondary, Tertiary and Post- tertiary (right) Strata.
1. Laurentian.
2. Cambrian.
3. Silurian.
4. Devonian.
5. Carboniferous.
6. Permian.
7. Tria.s.sic.
8. Jura.s.sic.
9. Cretaceous.
10. Eocene.
11. Miocene.
12. Pliocene.
13. Post-pliocene.
14. Recent.
Sea.)
The diagram (Figure 86.) will show the order of superposition of these deposits, a.s.suming them all to be visible in one continuous section. In nature, as before hinted (Chapter 6), we have never an opportunity of seeing the whole of them so displayed in a single region; first, because sedimentary deposition is confined, during any one geological period, to limited areas; and secondly, because strata, after they have been formed, are liable to be utterly annihilated over wide areas by denudation. But wherever certain members of the series are present, they overlie one another in the order indicated in the diagram, though not always in the exact manner there represented, because some of them repose occasionally in unconformable stratification on others. This mode of superposition has been already explained (Chapters 5 and 7), where I pointed out that the discordance which implies a considerable lapse of time between two formations in juxtaposition is almost invariably accompanied by a great dissimilarity in the species of organic remains.
FREQUENT UNCONFORMABILITY OF STRATA.
