IV. Cenolithic groups, or cenozoic (tertiary) groups of strata. XII. Pliocene (neo-tertiary) 34. Arverne 33. Subapennine Upper pliocene Lower pliocene XI. Miocene (middle tertiary) 32. Falun 31. Limbourg Upper miocene Lower miocene Xb. Oligocene (old tertiary) 30. Aquitaine 29. Ligurium Upper oligocene Lower oligocene Xa. Eocene (primitive tertiary) 28. Gypsum 27. Coarse chalk 26. London clay Upper eocene Middle eocene Lower eocene III. Mesolithic groups, or mesozoic (secondary) groups of strata. IX. Chalk (cretaceous) 25. White chalk 24. Green sand 23. Neoconian 22. Wealden Upper cretaceous Middle cretaceous Lower cretaceous Weald formation VIII. Jurassic 21. Portland 20. Oxford 19. Bath 18. Lias Upper oolithic Middle oolithic Lower oolithic Liassic VII. Triassic 17. Keuper 16. Muschelkalk 15. Bunter Upper triassic Middle triassic Lower triassic II. Paleolithic groups, or paleozoic (primary) groups of strata. VIb. Permian 14. Zechstein 13. Neurot sand Upper permian Lower permian VIa. Carboniferous coal-measures) 12. Carboniferous sandstone 11. Carboniferous limestone Upper carboniferous Lower carboniferous V. Devonian 10. Pilton 9. Ilfracombe 8. Linton Upper devonian Middle devonian Lower devonian IV. Silurian 7. Ludlow 6. Wenlock 5. Llandeilo Upper silurian Middle silurian Lower silurian I. Archeolithic groups, or archeozoic (primordial) groups of strata. III. Cambrian 4. Potsdam 3. Longmynd Upper cambrian Lower cambrian II. Huronian I. Laurentian 2. Labrador 1. Ottawa Upper laurentian Lower laurentian The primordial period falls into three subordinate sections—the Laurentian, Huronian, and Cambrian, corresponding to the three chief groups of rocks that comprise the archaic formation. The immense period during which these rocks were forming in the primitive ocean probably comprises more than 50,000,000 years. At the commencement of it the oldest and simplest organisms were formed by spontaneous generation—the Monera, with which the history of life on our planet opened. From these were first developed unicellular organisms of the simplest character, the Protophyta and Protozoa (paulotomea, amœbæ, rhizopods, infusoria, and other Protists). During this period the whole of the invertebrate ancestors of the human race were evolved from the unicellular organisms. We can deduce this from the fact that we already find remains of fossilised fishes (Selachii and Ganoids) towards the close of the following Silurian period. These are much more advanced and much younger than the lowest vertebrate, the Amphioxus, and the numerous skull-less vertebrates, related to the Amphioxus, that must have lived at that time. The whole of the invertebrate ancestors of the human race must have preceded these.