[Footnote 8: As we have every reason to believe that dry land and sea have existed, at any rate from the commencement of the Laurentian period to the present day, it is quite obvious that no one of the great formations can ever, under any circumstances, have extended over the entire globe. In other words, no one of the formations can ever have had a greater geographical extent than that of the seas of the period in which the formation was deposited. Nor is there any reason for thinking that the proportion of dry land to ocean has ever been materially different to what it is at present, however greatly the areas of sea and land may have changed as regards their place. It follows from the above, that there is no sufficient basis for the view that the crust of the earth is composed of a succession of concentric layers, like the coats of an onion, each layer representing one formation.] In no case, however, do we ever find the Devonian resting upon the Carboniferous, or the Silurian rocks reposing on the Devonian. We have therefore, by a comparison of many different areas, an established order of succession of the stratified formations, as shown in the subjoined ideal section of the crust of the earth (fig. 17). The main subdivisions of the stratified rocks are known by the following names:-- 1. Laurentian. 2. Cambrian (with Huronian ?). 3. Silurian. 4. Devonian or Old Red Sandstone. 5. Carboniferous. 6. Permian \_ New Red Sandstone. 7. Triassic / 8. Jurassic or Oolitic. 9. Cretaceous. 10. Eocene. 11. Miocene. 12. Pliocene. 13. Post-tertiary. [Illustration: Fig. 17. IDEAL SECTION OF THE CRUST OF THE EARTH.] Of these primary rock divisions, the Laurentian, Cambrian, Silurian, Devonian, Carboniferous, and Permian are collectively grouped together under the name of the Primary or _Paloeozoic_ rocks (Gr. _palaios_, ancient; _zoe_, life). Not only do they constitute the oldest stratified accumulations, but from the extreme divergence between their animals and plants and those now in existence, they may appropriately be considered as belonging to an "Old-Life" period of the world's history. The Triassic, Jurassic, and Cretaceous systems are grouped together as the _Secondary_ or _Mesozoic_ formations (Gr. _mesos_, intermediate; _zoe_, life); the organic remains of this "Middle-Life" period being, on the whole, intermediate in their characters between those of the palæozoic epoch and those of more modern strata. Lastly, the Eocene, Miocene, and Pliocene formations are grouped together as the _Tertiary_ or _Kainozoic_ rocks (Gr. _kainos_, new; _zoe_, life); because they constitute a "New-Life" period, in which the organic remains approximate in character to those now existing upon the globe. The so-called _Post-Tertiary_ deposits are placed with the Kainozoic, or may be considered as forming a separate _Quaternary_ system. CHAPTER IV. THE BREAKS IN THE GEOLOGICAL AND PALÆONTOLOGICAL RECORD.