in that area. 2.  The Devonian might have been deposited upon the Silurian, and then the whole might have been elevated above the sea, and subjected to an amount of denudation sufficient to remove the Devonian strata entirely.  In this case, when the land was again submerged, the Carboniferous rocks, or any younger formation, might be deposited directly upon Silurian strata.  From one or other of these causes, then, or from subsequent disturbances and denudations, it happens that we can rarely find many of the primary formations following one another consecutively and in their regular order.

[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