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.