Adjacent strata in any region are arranged according to the law of superposition, i.e. any stratum is younger than that on which it was deposited, just as in a pile of paper, any sheet was laid later than that on which it rests.  Where rocks have been disturbed, their original attitude must be determined before the law can be applied.  Nor can the law of superposition be used in identifying and comparing the strata of different regions where the formations cannot be traced continuously from one region to the other.

The formations of different regions are arranged in their true order by the law of included organisms; i.e. formations, however widely separated, which contain a similar assemblage of fossils are equivalent and belong to the same division of geological time.

The correlation of formations by means of fossils may be explained by the formations now being deposited about the north Atlantic.  Lithologically they are extremely various.  On the continental shelf of North America limestones of different kinds are forming off Florida, and sandstones and shales from Georgia northward.  Separated from them by the deep Atlantic oozes are other sedimentary deposits now accumulating along the west coast of Europe.  If now all these offshore formations were raised to open air, how could they be correlated?  Surely not by lithological likeness, for in this respect they would be quite diverse.  All would be similar, however, in the fossils which they contain.  Some fossil species would be identical in all these formations and others would be closely allied.  Making all due allowance for differences in species due to local differences in climate and other physical causes, it would still be plain that plants and animals so similar lived at the same period of time, and that the formations in which their remains were imbedded were contemporaneous in a broad way.  The presence of the bones of whales and other marine mammals would prove that the strata were laid after the appearance of mammals upon earth, and imbedded relics of man would give a still closer approximation to their age.  In the same way we correlate the earlier geological formations.

For example, in 1902 there were collected the first fossils ever found on the antarctic continent.  Among the dozen specimens obtained were some fossil ammonites (a family of chambered shells) of genera which are found on other continents in certain formations classified as the Cretaceous system, and which occur neither above these formations nor below them.  On the basis of these few fossils we may be confident that the strata in which they were found in the antarctic region were laid in the same period of geologic time as were the Cretaceous rocks of the United States and Canada.