If we find strata containing identical fossils within the limits of a single geographical region--say in Europe--then there is a reasonable probability that these beds are strictly contemporaneous, in the sense that they were deposited at the same time.

There is a reasonable probability of this, because there is no improbability involved in the idea of an ocean occupying the whole area of Europe, and peopled throughout by many of the same species of marine animals.

At the present day, for example, many identical species of animals are found living on the western coasts of Britain and the eastern coasts of North America, and beds now in course of deposition off the shores of Ireland and the seaboard of the state of New York would necessarily contain many of the same fossils.

Such beds would be both literally and geologically contemporaneous; but the case is different if the distance between the areas where the strata occur be greatly increased.

We find, for example, beds containing identical fossils (the Quebec or Skiddaw beds) in Sweden, in the north of England, in Canada, and in Australia.