Knowledge of the successive faunas of North America is still imperfect. Far more is known of the marine invertebrate faunas than of the fresh water and terrestrial ones, as the former are more abundant in the separate strata, and have, in most cases, wider regional extent. Thus, the correlation of formations of an epeiric sea is relatively simple, even though the sea may have covered a large portion of North America. There is, accordingly, comparatively little difficulty in the correlation of the majority of formations from Lower and Upper Ordovician, Middle Silurian (Niagaran), Middle Devonian (Ulsterian, Erian, and Senecan), and, in the West, Upper Devonian, Lower and Upper Mississippian, Lower and Middle Permian, Upper Triassic, Late Jurassic, and marine Upper Cretaceous.

Formations laid down during times of enlarged continents and of limited marine incursions are difficult to correlate unless sections are found where the two types of deposits interfinger. . . .

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