The following discussion is intended to present the currently accepted correlation of the standard succession of western Tertiary mammal horizons, with an attempt to distinguish between its fixed and well proven features and those which are in varying degree doubtful or provisional, or in which recent evidence appears to indicate certain modifications. It is hoped that this method of presentation may be of aid in reconciling the evidence of the vertebrate faunae with that of marine invertebrates and plants. It is also necessary to discuss the criteria used in the methods of comparison, necessarily based on certain underlying assumptions which may not always be even approximately true.
1. Direct superposition (in beds not overturned) is the fundamental evidence.
2. Stratigraphic continuity of lithologically similar strata or sequences of strata is usually, but not always, evidence of identical age. A certain type or sequence of . . .