Stratigraphic geology operates concurrently with two different time scales. One is absolute and based on measurements in units of equal duration (years), the other is relative and based on arbitrarily selected and unequally spaced fixed time markers of known or unknown position in the absolute scale, established as a discontinuous sequence of unrelated events which have left traces in the geological record. These events which may be classed as lithogenetic, geotectonic, or biological, serve as fixed points for the establishment of a hierarchic system of arbitrary divisions of geological time. The first regional appearance of a new fauna, analyzed from the viewpoint of evolution as well as ecologically, is the basic time-marking event. Major subdivisions are marked by horizons at which a number of events occur more or less contemporaneously, indicating major changes. Stratigraphic nomenclature expresses the resulting classification. The principle of priority is applied to it with certain limitations.
On the basis of these principles, it is found that the existing discrepancies in the placing of the lower and upper boundaries of the Miocene can only be resolved by agreement, as various arguments for different placings are inconclusive. Ambiguity of terminology has further confused the Miocene-Pliocene boundary problem. In the Caribbean region, the terms Lower, Middle, and Upper Miocene seem to have acquired an agreed meaning. Faunal relations with the Indo-Pacific Miocene seem at first glance confusing, but a suggested line of analysis leads to the conclusion that discrepancies are small. The Indo-Pacific “letter-classification” is reviewed in some detail and rejected for regional or general use outside Indonesia as seriously defective. The introduction of Lower, Middle, and Upper Miocene as divisions of the standard time-stratigraphic scale is advocated. Most authors using at present European Stage terms for reference place the Burdigalian in the Lower, the Vindobonian in the Middle, and the Sarmatian in the Upper Miocene. There is no agreement about the desirability of including the Aquitanian in the Lower Miocene and the post-Sarmatian interval, which is currently but ambiguously termed Pontian, in the Upper Miocene.