This paper is an attempt to analyze the meaning of the basic concepts of stratigraphy. It is, by its very nature, a personal and subjective interpretation, even if throughout the whole essay a deliberate effort has been made to maintain an objective and unbiased attitude and a rigorous and exacting method in the analysis of the somewhat complex problems involved.

The first part of the essay deals with the concepts of stratigraphy, space, locality, material things, formations, time, events, simultaneity, and succession. This part lays down the basis, and states the premises, on which the second part of the essay is built up by a simple process of deductive reasoning. The fundamental premises set forth are: (1) stratigraphy is "the study of the shape, dimensions, and spatial relations of bodies of rock, and of the space-time relations of the lithogenetic events which the bodies of rock represent"; (2) "bodies of rock" or "formations" are purely three-dimensional material things divorced from the idea of time; (3) "physical time" means both "date" and "duration," the first concept being the fundamental one, as the second concept can be expressed n terms of the first one, whereas the reverse is not true; (4) "physical time" is a system of imaginary infinitesimal world-wide instants in the space-time continuum, constituting a "kind of frame in which we locate the events of the external world"; (5) the events of geology are "happenings in the external world"; (6) there are two kinds of geological events: "instantaneous events" and "flow events"; (7) "bodies of rock," though purely material things, are the record of past "lithogenetic flow events"; and (8) the time relationships between "lithogenetic flow events" can be classed as simultaneous, partly simultaneous, successive, or subsequent.

The second part of the essay deals essentially with the concepts of correlation, geological time-table, geological key-events, relationship between stratigraphy and paleontology, zones, stages, "time-rock units," and "time-units." A strict deductive reasoning, based on the premises set forth in the first part of the essay, leads me to conclude that (1) geological correlation means "the establishment of the temporal relations between geological events" and that this can only be achieved "indirectly," by means of "linking events" connecting unrelated events; (2) "geological time-table" is a "tabular statement of the chronological order in which geological events took place"; (3) the "key-events" of the geological time-table are "paleontological events"; and (4) we can "correlate" the se imentary flow events represented by bodies of rocks with the unrelated paleontological events of the geological time-table only by means of a "linking event": the "fossil content" of the bodies of rock. This leads naturally to regard "zones" as "events of historical paleontology," and to define "zone" as "the recorded life-range of a species, or of a group of associated species," and to accept Oppel's definition of "stage" as a "grouping of zones." However, the same deductive reasoning shows that the present concept of stage transcends Oppel's definition and has developed into a "correlation concept," with the net result that a "stage" is now conceived as the image of "a dynamic fauna," as opposed to the mere association of "static" species in successive zones. A stage, as a "correlation concept," is the "recorded life range" of a whole dynamic fauna, which though diversified in phyla and classes, unevenly distributed in space and time, and undergoing changes with the passing of time, yet forms a coherent whole, a congruent totum characterized by a "state of evolution" once given and never repeated. Both concepts, those of "zone" and "stage," are, therefore, concepts of historical paleontology and not concepts of historical stratigraphy. Pursuing the subject further, the same deductive thinking leads me to the conclusion that what many stratigraphers call "time-rock units" are, in truth, "paleontological zone event-stratigraphical events units," i.e., a correlation concept that ties together a number of past lithogenetic events through their results (rocks), with a given paleobiological event, also through its results (fossils). And pushing the argument still further, to its ultimate end, I see no intrinsic difference between the so-called "time-units" and "paleobiological events." To my way of thinking, "Tremadocian Age" and "Tremadocian Stage," for instance, are interchangeable expressions, because both concepts are based not on pure time, but on a succession of paleontological events. To summarize this abstract: physical stratigraphy is a science that deals with the spatial relationship of material things; historical stratigraphy deals with the spatial and temporal relationships of events.