In order that stratigraphic correlations of a geologic system may approach completeness and perfection, it is necessary to make repeated trials, each based on an advance in knowledge; but before a trial is possible, there must be a generalized time scale with which comparisons may be made. This general scale is itself necessarily a matter of growth, since no natural section is ever complete. Usually, the first, or discovery, sections of a stratigraphic division are accidentally encountered, and subsequent field investigations cannot follow a logical course until a fairly comprehensive survey has furnished some clues to the major features and the delineation of the system as a whole. After such a survey, field studies can be directed toward definite goals, and a tentative general time scale can be attempted and subjected to the test of use.
One hundred years ago Sedgwick and Murchison . . .