Although it may already have been more ably done by the preceding speakers in this symposium, a brief outline of the general underlying principles may not be wholly out of place as an introduction to the topic assigned me.
Classification is the orderly grouping together of those beings or things that have certain characteristics in common. Correlation is the more or less technical designation employed in geology for the establishment of an orderly relationship between the geologic units of separate areas or regions. Back of all this is the desire to establish a geologic chronology or time scale—that is, a properly subordinated sequence of geologic events. When such a time scale has been established for a limited area as well as the perfection or the imperfection of the geologic record will permit, it becomes a large and important part of the work of geology to carry that record into . . .