Page:The Journal of geology (IA journalofgeology21894univers).pdf/174

Marine invertebrates for the Paleozoic, and later marine vertebrates constitute the most satisfactory means for discerning geologic age, because the oceans, in which they live, are of world-wide extent and inter-communication. They are distributed around the globe, and from pole to pole, so far as the adaptation to conditions of environment will permit. Hence, we find it to be a fact that, at every period of geological time, there are representatives in all regions of the globe of the same species, or of closely allied species such as to mark the close correlation of time of their living. While this is true, it must be borne in mind that correlation cannot be made by fossils to a finer degree of discrimination than the facts of specific integrity allow. The life-period of species is not uniform, and the wider the distribution, the longer is, as a rule, the range; so we find, in practice, that with the present knowledge of fossils it is rarely possible to discriminate age to a finer degree than that indicated by the periods, Eocambrian, Mesocambrian, and Neocambrian, or similar periods for the other eras. Within such general limits, time relations can be discriminated by the fossils alone, and the progress of science will enable us to recognize more minute divisions of time as we proceed.

On the other hand, formations, being local phenomena, are discriminated with greater precision locally, and the indefiniteness in application of formation names increases with the wideness of the territory covered. No classification of terranes can be of universal application, for no formation, or formation character, or criterion of determination, is of world-wide extent. The time divisions are more precisely defined the larger they are; the formation divisions are more precisely defined the smaller they are, and progress of knowledge will extend the precision in opposite ways for the two scales.