Page:The Journal of geology (IA journalofgeology21894univers).pdf/387

never, if we are to judge by the recent trend of attempts at widespread correlation, hold the position of importance that correlation within circumscribed areas does. The minor subdivisions of the geological time-standard established for Europe, for example, is found to be of only limited application in North America, and attempts to bring them into complete harmony are little short of wasted energy. But with limited or natural areas the case is far different.

Organic remains are unquestionably of first importance in identifying formations. The study of the mineral composition and lithological characteristics of formations must be abandoned as the sole means necessary for their identification. Recourse must be had to the fossils to set the stratigraphist aright, for as Professor J. W. Judd has said,^[1] "We still regard fossils as the 'medals of creation,' and certain types of life we take to be as truly characteristic of definite periods as the coins which bear the image and superscription of a Roman emperor or of a Saxon king." Of the various kinds of such remains fossil plants occupy relatively as important a position as those afforded by most of the other biological groups.

It is by no means uncommon to find that fossil plants are almost the only organic remains present in a formation, but if they are not, the evidence they afford, when properly interpreted, confirms that obtained from other groups of organic life, as the following examples will show.

As an illustration of the first mentioned condition, viz.: that in which plants only are present in numbers sufficient to entitle them to exclusive consideration, the Dakota group offers an exceptionally fine example. This formation is four or five hundred miles wide, more than a thousand miles long and of considerable thickness, yet not a single vertebrate fossil, and hardly ten species of invertebrates have thus far been detected throughout its vast extent. The Dakota flora, however, is one of the most extensive and thoroughly known fossil floras. According to Lesquereux^[2]