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have seen the importance of classifying terranes and periods seperately.

James D. Dana,^[1] in 1855, set forth, with great clearness, the importance of the chronological classification of rocks, and in his manual^[2] all geologists have been made familiar with the meaning of a chronological classification of stratified rocks, but the classification is a classification of rock strata, and the time-divisions are those determined by the strata, so that but one nomenclature has been needed or used.

The International Congress of Geologists was the first to distinctly formulate a dual method of classification, but here, too, it was only two ways of classifying one set of facts that was proposed. The divisions of the scale have been identical and in the terms of strata. The Congress was organized for the purpose of unifying nomenclature, but one of the most important results of the Congress has been the discovery that uniformity, in the sense first proposed, is not practicable.

In advocating a dual nomenclature, I would carry the differentiation one step farther, and propose that we give a different nomenclature to the time-scale, and classify it independently from the terrane-scale, because the fossils by which its divisions are determined contain, in themselves, the evidence of their time relations. In an article in The Journal of Geology^[3] I described the history of the elaboration of the system of nomenclature and classification now in use, and showed how the geological formation is the actual unit of classification in the present system. Having called attention to the fact that the geological formation and the geological period have become thoroughly differentiated, I remarked in closing that "the elaborating further and making more precise the geological time-scale must come from a direct study of the life history of organisms as