146 THE JOURNAL OF GEOLOGY. 



have seen the importance of classifying terranes and periods sepa- 

 rately. 



James D. Dana,^ in 1855, set forth, with great clearness, the 

 importance of the chronological classification of rocks, and in 

 his manual^ 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 impor- 

 tant results of the Congress has been the discovery that uni- 

 formity, in the sense first proposed, is not practicable. 



In advocating a dual nomenclature, I would carry the differ- 

 entiation 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 rela- 

 tions. In an article in The Journal of Geology3 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 sys- 

 tem. 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 fur- 

 ther and making more precise the geological time-scale must 

 come from a direct study of the life history of organisms as 



' See Proceedings of the American Association for tfie Advancement of Science 

 for 1855 ; also On American Geological History, Am. Jour. Sci., II., Vol. XXII., 

 PP- 335-344- 



'^ Dana : Manual of Geology, ist edition 1863, 2d edition 1874, 3d edition 1879. 



3 The Making of the Geological Time-Scale. Jour, of Geol., Vol. I., pp. 180- 

 196. See also Elements of the Geological Time-Scale, Vol. I., pp. 283-295, 1893. 



