Page:The Journal of geology (IA journalofgeology11893univ).pdf/213

acters change."..."The time divisions shall be defined primarily by palæontology and secondarily by structure, and they shall be called periods" (p. 65). We have thus reached the stage in the making of the geological time-scale in which the ideas of the geological formation and the geological period have become thoroughly differentiated. The geological period as a time-unit is primarily defined by the characters of the fossil remains in the rock, so that the elaborating further and making more precise of the geological time-scale must come from a direct study of the life history of organisms as recorded in the stratigraphical forma ions.