THE GEOLOGICAL TLME- SCALE. 1 97 



acters change." . . . "The time divisions shall be defined pri- 

 marily by palaeontology 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 

 \.\\Q geological formation and the geological period h^iWQ become thor- 

 oughly 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- 



H. S. Williams. 



