THE MAKING OF THE GEOLOGICAL TIME-SCALE. 29 



each other and by their lithological characters ; and (2) cJirono- 

 logical time-periods, which may be locally marked by the 

 stratigraphical division-planes, but which depend, fundamen- 

 tally, upon biological evidence for their interpretation and 

 classification. Gilbert * has concisely expressed the impor- 

 tant fact of the purely local nature of the division-planes 

 separating the formations stratigraphically into stages, series, 

 systems, or groups in the words : "There does not exist a 

 world-wide system nor a world-wide group, but every system 

 and every group is local." "The classification developed in 

 one place is perfectly applicable only there. At a short dis- 

 tance away some of its beds disappear and others are intro- 

 duced ; farther on its stages cannot be recognized ; then its 

 series fail, and finally its systems and its groups." 



If we accept the correctness of this statement, it is evi- 

 dent that geological terranes and the stratigraphical division- 

 planes by which they are marked, although indicative of 

 time succession, present nothing in themselves to indicate 

 the particular place they occupy in a time-scale. Even were 

 the age of a particular stratum in one section accurately de- 

 termined by other means, there is no stratigraphical or litho- 

 logical mark upon the rock stratum by which the correspond- 

 ing age can be recognized in another section. This is not 

 meant to imply that it is impossible to trace a stratum or 

 formation from one section to another in the same general 

 geological province, for in such case it is a process of tracing 

 with slight interruption the continuity of the one terrane. 

 But when we pass from one basin to another the physical 

 continuity is broken, and the stratigraphy and lithology were 

 made on a separate basis. Hence we reach the conclusion 

 that the perfecting of the geological time-scale must be 

 wrought by the means, primarily, of organic remains. Chro- 

 nological time-periods in geology are not only recognized by 

 means of the fossil remains preserved in the strata, but it is 

 <"o them chiefly that we must look for the determination and 

 classification of the rocks on a time basis. 



* G. K. Gilbert, "The Work of the International Congress of Geologists." 

 Proc Am. Assoc. Adv. Sci., August, 1S87, vol. .\xxvi. p. igi. 



