THE DIVISIOiYS OF THE GEOLOGICAL TIME-SCALE. 55 



a scheme which is actually recognized in the classification of 

 the geological formations throughout the world, where the 

 criteria of classification are the fossils contained in them. 

 Geologists dealing with distinct series of strata have named 

 the individual members of the series differently for different 

 regions of the earth. Therefore, as the systems are made up 

 of formations presenting local features, of stratification, of 

 petrographic composition, of structure, and of thickness, which 

 are given local names, the fossil fauna-floras representing each 

 one of the periods are found in formations which have different 

 names in separate regions. 



In using such a scale it becomes necessary to correlate 

 the faunas of formations having different names. While the 

 formation names may well be retained, in the discussion of 

 the time-relations of organisms it is essential to use a uniform 

 scale of time-divisions expressed in a single series of names: 

 the scale and names above given supply us with such a 

 standard time-scale. 



Actual Length of Geological Time. — That geological time is 

 immensely long, as compared with any human standards, all 

 modern geologists admit ; but as to how much time, in cen- 

 turies or years, has elapsed since the beginning of the series 

 of sedimentary rocks, opinions greatly differ. A few facts 

 may be mentioned to illustrate what is meant by great length 

 of time in terms of geological work accomplished : 



(i) Since the close of the Cretaceous Period the greater part 

 of the mountain elevation along the southern part of Europe 

 and extending to the extreme southeastern part of Asia was 

 accomplished ; and the Himalayas were raised, so that at least 

 16,000 feet thickness of their mass is composed of marine 

 strata of Tertiary or earlier era. 



(2) The large part of the Rocky INIountain region was 

 under marine water in the Cretaceous time. Since the 

 close of the Eocene, or beginning of the Middle Tertiary, 

 as Captain Button estimates, the region of the Colorado 

 canons has been elevated approximately 10,000 or 11,000 

 feet, and 10,000 feet of erosion has taken place. G. M. Daw- 

 son estimates the total amount of elevation which has taken 

 place since Cretaceous time, in British Columbia, to have been 



