MODERN GEOLOGY 



ogy. All manner of moot points still demanded atten- 

 tion the cause of the ice age, the exact extent of the 

 ice sheet, the precise manner in which it produced its 

 effects, and the exact nature of these effects; and not 

 all of these have even yet been determined. But, de- 

 tails aside, the ice age now has full recognition from 

 geologists as an historical period. There may have 

 been many ice ages, as Dr. Croll contends ; there was 

 surely one; and the conception of such a period is one 

 of the very few ideas of our century that no previous 

 century had even so much as faintly adumbrated. 



THE GEOLOGICAL AGES 



But, for that matter, the entire subject of historical 

 geology is one that had but the barest beginning before 

 our century. Until the paleontologist found out the 

 key to the earth's chronology, no one not even Hut- 

 ton could have any definite idea as to the true story 

 of the earth's past. The only conspicuous attempt to 

 classify the strata was that made by Werner, who di- 

 vided the rocks into three systems, based on their sup- 

 posed order of deposition, and called primary, transi- 

 tion, and secondary. 



Though Werner's observations were confined to the 

 small province of Saxony, he did not hesitate to affirm 

 that all over the world the succession of strata would be 

 found the same as there, the concentric layers, accord- 

 ing to this conception, being arranged about the earth 

 with the regularity of layers on an onion. But in this 

 Werner was as mistaken as in his theoretical explana- 

 tion of the origin of the " primary " rocks. It required 

 but little observation to show that the exact succession 



155 



