THE STORY OF NINETEENTH-CENTURY SCIENCE 



tered controversy and the inevitable full generation of 

 probation, the idea of an Ice Age took its place among 

 the accepted tenets of geology. All manner of moot 

 points still demanded attention 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, details 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 concep- 

 tion 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. 



IV 



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 Hutton 

 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 divided the 

 rocks into three systems, based on their supposed order 

 of deposition, and called primary, transition, and sec- 

 ondary. 



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- 



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