G. PRINCIPLES OF CLASSIFICATION AND COR- 

 RELATION OF GEOLOGICAL FORMATIONS. 



CHAPTER XXXI. 



NOMENCLATURE AND CLASSIFICATION OF GEOLOGIC 

 FORMATIONS. 



Development of Classifications. 



The history of the earth is written in the strata of the earth's 

 crust. Like all histories, it is a continuous succession of events, but 

 the record of these events is never complete and seldom even un- 

 broken in any given region. It is of the first importance to the 

 chronographer of earth history that he should find a continuous 

 record, in order that he may have a measure by which to judge 

 the partial records of any given region and to discover the breaks 

 and imperfections in the local records thus presented. (Grabau-5.) 

 The question then arises : under what conditions may we expect to 

 obtain a continuous record and how are we to guard against the 

 introduction of errors? 



We have, in the first place, to deal with the time element in the 

 history of the earth. In human history the time element is a mea- 

 surable factor, its duration being recorded in years and centuries. 

 No such precise measurements are possible in earth history, al- 

 though several attempts have been made to reduce geologic time to 

 units of human chronology. (For methods and results, see beyond.) 

 But while we cannot now, and probably may never, hope to divide 

 geologic time into centuries and millenniums, we can divide it into 

 periods, each of which has its own special significance in the his- 

 tory of the earth. The basis for such subdivision was long ago 

 found in the succession of organic types from relatively simple to 

 highly complex forms. As long as the doctrine of special creation 

 of organic types was held, and with it the belief in successive acts 

 of creation, and more or less complete extinction of the faunas and 



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