INTRODUCTION 457 



were larger and the continental shelves correspondingly narrower than in 

 the middle stages of the periods. 



TRUE FUNCTION OF FOSSILS IN STBATIGBAPHIC TAXONOMY 



Eegarding the function of fossils in correlation, I hold that it consists 

 chiefly of the means they afford of identifying geological horizons. This 

 conclusion is indicated by the fact that life is a continuous process, and 

 that the fossils represent merely occasional stages in its evolution. The 

 first appearance of a species in a stratigraphic sequence does not mark its 

 inception, nor is it likely that its latest appearance in the column marks 

 its extinction. As a rule, the species continued to exist somewhere, and 

 under the. stresses incident to sea retreat it gave rise to modified de- 

 scendants which, when opportunity again offered, invaded the reestab- 

 lished continental seas and left remains that became the guide fossils of 

 the succeeding age. 



Furthermore, the life on the several continents and in each of the 

 oceanic basins differed greatly in character and evolution, and its develop- 

 ment in each was not uniform in direction and rate of modification. 

 Many types were confined to a single area or basin, some lingered much 

 longer in one realm than in another, and others, before becoming extinct 

 in their original habitats, migrated to distant areas, in which they then 

 continued to live for a long time after their extinction elsewhere. 



Even when all these complicating factors have been accounted for and 

 the age of a fossiliferous bed has been properly determined, the purely 

 paleontological method of classifying deposits into formations, groups, 

 series, and systems commonly fails in a most important particular, 

 namely : it does not exactly locate the contact between directly superposed 

 sediments of different ages. As the contact commonly marks some time 

 break whose exact location in the section is essential to a proper under- 

 standing of the geological history of the region, the need of some other 

 criterion that will assure greater defmiteness in the delimitation of strati- 

 graphic units is obvious. To meet this demand, we are obliged to revert 

 to those physical criteria of diastrophism which indicate displacement, by 

 advance or retreat, of the strand-line. 



Methods of Correlation 



uniformity and consistency in practice essential 



The most prolific source of disagreement among systematic stratigra- 

 phers lies in the prevailing disregard of uniformity in taxonomic methods. 

 One geologist bases his judgment regarding the position of a given bed 



