560 BAILEY WILLIS 



stratigraphic association and its contaified fossils.'^ (Italics the 

 present writer's.) 



Formations aiid fossils. — That the purpose of the definition is 

 to emphasize lithologic character, and make it the essential of 

 individuality of a formation, is clear, but the habit of classifying 

 roughly by fossils is strong, and the few words here italicized 

 have led man}^ to lose sight of the distinction intended. There 

 is a difference between using fossils as one of several means of 

 identification and employing them as essential characters. In 

 the former case, other characters being the same, the occurrence 

 of a known fossil is an aid, but its non-occurrence sets no limit. 

 In the latter case the range of the characteristic fossils defines 

 the extent of the division. By the one method we may define 

 a lithologic formation according to all its characters. By the 

 second method we limit the lithologic unit by a fauna and a 

 fauna by the lithologic unit, when either may occur beyond the 

 other; and thus, combining two unlike things in one definition, 

 we can recognize neither. 



The writer by no means advocates disregard of fossils in the 

 identification of formations. But abundant experience of the 

 ablest stratigraphers shows that classification by faunas requires 

 most refined investigation based upon thorough knowledge of 

 the rocks. To map the formations is a necessary preliminary 

 to determining the faunal units. We should proceed from the 

 simple and obvious to the complex and obscure. We should 

 trace out lithologic individuals, according to their constitution 

 and stratigraphic associations. If these leave us in doubt, fos- 

 sils may prove valuable ear-marks ; but they should not set 

 limits in the discrimination of formations. When the map of 

 formations is made, the way is prepared for the paleontologist 

 to ascertain the number and bounds of the faunal units and to 

 draw the faunal map. 



In mapping formations it is convenient to combine them in 

 systems — Cambrian, Cretaceous, Eocene, etc. Fossils are the 

 means of this preliminary rough classification, and the little 

 evidence required is usually obtained in studying the lithology. 



