568 BAILEY WILLIS 



formation may belong to any geologic age ; a fauna belongs 

 only to that age in which it was evolved and died out. 



A principal object in studying formations is to read the 

 physical history of the earth, but the principal object of investi- 

 gating fossils is to get at the life history. Each is an essential 

 aid to the other, and the time relations of fossils are funda- 

 mental. But the two lines of inquiry should not be confused. 

 They are confused when we call a stage a formation, or vice 

 versa, or place the limitations of the one upon the other. 



STEPS IN GEOLOGIC SCIENCE. 



The question with which this article opens may now be given 

 a fuller answer. The first step in the geologic study of any 

 locality is to ascertain the relations of the rock masses, and 

 those relations are most happily expressed in a map which 

 exhibits the distribution of formations. This map illustrates 

 local facts. It can be complete in itself, even though no con- 

 nection is established between the facts of that particular 

 district and those of the world at large, but if it be a correct 

 map it will fit into the general record when the connections are 

 traced. 



The second step in geologic investigation is the detailed 

 study of faunas and their distribution in stages. To this second 

 step the first is an essential, as a good topographic map is to 

 both. Through close analysis of the faunas and comparative 

 study of their distribution, the data may be gathered for a map 

 of the stages represented in the district. This map of stages 

 may in some cases resemble closely that of the formations, but 

 in others there will be marked differences. 



With the development of maps showing the distribution of 

 stages we may arrive at correlation of events, and thus be able 

 to compare physical conditions the world over, fitting into its 

 place with some degree of exactness the record of the forma- 

 tions for any locality. Thus correlation is the third step, a step 

 which may be facilitated through other lines of research, but 

 which is fundamental in broad studies of the earth's history. 



