558 BAILEY WILLIS 



with these features, and an identical sediment accumulate in the 

 migrating zone successively, not simultaneously, over adjacent 

 areas. 



After the days of William Smith a second class of divisions 

 arose — divisions based on the fossils which the rocks contain. 

 Fossils were found to occur in certain associations, which were 

 called faunas, and these became the basis of a classification of 

 rocks. On the hypothesis of special creations and destructions 

 each fauna, wherever occurring, represented a certain date, and 

 thus faunas became the significant figures expressing age. But 

 special creation has given way to evolution, and we recognize 

 migration of faunas as a fact. For instance, Walcott has stated 

 that it took a long time for the Olenellus fauna to move round 

 the globe. A greater or less time interval must elapse between 

 the earliest appearance of a fauna at one place and its earliest 

 appearance at another place remote from the first, hence, a 

 fauna does not indicate a precise date in the narrow sense in 

 which it was once taken. Professor H. S. Williams has pointed 

 out that in a wider sense any fauna endures a length of time, 

 from its initial appearance somewhere to its extinction every- 

 where ; and this interval is an episode of evolution which has a 

 fixed place in geologic history. In that sense faunas have defi- 

 nite time values, but the discovery of that value in any case is 

 dependent on refined and extensive paleontologic research. 



Accordingly, when studying a stratigraphic series, a geologist 

 may recognize distinctions of lithologic character, variations of 

 faunal content, and succession of physical or faunal changes. 

 The differences enable him to define lithologic individuals, faunal 

 individuals, and time intervals. Though often intimately related, 

 sediments and faunas are by no means necessarily bounded by 

 the same limits in space or in time; they constitute not identical 

 but unlike things. They may migrate together or independently. 

 Either may cease and the other continue. When each of them 

 has been described and discusssed in its local and general rela- 

 tions, the problem of correlation in terms of earth history may 

 be hopefully attacked ; but when strata and faunas are treated 



