148 H. S. WILLIAMS — NOMENCLATURE AND CLASSIFICATION 



in the field. In the examples selected the line between the Romney and 

 Jennings and that between Jennings and Hampshire are frankly de- 

 scribed in the folios as so indefinite as to be recognized with difficulty. 

 Geologists are not always so frank on this point when they return from 

 the field. The evidence brought forward by the analysis of the faunas 

 shows that if the lithologic transitions are perplexing in the case in 

 question the sequence of faunules is also unsatisfactoiy for establishing 

 precise lines of separation between two contiguous formations. 



In the course of the present investigations the recurrence of faunules 

 has become an established fact, and not only for a short vertical distance 

 through the beds, but recurrences of faunules of the same fauna have 

 been traced for a thickness of hundreds and in one region up to about 

 2,000 feet of sediments in which intercalation of entirely distinct faunas 

 has taken place. With these facts in view, we are deceiving ourselves 

 when we presume that half a dozen fossils of particular species occur- 

 ring together determine the stratigraphic horizon, so that the local line 

 below or above them may always serve for the limits of the formation. 

 The fossils do indicate a general portion of the geologic column, but it 

 is only indefinitely, somewhere within one or two thousand feet of thick- 

 ness of strata. They do not alone establish equivalence of formation, 

 when by formation is meant a definite part of the stratigraphic column 

 set off by definite boundaries below and above. 



It may also be stated that the sharper the paleontologic transition 

 from one fauna to another (in ascending through a series of strata) the 

 stronger is the certainty that the local limit thus assigned is not the 

 stratigraphic equivalent of a similar sharp transition between the same 

 two faunas elsewhere. The very fact of the definiteness of the faunas 

 is also sure evidence that they had lived a long time before the first 

 trace of them in the section and lived a long time after the highest traces 

 in the local section, and the sharpness of the transition from the one to 

 the other is evidence that the superior fauna was not derived from the 

 lower one, but that the local succession is a result of movement and 

 replacement of the faunas themselves. 



The conclusion of the matter to which the facts force us is that not 

 only lithologic but paleontologic facts are local. The fossil contents 

 may completely change, often very rapidly and often in a few miles. 

 The fossils undoubtedly are the means on which we chiefly rely for 

 determining that kind of equivalence which is called contemporaneity and 

 homotaxy ; but it must not be overlooked that the characters of fossils 

 (that is, the marks by which species and genera are distinguished) are 

 extremely long ranging. Fossil species were not ephemeral things which 

 changed every few feet of thickness of sediments. 



