526 F. H. KNOWLTON FOSSIL PLANTS IN GEOLOGIC CORRELATION 



to correlate the one with the other. Of the two sets of criteria that must 

 supply the basic data for correlation — namely, the physical and the bio- 

 logical criteria — the latter is by far the more important, and it is safe to 

 say that without its aid there never could have been established anything 

 like a rational chronology and system of correlation. To prove that this 

 is in general true, we have only to review the earlier history of geology to 

 find the pathway strewn with what now seem very crude and imperfect 

 conceptions of the sequence of events, and when the attempt was made to 

 carry thjs sequence, based only on physical criteria, into other areas its 

 inadequacy became strikingly apparent. It is not to be implied, however, 

 that the physical criteria are absolutely without any value, for in not a 

 few cases they are the only data available and, perforce, the most must be 

 made of them. Nor, on the other hand, is it to be affirmed that the bio- 

 logic criteria are infallibly to be relied on in all cases, but rather of the 

 two sets the latter has been found to be of most practical value. The 

 truth of the matter is that not one but all possible sources of information 

 must be brought under tribute in our attempted solution of these intricate 

 problems, and when this has been done substantial harmony will result. 

 For instance, if the two, or even three, types of biologic data are in ap- 

 parent disagreement, we may rest assured that it is not necessarily the 

 several kinds of fossils that are at fault, but rather our faulty interpreta- 

 tion of them. 



Standard Sections 



The establishment of a standard section is an essential basis for geologic 

 correlation, and, as might be presumed from the nature of the controlling 

 factors, the marine section has been found to give the best results, since 

 long-continued, relatively stable environmental conditions are oftener to 

 be met in marine deposits than in any other class of sediments. At first, 

 and indeed for many years, it was supposed that each unit was character- 

 ized by an assemblage of life forms that were more or less completely con- 

 fined to it; but as knowledge progressed, what at first appeared to be a 

 very disconcerting discovery was made, namely, that in an increasing 

 number of cases it was found that a fauna might recur, in a greater or 

 less degree of purity, at a second, a third, or exceptionally even at a fourth, 

 horizon above where it first appeared. This condition of affairs was at 

 first very disconcerting, since it seemed to undermine the very founda- 

 tions of stratigraphic paleontology; but subsequent critical study has 

 shown that it is not so serious as it at first seemed, for while a very large 

 proportion of the species of a marine fauna may sometimes "come back," 

 careful study discloses that there are always detectable differences — in 



