IOWA ACADEMY OF SCIENCES. 135 



presented may be: (1) Local, (2) provincial, (3) regional and (4) 

 general. The last should be clearly distinguished from the 

 others. With the various methods which have been followed 

 from time to time in correlative inquiry, the almost universal 

 practice has been to attempt to base the broader generaliza- 

 tions upon criteria that are, in reality, applicable only to 

 limited areas. Hence, in passing from the more local to the 

 more general, difficulties have always arisen which have 

 become more and more formidable in direct proportion to 

 the extension of the local scheme. Most of the methods that 

 have been applied, and that have been found to answer locally, 

 have fai ed when extended over larger districts. The real 

 problem, then is to find some means of solving the difficulties 

 of the latter, or more general. In the attempts to do this, 

 or when broadly applied, most of the correlation criteria have 

 proved very inadequate. A little consideration will make the 

 reasons evident. As the specific distinctions that are regarded 

 as decisive in a given locality are extended more and more 

 widely, they change and all are gradually replaced by others 

 which may be very different. The physical conditions that 

 have given rise to the various distinctive features, or the 

 processes involved in their production, themselves change 

 from place to place and from time to time. In seeking for 

 a suitable means of carrying on correlation it is manifest, at 

 the outset, that in no case should the critical criteria deal with 

 the intrinsic features as such, but with the causes producing 

 them. Moreover, the great factor to be taken into account in 

 every standard of comparison which has to do with the 

 correlation of strata, is a definite or absolute basis to which 

 the various minor, or local and provincial, successions can be 

 referred. This fundamental conception grows out of a consid- 

 eration of the nature of sedimentation itself. 



The features which have in the past had the greatest weight 

 in geological correlation, have been those which, in reality, 

 are partly or entirely unrelated to the deposition of strata. 

 In attempting to seek a criterion that is fundamental in strati- 

 graphy, it is pertinent at the start to inquire into the real 

 nature of sedimentation, into the causes producing it, modify- 

 ing it and limiting it, into the forces called into action, in sub 

 sequently obliterating their results, in fact, into all of the 

 primary processes involved, and into the secondary processes 

 which tend to obscure the actual workings of the real and 



