IOWA ACADEMY OF SCIENCES. 
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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 
