140 
IOWA ACADEMY OP SCIENCES. 
belong, more or less, to one set of conditions as shown by the 
similarity of the inhabitants, as well as of the country occupied, 
and of the structures which remain; that is of the fossils, the 
stratigraphy and petrology of the district. Our greater divi- 
sions must be based on the more complete changes and the 
smaller upon the minor fluctuations which will be indicated only 
by the more sensitive and specially adapted forms of life, or by 
the more minute structural changes. ” 
EXTENSION OF THE USUAL CRITERIA TO GENERAL APPLICA- 
TION. 
Main Consider at ions . — Among the various methods of parallel- 
ing strata, and in the broader phases of their consideration, 
there are certain points in several of them to which attention 
should be directed. The methods referred to are those which 
have, of late, received the greatest consideration. They may 
be included under the titles of (1) biological relationships, (2) 
unconformity, (3) community of genesis, (4) historical similar- 
ity, and (5) physiographic development. 
Biological EelationsMi^s . — As the various standards that have 
been usually used in geological correlation have been finally 
found to be useful only in limited areas, instead of being world- 
wide, or even of continental application, so also the latest one, 
which has so long held prestige, has been found at last to have 
no longer the unerring certainty in exact correlation that was 
once claimed for it, and in this respect to be no longer keeping 
pace with the advance of geological science. Like the other 
methods or schemes, it too is having its usefulness restricted 
to limited districts, and to be relegated to the subordinate posi- 
tion of a local criterion. Its accuracy remained unquestioned 
in the absence of more reliable criteria with which to check its 
results. With, however, the advent of more refined methods 
of working, its unreliability in exact general correlation has 
become very manifest. As a striking example, stands the 
eastern sea-board of the United States. Of it, McGee says that 
‘ ^ nearly as much information concerning the geological history 
of the Atlantic slope, has been obtained from the topographic 
configuration of the region within two years as was gathered 
from the sediments of the coastal plain and their contained 
fossils in two generations. ” 
It has come to be widely recognized that there are no more 
grounds for the claim that the succession of organic forms and 
faunas is an expression of the geological course of events and 
