140 IOWA ACADEMY OF 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 Considerations. — 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. 



Biol 0(1 ical Uelationships. — 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 



