106 IOWA ACADEMY OF SCIENCES. 



reasons geologists have been slow in applying in the field 

 what they have been convinced of in their theoretical 

 musings. 



In those cases in the Mississippi valley in which several 

 correlative tests have been made simultaneously, some 

 very instructive features have been disclosed. A paper on 

 the results of one of their multiple tests was read before 

 the Academy last 3"ear, and a more extended memoir, on a 

 closely similar topic was also read before the Geological 

 Society of America, entitled the "Devonian Interval in 

 Missouri." 



In geological correlation the most important of the cri- 

 teria which have been most generally employed may all be 

 assigned to two main groups, the biological, or biotic, and 

 the physical. At one time or another each one of the sub- 

 ordinate methods of both groups has been made all-decisive 

 in the various plans of geological work. At the present 

 time all of these are used to some extent, either directly 

 or indirectly. These minor methods have been recently 

 arranged by Gilbert in the following way: 



I. Physical, through 



(1) Visible continuity. 



(2) Lithological similarity. 



(3) Similarity of lithological sequence. 



(4) Unconformities. 



(5) Simultaneous relations of diverse deposits to 



some physical event. 



(6) Comparison of changes deposits have experi- 



enced from the action of geological processes 

 supposed to be continuous: and 



II. Biotic, through 



(7) Relative abundance of identical species. 



(8) Relative abundance of allied or representative 



species. 



(9) Comparisons of faunas with present life. 

 (lOj Relations of faunas to climatic episodes. 



With possibly one exception, all the methods of correla- 

 tion which are included in the two principal categories are 

 strictly local in their scope, though it is the custom to 



