TOWA ACADEMY OF SCIENCES. 137 



numerous. As they come to be tested practically in the field 

 they have been, one by one, abandoned entirely, passed over, 

 or, in lieu of something better, have been used only provision- 

 ally, or with reservation. No single criterion has yet been 

 proposed that answers the purpose successfully. Although 

 some one of the various methods is commonly used as the 

 principal one, others are almost invariably also taken into 

 account at the same time. Hence, it is universally recognized 

 that few correlation problems can be now settled by a single 

 standard alone. 



Chief Methods. — In geological correlation the most important 

 of the criteria that 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. 

 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 manner: 



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 experienced 



from the action of geological processes supposed 

 to be continuous. 

 //. Biotic, through, 



7. Relative abundance of identical species. 



8. Relative abundance of allied or representative 



species. 



9. Comparisons of faunas with present life. 

 10. Relations of faunas to climatic episodes. 



With possibly one exception all the methods of correlation 

 which are included in these two catagories are strictly local in 

 their scope, though it is the custom to regard them as applying 

 widely, if not universally. For many years general correla- 

 tions have been carried on almost entirely by the biotic 

 methods. At the present time they predominate over all others. 



*Oong. geol. international, Compte Rendu, 5me Sess., 1891, pp. 151-155, 1893. 



