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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: 
/. 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. 
II. 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. g^ol. international, Compte Rendu, 5nie Sess., 1891, pp. 151-155, 1893. 
