25J- The A iiiefirtin Geoh/f/inf. NovcihIkt, im96 
of sojiifcthing better have been used only provisionally, 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, the others are also followed at the same time. Hence, 
few correlation problems are now settled by a single standard 
alone. 
The most im])ortant of the criteria which hn\r been em- 
ploj'ed are: Mineral deposits contained, lithological similari- 
ty, organic contents, stratigraphic continuit}'", and discordant 
sedimentation. At one time or another each of these has had 
supremacy in geological w^ork, and at the present time all of 
them are used to some extent, either directly or indirectly. 
At the Washington meeting of the International Congress of 
Geologists, Gilbert* arranged the common methods of correla- 
tion in the following way: 
I. Physical, through 
1. Visible continuity. 
'2. Lithological similarity. 
.'}. Similarity of lithological sequence. 
■4. Unconforniities. 
•'>. Simultaneous relations of diverse deposits to some physical 
event. 
<1. Comparison of clianges dej^osits haveexijcricnced from the 
action of geological processes supposed to l)e continuous. 
II. Bi(jtic, 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. 
For a long time general correlations have been almost uni- 
versally carried on by the biotic methods. .Vt present the fac- 
tors of organic remains ))redominates over all others and is in 
fact the foundation of the commonly accepted system of geo- 
logical synchrony. However, it is beginning to be recognized 
more and more clearly that organic remains are not the all- 
deciding factors in questions of correlation, that they are in 
reality accidental characters, and that when depended upon 
they must always be taken in connection with the physical 
features. In actual practice they are regarded as corrobora- 
*Cong. geol. international, Compte Rendu, 5me Sess.. 1891, pp. 151- 
1.").-.. 1893. 
