•1\){\ The American Geohnjist. .Nnvomix-r. 1^% 
may be devis-ed which will put geological correlation and sys- 
tematic stratigrai)hy on a purely physical basis, that will be 
at once rational, practical and elastic. At the same time it 
will accord Avith the physical historj^ of events, and will not 
inhibit the usage of all features of other plans so far as they 
are found to be trustworthy and of real service. 
The primary object of geological correlation is the establish- 
ment of a general system of chronology. It finds expression 
in the classification adopted. In the development of every 
branch of natural knowledge one of the first considerations to 
receive attention is a systemization of the facts obtained. 
This orderly arrangement is one of the earliest prerequisites 
demanded of the branch in its attainment of scientific recogni- 
tion ; while its advancement is measured by the degree of tax- 
onomic completeness and by the nature of the criteria regarded 
as critical. The bringing together of the various i)henoniena 
so that some sort of systematic relationship is made to exist 
among them all is the initial step in raising a particular de- 
partment of knowledge to the dignity of a science. As pro- 
o-ress is made a gradual evolution takes in the fundamental 
plan of grouping the facts. In the beginning, a classification, 
rude though it may be, is fashioned from the superficial fea- 
tures which are most striking at first glance. It is at a later 
stao-e modified to one in which similarity of the common 
characters, irrespective of natural relations, is taken into ac- 
count. A vastly more advanced conception is classification 
based upon attinity, in which for similarity of features there 
is substituted similarity of plan. The final stage is the ge- 
netic, in which origin or causal relationship is the governing 
or predominant factor. 
The classification of geological phenomena is no exception 
to the rule. In stratigraphy, as in other branches of geology, 
the various standards of comparison have given way, one 
after another, to other standards more expressive of the ad- 
vancement of the science. But in the successive replacement 
of one set of criteria by another the abandoned groups are not 
found to be altogether wrong, and they continue to exert more 
or less influence, long after they are thought to be forgotten. 
In practice, then, the establishment of a rational system of 
geological chronology is not to be sought in the comparison of 
