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IOWA ACADEMY OF SCIENCES. 
secondary, or even accidentally associated. That this is true 
in every department of science is clearly shown, not only 
by its history, but particularly by the classifications of the 
phenomena that have been followed during the different stages 
of its development. 
In the progress of every branch of knowledge, one of the 
first considerations to receive attention is a systematization of the 
known facts. This orderly arrangement is one of the earliest 
prerequisites demanded of the branch in its attainment to 
recognition; while its advancement is measured by the degree 
of taxonomic completeness and the critical criteria adopted. The 
bringing together of the various phenomena, so that some sort 
of systematic relationship is made to exist among them all, is 
the initial step in raising the particular department of knowl- 
edge to the dignity of a science. As progress is made, a 
gradual evolution takes place in the fundamental grouping of 
the facts. In the beginning, a classification, rude though it 
may be, is fashioned according to the superficial features, 
which are most striking at first glance. It is, at a later stage, 
modified to one in which similarity of characters, irrespective 
of natural relations, is taken into account. A vastly more 
advanced conception is classification based upon affinity, in 
which, for similarity of features, there is substituted similarity 
of plan. The final stage is the causal, in which origin and the 
processes become the dominant and determining factors. 
In the expansion of the multifaceted science of geology, the 
classification of the phenomena presented has been no excep- 
tion to the rule. In the department of stratigraphy, that part 
of the general subject which has to do with the history of the 
changes which have taken place in the lithosphere, that part 
in which we find a measure of geological time, and in which we 
determine the sequence of geological events, there has been 
the same growth as in the other branches of the science. As 
in those other branches various standards of comparison have 
given away, one after another, to new standards more in accord 
with the general advancement of human knowledge, so also in 
stratigraphy has there been a passage from one criterion to 
another. In the successive replacements, however, of one set 
of criteria by another, the abandoned ones have not always been 
found to be altogether wrong; and they usually continue to 
exert a more or less profound influence long after they are 
thought to be forgotten. These various classifications, based 
