132 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 tBuXonomic 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 j)art 

 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 



