IOWA ACADEMY OP SCIENCES. 147 



with other aberrant phenomena, are generalized, and from them, in con- 

 nection with the normal deposits, the conditions of genesis {i. c , the atti- 

 tude of land, proximity of rivery, etc.) are inferred. By these means a 

 tangible and definite picture of the topography, geography and geologic 

 agencies of the area is produced: and the various inferred features are 

 tested by their consistency and the inconsistent eliminated or withheld for 

 more extended comparison. Then the history of the contiguous area is 

 wrought out in similar fashion and the episodes are compared severally 

 and jointly, and the deposits and unconformities are interpi^eted in the 

 light of this comparison. The comparison is eventually extended to other 

 portions of the province and to the eontiguous provinces, and in each area 

 the significance of the sum of phenomena is sought and the inferred his- 

 tories are generalized by combination of the congruous and elimination of 

 the incongruous until finally the history of a given period throughout the 

 entire province is interpreted in terms of episodes each inferred from the 

 sum of phenomena representing the period." 



As orig'inally suggested correlation by homogeny had long 

 been in general use, in one phase or another, but the method 

 had lacked definite formulation. Its main distinctive features 

 were in emphasizing the importance of the recognition of a 

 commitant period of land degradation, with each period of 

 deposition and in the inference of the agencies from the 

 materials of deposition. As set forth in its latest form, the 

 theory has been so expanded that its vei^y name loses its 

 significance and becomes a misnomer. Instead of designating 

 a method of correlation it is a synonym of geological history 

 itself. Its foundation is entirely new from what it was in the 

 beginning and its distinctions are taken almost out of the realm 

 of observational science and are placed in the metaphysical. 



In its most acceptable form, it is a special case of a more 

 general proposition, in which refinement of determination is 

 carried out far beyond a point where the method can be of 

 general utility in geological work. With the older formations 

 its use will be very limited. It is better adapted to the latest 

 deposits, but even among these its olfice will be necessarily 

 restricted, for with its practical use there is postulated com- 

 paratively little effacement of the geological record of the 

 region. Moreover, it is a method that is local in application 

 and not general. As in the cases of lithological similarity, the 

 biotic, and most other methods that have been commonly used, 

 it makes no provision for correlation of provinces the geologi- 

 cal histories of which have been not similar, but dissimilar. 



Historical Similarity. — Practically, this method has been in 

 use for a long time, though not always clearly emphasized. 



