IOWA ACADEMY OP SCIENCES. 
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with other aberrant phenomena, are generalized, and from them, in con- 
nection with the normal deposits, the conditions of genesis [i. e , 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 interpreted in the 
light of this comparison. The comparison is eventually extended to other 
portions of the province and to the contiguous 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 originally 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 very 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 office 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. 
