146 IOWA ACADEMY OF SCIENCES. 



were of very wide, and in other cases of very limited extent, 

 and, in the geological classification of the non-fossiliferous 

 rocks of a whole province, to propose a plan in which uncon- 

 formities occupied a prominent place Short though the 

 period has been, since Irving 's time, there has sprung into 

 existence a new department of geological inquiry, that not 

 only reads later geological history in the geographical forms 

 presented, but gives an entirely new insight into the real 

 significance of uncomformable relations between the older 

 rock masses. The bringing in of the geographic aids, to 

 unravel stratigraphy, finds a hearty support and a wide 

 expression. It is in the extension of Irving "s theme, as out- 

 lined under the guidance of modern geographic interpretation, 

 that stratigraphy is believed to have found a rational and 

 practical method of correlation and classification that, in its 

 fundamental concepts, is entirely independent of the usual and 

 almost universal paleontologic standard. The specific appli- 

 cations are referred to in another place. 



('o)itjiitmlty of Geneais. — Correlation by community of genesis 

 is a "simple application of the well known principles (1), that 

 geologic processes may be inferred from their products, and 

 (2), that geologic processes are universally inter-related." It 

 is a method that was elaborated by McGee* for the more 

 recent deposits of the coastal plain of eastern United States. 

 In its more mature statement, f correlation by this principle 

 "becomes a juxtaposition of episodes or is a correlation by 

 historical similarity. " 



"The applicatioQ of this mode of correlation involves such astudy of 

 agencies and conditions of geologic action as to enable the geologist to 

 determine provisionally the origin of each phenomenon examined, whether 

 deposit or topographic feature, formation or land form: and the subsequent 

 comparisons involved in the correlation are comparisons of genetic records, 

 which may be made in such manner as to eliminate the incongruous and 

 preserve the congruous, and thereby develop a consistent history for the 

 entire province under examination. This method has already been charac- 

 terized as homogenic, i. e., correlation by homogeny, or identification by 

 origin. 



' ' In the practical app' ication of the method, the deposits of given sections 

 and circumscribed areas are first correlated empirically by visible con- 

 tinuity and lithologic similarity, and to some extent by similarity of 

 sequence, in order that their relations may be generalized; next, the 

 agencies of genesis are inferred from the materials of the deposits viewed 

 individually and collectively; then the unconformities and pebble-beds, 



* Am .lour. Sci., (3), Vol. XL, p. 36, 1890. 



tCong geol. ioternational, Sme Sess., p. 164, 1893. 



