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. 
Community of Genesis . — 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,! 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 application 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 .Jour. Sci., (3), Vol. XL, p. 36, 1890. 
•VCong. gdol. international, 5me Sess., p. 164, 1893. 
