144 
IOWA ACADEMY OF SCIENCES. 
scarcely yet capable of unraveling. How infinitely greater are 
the difficulties when not only space is to be considered but dis- 
tribution in time as well, and all with material that at best is 
but fragmentary. No biologist would have attempted it. 
Unconformity . — In its widest sense an unconformity is any 
discordance in sedimentation in which younger beds reposing 
upon older rocks give evidence of no direct connection, of 
interrupted deposition, or of a change in the prevailing physi- 
cal conditions whereby nonparallelism is developed in the 
stratification planes of the two formations. The phenomenon 
carries with it the idea of more or less pronounced warping of 
the older strata before the younger are ] aid down. In a some- 
what narrower sense, and in the one that it is perhaps most 
generally used, unconformity implies a tilting of the strata, their 
elevation above sea-level, and subjection to more or less pro- 
found erosion before being covered by the younger sediments. 
The irregularity in the juncture between two uncomformable 
formations is, in the majority of cases, a well defined line 
denoting a break in the continuity of conditions. In Europe, 
where modern stratigraphical science originated, the time-gaps 
indicated by unconformities have been regarded chiefly from a 
biological rather than a physical standpoint. The full sig- 
nificance of the interruption in sedimentation has not been 
appreciated so much in its bearing upon the conditions which 
could have given rise to such effects, as in its production of a 
well-marked hiatus in the faunal successions, or rather, by the 
introduction of entirely new faunas, perhaps, in the younger 
beds above. In the European countries also, the abrupt faunal 
breaks have been at the foundation of the separation of geo- 
logical history into its grander divisions, and of the strata into 
systems. In America the attempt to transfer the classification 
of Europe has not proved so successful, and the application of 
the same principles does not find the great gaps in the same 
places. The inferences are obvious. • 
As a purely physical feature, application of unconformity to 
anything smaller than the great systems, has usually been sub- 
ordinated to the faunal or lithological criteria. It has been 
done, however, in a few cases with success; though not in such 
a way as to attract very much attention. 
Although marked physical breaks in the continuity of sedi- 
mentation have, from time to time, received some attention in 
