Pre HENRY SHALER WILLIAMS 
We are thus forced by the study of the facts, to the opinion that 
the horizon which is thus drawn on the basis of a change of the fossil 
faunas in the strata, while it represents at any one spot a definite 
point in geologic time, does not represent the same point of time in 
the sections of separate regions. 
We are warned by the reappearance of many of the species of the 
Independence shale of Iowa in the fauna at the top of the Iowa 
Devonian series (Lime Creek shales), against the assumption that the 
same horizon is always indicated by the same fauna.t Correlation 
by faunas is always subject to the criticism that the mere presence of a 
fossil indicates only some point of time during the life history of that 
species. Nevertheless, in a restricted region, such as the one now 
under investigation, the first appearance in the stratigraphic column 
of a definite fauna, defines a definite geologic horizon with a high 
degree of precision. And so long as correlations are extended from 
the typical outcrop within the same geologic basin, the transition from 
one fauna to the next higher may be depended upon as marking the 
same point in the faunal histories whether they be considered as 
synchronous or only homotaxic. 
t Calvin, Iowa Geol. Surv., Vol. VIII, p. 222. 
