IOWA ACADEMY OP SCIENCE 
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were deposited upon their upturned edges. The completed cycle of mountain- 
making is the measure of orotaxial chronology. The division-planes cutting 
the geological column into series, terranes and the smaller subdivisions are 
actually, as well as theoretically, the lines of unconformities and their represen- 
tatives. In the cases of the more extensive features of discordant sedimenta- 
tion they represent no doubt base-leveled surfaces, or peneplains. 
As a concrete illustration there is probably no better one than that pre- 
sented by the stratigraphy of the southern end of the Rocky mountains. There 
are in the geologic column more than a score of well defined terranes having 
the taxonomic rank of series. With possibly one or two exceptions they are all 
separated from one another by marked planes of unconformity. The most exact 
means of correlation over wide areas are given. Of not the least interest are 
the comparisons that are able to be made with the geologic sections both to the 
eastward in the Mississippi valley and to the westward in the Great Basin 
region. 
Figure 1. Oscillation of Continental Sliore Lines. 
In the domain of geologic correlation the most important deduction of recent 
years is that there is an erosional history of continental borders equally sig- 
nificant as that expressed in the sedimentary record. At best the sediments 
present only one-half of the stratigraphic history. The historical testimony 
of the fossils is not only fragmentary so far as the sediments in which they 
occur are concerned but it gives no suggestion of the erosional sequence of 
events which are of even greater consequence. 
If we represent in diagram (Fig. 1) an ideal cross-section of a continental 
border, complete sedimental history stands at one side (section A), and com- 
plete erosional history at the other side (section C). In terms of the sedi- 
ments the latter is an hiatus, or as the older text-books on geology call it a 
time-gap. The section B represents about as much of the complete history as 
the fossils ordinarily record. In reality the history which organic remains 
portray is merely that of faunal sequence, with no necessary relationships of 
physical episodes suggested; in its entirety it is only a small and imperfect 
fragment of the actual record of geologic events. 
The zig-zag line may be further taken as indicating the oscillations of a 
shore-line; and also the course of the migration of a specific fauna during geolo- 
