THE VALUATION OF UNCONFORMITIES 203 
the two sections, the time-value is very different, being equivalent to 
five to seven periods of geologic history in the Georgian region, but 
to only one period, or a fraction of a period, in Wyoming. 
The same thing may be brought out by examining one of these 
two unconformities at different points. Fig. 1 represents diagram- 
matically the pre-Oligocene unconformity of the Black Hills region. 
In a section taken at (A) horizontal Oligocene silts (solid black) 
rest on folded Algonkian slates and granite; at (B) upon tilted 
Permian shales, and at (C) upon horizontal late Cretaceous sandstone 
and shale. Judged from the standpoint of structural discordance, the 
unconformity is very great at (A), moderate at (B), and.nil at (C). 
Regarded from the basis of stratigraphic hiatus, it is greatest at 
(A), less at (B), and least, although still noteworthy, at (C). But 
the time-value is probably much the same at all three points. The 
PEL A 
HL Hh 
Cc B 
Care, Hn 
= LUG AGLI 
Fig. 1.—Pre-Oligocene unconformity in the Black Hills. The section is dia- 
grammatic and generalized. 
history of the region appears to be roughly this: Sedimentation was 
continuous from the Cambrian to the close of the Cretaceous period 
save for temporary episodes of erosion in mid-Paleozoic, and 
Jurassic times. No deformation attended these early changes, and 
the final result of the deposition was a thick blanket of strata lying 
horizontally across the region of the Great Plains. At the close of 
the Cretaceous period a low dome was bulged up, and during the 
Eocene the top of this was beveled off so that the pre-Cambrian 
rocks were exposed within encircling rims of younger beds. To 
this epoch of erosion the entire unconformity under discussion is due; 
and it would seem therefore that the time-value of the break is to be 
measured in this way rather than by the time-equivalent of the strata 
which are missing in any one section. 
It appears, then, that unconformities seen in isolated sections 
may be prominent or obscure structurally, that they may represent 
a large or a small gap in the sedimentary column, and that they may 
