THE VALUATION OF UNCONFORMITIES 209 
Traced toward the old land nucleus, the many little unconformities 
merge into a few. and finally into one all-inclusive unconformity. 
Traced seaward, the sedimentary wedges of the record expand into 
continuous piles of marine strata; for over much of the oceanic 
abysses sedimentation has probably been uninterrupted since the 
Archean period or before. 
Our more familiar unconformities are to be thought of, then, as 
temporary expansions or wedge-shaped extensions of greater uncon- 
formities, and we must not be surprised if, when traced in one direc- 
tion, they dwindle to nothing,* or if in another direction they expand 
so as to swallow up the entire sedimentary record. 
SUMMARY 
In the preceding discussion, the writer seeks to show that the 
words “great” and “slight” as applied to unconformities are often 
ambiguous and in need of definition; that, where these things can be 
determined, it is important to know whether the structural discordance, 
or the stratigraphic hiatus, or the duration of erosion, is the thing that 
is great or small. 
It appears that the stratigraphic hiatus or lost record is not neces- 
sarily a measure of the time which elapsed while the unconformity 
was being produced. ‘The two may be nearly equal, but on the other 
hand the lost time may be much less than the lost record. It cannot 
well be greater. 
Also, all three factors change from place to place—the discordance 
and hiatus often suddenly and capriciously, the time-value usually 
more gradually. 
Many, if not most, unconformities are merely lateral extensions 
of much more persistent unconformities. ‘The main unconformity 
denotes a very long duration of terrestrial erosive conditions, while 
the projecting wedges record the backward and forward migrations 
of belts of sedimentation around the borders of that land. 
The entire geologic record then is not to be conceived of as a pile 
of strata, but as a dovetailed column of wedges, the unconformities 
and rock systems being combined in varying proportions. The 
former predominate in some places and periods, while the latter 
prevail in others. 
t This conception is admirably explained by Chamberlin and Salisbury in Geology, 
IT, chaps. iv, v. 
