290 W. O. CROSBY 
slow, long periods of geologic time being, for the particular region, 
virtually, though not absolutely, unrecorded; but such an apparent 
or even actual gap in a sedimentary series is consistent with perfect 
conformity, for the one absolute essential of a conformable series 
is that it be in no part a record of erosion. 
Unconformity, on the other hand, implies a true hiatus, an 
actual interruption of deposition by erosion, followed by further 
deposition. The erosion may be terrestrial (peneplanation) or 
littoral (marine planation). But it is necessary to emphasize the 
importance of the time break in order to exclude from consideration 
the localized erosion, both terrestrial and marine, due to shifting 
and variable currents and often closely accompanying deposition 
in both space and time. This phenomenon, known as contempo- 
raneous erosion and deposition, giving rise locally, in one and the 
same sedimentary series, to numberless examples of the appear- 
ance, but never the reality, of true stratigraphic unconformity, is 
best relegated, with cross-bedding or current lamination, to the 
category of the irregularities of stratification. The one is not true 
unconformity any more than the other is true delta structure. 
The paramount, the vital or dynamic, interest of unconformity is 
found in the clear and unquestionable proof which it affords of a 
double interchange of land and sea, or at least of wide or general 
areas of erosion and of deposition. An important time break or 
hiatus is a necessary implication; and thus arises, somewhat inci- 
dentally, the great value of unconformity in stratigraphic demarka- 
tion and classification. 
In order to avoid other possible misconceptions, it is needful, 
also, that attention be directed particularly to the fact that both 
conformity and unconformity are absolutely definite structures, 
definite in the sense that each is always sharply localized strati- 
graphically and may be predicated of a particular stratigraphic 
plane or contact. In other words, the student may put his finger 
on a definite line and say, ‘‘ Here is conformity”’ or “‘ Here is uncon- 
formity,” as the case may be. This point is, perhaps, clearest for 
unconformity; but it must be obvious, on reflection, that conformity 
does not require that the top and bottom of a series should be 
parallel. Conformity exists throughout if at each line of stratifi- 
