
STRUCTURES DUE TO DENUDATION 181 
interruption of sedimentation is hard to detect in individual 
exposures or sections. But when the strata are traced over 
some considerable area the actual discordance will be shown 
by the manner in which the upper gradually steals over the 
outcrops of the lower series. In cases of this kind the 
presence of an unconformity is often indicated by the occur- 
rence of rolled or angular fragments of the lower rocks 
enclosed in the lowest bed or beds of the upper series. 
Indeed, conglomerate and grit frequently appear along every 
kind of unconformable junction. Again, the presence of 
dykes of igneous rock and dislocations in the lower series 
and their absence from the overlying beds—dykes and faults 
terminating abruptly at a given line of junction—would be 
convincing evidence of a “break in the succession” (see 
Fig. 59). For it is highly improbable that two or more dykes 
should terminate upwards at exactly the same level; while 
we may feel assured that if the dislocations visible in the 
lower beds do not extend into the overlying strata, the latter 
must be resting upon a denuded surface. 
When none of these incidental proofs of unconformity is present, the 
evidence of fossils may yet be available. The assemblage of fossils 
occurring in the lower beds may be more or less strongly contrasted 
with that of the overlying series, and so lead to the conviction that the 
appearance of conformity is deceptive. Such an abrupt break in the 
continuity of life-forms is termed by paleontologists a “break in 
succession,” and indicates a gap or imperfection in the record, which 
usually implies a long lapse of time. We must allow for the gradual 
extinction of the old forms of life occurring in the lower beds, and for 
the gradual introduction of the different series of types which appear in 
the upper beds. In short, the apparent conformity in such a case is 
deceptive—it is in reality an unconformity. 
Usually, however, unconformity is marked by some dis- 
cordance of inclination—one set of beds often resting upon 
the upturned and denuded edges of an older series (see Figs. 
60, 61). Thus the lower beds may be inclined and the 
overlying strata horizontal; or both may be inclined in the 
same or in different directions. Strongly marked discord- 
ances of this kind are not hard to trace, even when there is 
no section to show the actual junction of the two sets of 
strata. 
Conformity, as a rule, indicates more or less continuous 

ge re mae ee 

‘ 
! 
————— 
