
182 STRUCTURAL AND FIELD GEOLOGY 
sedimentation or accumulation—a persistence, upon the 
whole, of the same physical conditions. It does not, 
however, prove that the area of deposition was stable. On 
the contrary, a “thick 
series of conformable 
strata of shallow-water 
- origin could only have 
+ been accumulated during 
gradual subsidence of 
Fic. 60.—STRONG UNCONFORMITY. the area. The evidence 
supplied by palzonto- 
logical “breaks in the succession,’ further shows that the 
accumulation of apparently conformable strata has sometimes 
been interrupted for prolonged intervals of time. But these 


Fic. 61.—T wo UNCONFORMITIES. 
are really cases of unconformity, and do not invalidate the 
general rule that true conformity indicates a persistence of 
the same physical conditions. 
It must be remembered, however, that conformable strata 
have not necessarily accumulated during one continuous 
movement of subsidence. As already pointed out (see p. 110), 
both downward and upward crustal movements may take 
place during the deposition of a long series of perfectly 
conformable strata, and these changes may sometimes lead 
to longer or shorter pauses in the process of accumulation. 
While, therefore, it holds generally true, that conformity is 
the result of more or less continuous sedimentation, we must 
allow for such interruptions of the process as those discussed 
in Chapter VII. 
Unconformity, on the other hand, obviously implies an 
interruption of sedimentation or accumulation, and the 
supervening of erosion and denudation—or, in other words, 
a change of physical conditions, [In short, unconformity 

