

294 STRUCTURAL AND FIELD GEOLOGY 
however, that the mutual approach or retreat of the out- 
crops isnot due ‘either to the form of the groune jogaee 
increase or decrease in the degree of inclination, then we 
may conclude that it is owing to an actual thinning-away 
or thickening-out of the intermediate strata. 
Unconformity.—This structure is readily revealed by 
mapping the ground, even although it may never be shown 
in any actual section. The accompanying diagram (Fig. 
113) represents the ground-plan of an unconformity. Here 
there are two series of strata inclined in different directions 
—one set is said to “strike at or against” the other. It is 
obvious that the series A cannot possibly belong to the 
ONG ae 
eee! 
new = ot 
. 

FIG. I112.—GROUND-PLAN OF AN UNCONFORMITY. 
Continuous lines=outcrops and boundaries exposed in section. Interrupted lines=inferred 
positions of outcrops and boundaries. Stippling=rocks exposed at surface, 
series B. There is no room, so to say, for the beds A to 
swing round (between @ and x) and dip underneath B. 
The junction between the two series must, therefore, be 
discordant, and, if not due to faulting, can only indicate 
an unconformity. If the observer have reason to suspect 
an unconformity, he must carefully look for such evidence 
as is referred to in Chapter XII.—where the phenomena of 
unconformity and overlap are described. 
Overlap is not readily shown upon a plan except when it 
accompanies well-marked unconformity. Mapping almost 
invariably discloses the structure, however, when it occurs on 
a considerable scale. Small local overlaps may readily be 
overlooked, but when the structure characterises a wide area 
