= See 
328 STRUCTURAL AND FIELD GEOLOGY 
conduct of the lines below the surface. In the case shown in 
Fig. 124 the strata between @ and 6 are obviously sym- 
metrically folded. We, therefore, should be justiiediiia 
continuing the dips of the synclinal strata downwards for 
some distance at the same angle as they show at the surface, 
and then gradually cause the degree of inclination to diminish 
until the beds should become horizontal in the core of the 
synclinal trough. If the rock-folds, instead of being “open” 
as shown in the diagram at a, were closely compressed as 
between 0 and «, the curves of synclinal and anticlinal cores 
would be more or less sharply angular; and in our section we 
should have to continue the limbs of a syncline downwards 

FIG. 124.—DIAGRAM-SECTION. 
Vertical lines=heights above datum-line (d). Continuous lines=outcrops. Interrupted and 
dotted lines =inferred direction of strata below the surface. 
for a relatively greater distance before the trough core was 
reached. In short, when gently inclined strata dip towards 
each other at approximately the same angle, we may be sure 
that the inclination of the limbs will rapidly lessen as they 
approach the trough core, while in the case of steeply inclined 
and closely compressed unsymmetrical folds, the limbs of a 
syncline must descend to a relatively greater depth before 
they meet. 
In regions which have not been topographically sur- 
veyed, or the maps of which, if such exist, give very few 
elevations, the. geologist must, of course, do his own level- 
ling, if he desires to plot a section on a true scale. In such 
a case he may select any line for the base of his section— 
either the sea-level, the surface of some lake, or the bottom 
of some valley, etc., or he may prefer to erect his sectigg 


