The imaginary line passing longitudinally through a fold, about which the strata appear to be bent, is the _axis_; and the plane lying midway between the two sides of a fold and including the axis is the _axial plane_. The two principal kinds of folds are the _anticline_ (Fig. 18, _A_), where the strata dip away from the axis; and the _syncline_ (Fig. 18, _B_), where they dip toward the axis. They are commonly, but not always, correlative, like hill and valley. Rock-folds are of all sizes, from almost microscopic wrinkles to great arches miles in length and breadth, and thousands of feet in height. The smaller folds, or such as may be seen in hand specimens and even in considerable blocks of stone, are commonly called contortions, and it is interesting to observe that they are, in nearly everything except size, precisely like the large folds, so that they answer admirably as geological models. Large folds, however, are almost necessarily curves, but contortions are frequently angular (Fig. 19). With folds, as with waves, the small undulations are borne upon the large ones; but the contortions are not uniformly distributed. An inspection of Fig. 18 shows that when the rocks are folded they must be in a state of tension on the anticlines (_A_), and in a state of compression in the synclines (_B_), and the latter is evidently the normal position of the puckerings or contortions of the strata, as shown in Fig. 20. Contortions are also most commonly found in thin-bedded, flexible rocks, such as shales and schists. And when we find them in hard, rigid rocks, like gneiss and limestone, it must mean either that the structure was developed with extreme slowness, or that the rock was more flexible then and possibly plastic. [Illustration: Fig. 19.—Contorted strata.] [Illustration: Fig. 20.—Contorted syncline.] [Illustration: Fig. 21.—Section of anticlinal mountains.] It is very interesting to notice the relations of anticlinal and synclinal folds to the agents of erosion. At the time the folds are made, the anticlinals, of course, are ridges, and the synclinals, valleys, and this relation sometimes continues, as shown in Fig. 21; but we have seen that the rocks in the trough of the synclinal are compressed and compacted, _i.e._, made more capable of resisting erosion, while those on the crest of the anticlinal are stretched and broken, _i.e._, made more susceptible of erosion. The consequence is that the anticlinals are usually worn away very much faster than the synclinals; so much faster that in many cases the topographic features are completely transposed, and in place of anticlinal ridges and synclinal valleys (Fig. 21) we find synclinal ridges and anticlinal valleys (Fig. 22). [Illustration: Fig. 22.—Section of synclinal mountains.] [Illustration: Fig. 23.—Monoclinal fold.] [Illustration: Fig. 24.—Unsymmetrical and inverted folds.]