=The elements of folds.=—Folds must always be thought of as having extension in each of the three dimensions of space (Fig. 23), and not as properly included within a single plane like the cross sections which we so often use in illustration. A fold may be conceived of as divided into equal parts by a plane which passes along the middle of either the arch or the trough, and is called the _axial plane_. The line in which this plane intersects the arch or the trough is the _axis_, which may be called the _crestline_ in an anticline, and the _troughline_ in a syncline. In the case of many open folds the axis is practically horizontal, but in more complexly folded regions this is seldom true. The departure of the axis from the horizontal is called the _pitch_, and folds of this type are described as _pitching folds_ or _plunging folds_. The axis is in reality in these cases thrown into a series of undulations or “longitudinal folds”, and hence pitch will vary along the axis. [Illustration: FIG. 23.—Anticlinal and synclinal folds in strata (after Willis).] [Illustration: FIG. 24.—Diagrams to illustrate the different shapes of rock folds.] =The shapes of rock folds.=—By the axial plane each fold is divided into two parts which are called its _limbs_, which may have either the same or different average inclinations. To describe now the shapes of rock folds and not the degree of compression of the district, some additional terms are necessary. Anticlines or synclines whose limbs have about the same inclinations are known as _upright_ or _symmetrical folds_. The axial plane of the symmetrical fold is vertical (Fig. 24). If this plane is inclined to the vertical, the folds are _unsymmetrical_. So soon as the steeper of the two limbs has passed the vertical position and inclines in the same direction as the flatter limb, the fold is said to be _overturned_. The departure from symmetry may go so far that the axial plane of the fold lies at a very flat angle, and the fold is then said to be _recumbent_. The observant traveler by train along any of the routes which enter the Alps may from his car window find illustrations of most of these types of rock folds, as he may also, though generally less easily, in passing through the Appalachian Mountains. [Illustration: FIG. 25.—Secondary and tertiary flexures superimposed upon the primary ones.]