The inclination of a bed of rocks to the horizon is its DIP. The amount of the dip is the angle made with a horizontal plane. The dip of a horizontal layer is zero, and that of a vertical layer is 90 degrees. The direction of the dip is taken with the compass. Thus a geologist's notebook in describing the attitude of outcropping strata contains many such entries as these: dip 32 degrees north, or dip 8 degrees south 20 degrees west,--meaning in the latter case that the amount of the dip is 8 degrees and the direction of the dip bears 20 degrees west of south. The line of intersection of a layer with the horizontal plane is the STRIKE. The strike always runs at right angles to the dip. Dip and strike may be illustrated by a book set aslant on a shelf. The dip is the acute angle made with the shelf by the side of the book, while the strike is represented by a line running along the book's upper edge. If the dip is north or south, the strike runs east and west. FOLDED STRUCTURES. An upfold, in which the strata dip away from a line drawn along the crest and called the axis of the fold, is known as an ANTICLINE. A downfold, where the strata dip from either side toward the axis of the trough, is called a SYNCLINE. There is sometimes seen a downward bend in horizontal or gently inclined strata, by which they descend to a lower level. Such a single flexure is a MONOCLINE. DEGREES OF FOLDING. Folds vary in degree from broad, low swells, which can hardly be detected, to the most highly contorted and complicated structures. In SYMMETRIC folds the dips of the rocks on each side the axis of the fold are equal. In UNSYMMETRICAL folds one limb is steeper than the other, as in the anticline in Figure 167. In OVERTURNED folds one limb is inclined beyond the perpendicular. FAN FOLDS have been so pinched that the original anticlines are left broader at the top than at the bottom. In folds where the compression has been great the layers are often found thickened at the crest and thinned along the limbs. Where strong rocks such as heavy limestones are folded together with weak rocks such as shales, the strong rocks are often bent into great simple folds, while the weak rocks are minutely crumpled. SYSTEMS OF FOLDS. As a rule, folds occur in systems. Over the Appalachian mountain belt, for example, extending from northeastern Pennsylvania to northern Alabama and Georgia, the earth's crust has been thrown into a series of parallel folds whose axes run from northeast to southwest (Fig. 175). In Pennsylvania one may count a score or more of these earth waves,-- some but from ten to twenty miles in length, and some extending as much as two hundred miles before they die away. On the eastern part of this belt the folds are steeper and more numerous than on the western side.