disturbed strata must necessarily pass. We see it in its youth in such mountains as the Alps, the Himalayas, the Cordilleras, and in its old age in the Appalachians and the mountains of Scandinavia and Britain. Let us now briefly consider some of the typical kinds of structure presented by the more steeply inclined strata. In regions of moderately inclined rocks the folds, as we have seen, are symmetrical anticlines and synclines. the axes of which are vertical, the beds AAAAN FIG. 27. ISOCLINAL FOLDS. Axes moderately inclined from the vertical. dipping away from or towards the axes at approximately equal angles. (See Fig. 22, p. 87.) Folds of this kind, however, are not restricted to areas of moderately inclined strata; they are met with also in regions where the rocks as a rule dip steeply. But in such regions the anticlines and synclines are usually more or less unsymmetrical—their axes are inclined. In Fig. 27 we have represented a series of moderately inclined folds. In Fig. 28 the inclination of the axes is still greater. As the folds in these two diagrams all lean in one direction, they are said to be isoclinal. Very frequently the inclination of the axes increases to such a degree that one fold may come to lie almost hori zontally upon another, as in Fig. 29. But when the axes are so highly inclined as that the folds usually tend to become disrupted. All folds are the result of horizontal push or tangential pressure, and when this is very great they may yield by shearing, and one limb be thrust forward over the other, producing what is known as a reversed fault. (Figs. 30, 31.) So overpowering has been the horizontal movement in some cases that masses of rock thousands of feet in thickness have been buckled up and sheared, or, simply yielding to pressure, have sheared without folding, and been thrust forward for miles along a FIG. 30. OVERFOLD PASSING INTO REVERSED FAULT OR OVERTHRUST. gently inclined or even an approximately horizontal plane. These great reversed faults are termed overthrusts or thrust-planes. Sometimes such thrust planes occur singly (Figs. 32, 33), at other times the rocks have yielded again and again, great sheets having been sliced off successively and driven forward one upon the other. (Fig. 34.) Another structure encountered in regions of much disturbed strata is the synclinal double-fold, shown in the annexed diagram. (Fig. 35.) In this case two anticlinal folds approach each other from different directions, the synclinal depression between the approximating anticlines being occupied by highly convoluted strata. The converse of this structure is the anticlinal doublefold as shown in Fig. 36. Here two synclinal folds DI D2, Lower and Upper Devonian; C, Carboniferous Limestone; Cr, Cretaceous; T Overfold and thrust-plane. Devonian and Carboniferous strata turned upside down above the thrust-plane. approach each other, while in the intervening space the strata are arched into a great anticline. The beds within the anticline, it will be observed, are much compressed below, while they open out above. This is known as fan-shaped structure. Reverse faults and thrust-planes have been referred to, but it must be noted that normal faults also now and again occur in complicated regions. The former, as we have seen, are the result of horizontal, the latter of vertical movements of the crust. Reversed faults, therefore, are almost entirely restricted to regions |