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

$

c.

Cr.

CL.

FIG. 33. SECTION ACROSS COAL-Basin of MonS. (M. Bertrand.) D' D', 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

N

QUINAIG

FIG. 34. SECTION FROM QUINAIG TO HEAD OF GLENBEG. (Geol. Survey.)

1, Lewisian gneiss; 2, Torridon sandstones (Pre-Cambrian); 3, 4, lower and upper quartzite; 5, fucoid beds; 6, serpulite grit ;
7, limestone; (3 to 7 Cambrian); TT, thrust-planes.

97

FIG. 35. SYNCLINAL DOUBLE-FOLD,

where the rocks are more or less steeply inclined and contorted. Normal faults, on the other hand, occur under all conditions of rock-structure-traversing alike horizontally arranged strata and inclined and folded beds of every kind.

So much, then, for the general types of structure met with among highly folded strata. So far as our present knowledge goes, complex folding, such as is

seen in true mountains of uplift, has resulted from horizontal movement in one direction. This is shown by the manner in which most of the more closely compressed and steeper folds of a mountain-chain tend to lean over one way. Under the influence of an irresistible horizontal thrust the strata find relief by folding, and the crust bulges upwards, the flexured. rocks naturally bending over in the direction of least resistance. The resulting structure may be shown diagrammatically as in Fig. 37. In this diagram only