a thrust. Thrusts can only occur in areas of compression, and are of stupendous dimensions in old mountain lands. The most famous area for thrusts is in the north-west Highlands of Scotland, where segments of the earth's crust have been pushed over one another, as the scales of half-congealed wax may be pushed over one another on a polished table, only in the Highlands the scales have ridden over one another with a
Fig. 34. Fold in the Zwartberg mountains, Angelier's Bosch, near Prince Albert, Cape Colony. It will be noticed that the lower portion of the fault has been thrust under the upper; it is therefore a "lag fault".
displacement of as much as 10 ml. The term lag fault is applied to the unusual case where the lower portion of the fold is pushed forwards under the upper.
Outcrop.—Rocks laid down originally in horizontal layers may be folded and faulted. The tops of the folds and faults may be planed off by weathering and erosion, or cut into peak and valley. The beds, therefore, appear at any one place tilted, the angle of tilt being measured from the horizontal, and called the dip. As one follows any one bed over hill and valley it will be exposed in a certain undulatory line, according to the
