by the intrusion of igneous rocks, exert sufficient thrust to form mountains.
Fig. 29.—Diagram showing why a Normal (tension) Fault must always Hade towards the Downthrow as in A. In B, had the central block merely fallen in, there would be spaces between it and the enclosing segments. As there are no spaces the segments on either side must have pushed over the central block. The fractures in B, therefore, are reversed or thrust (pressure) faults, and the faults hade towards the upthrow.
Faults.—The earth's crust is like an immense arch spanning the central nucleus. If one pulls a little apart the two sides of an arch, the keystone will fall downwards; if the earth's crust be pulled apart, a strip representing the keystone will fall downwards. The block that has sunk is said to be faulted, and the breaks on either side are called faults. In the matter of folds we saw that the beds became inclined, and the inclination or dip was measured from the horizontal; that is because folds are most conspicuous in mountains and on the surface of the earth, where the most important plane by which we orientate ourselves is the horizontal. Faults, on the other hand, are seldom very well marked on the surface,
