4. The uplifting force by which the fracture is produced has frequently raised the rock on one side higher than it has on the other. This is called a fault. ([Fig. 50.] The unequal movements by which the fault is produced seem in some instances to have been repeated several times, and the grinding of the broken edges upon each other has polished and striated the sides of the fracture.
5. Sedimentary rocks are often found with the planes of their strata more or less inclined. It is evident that they were not thus formed. The depositions of sediment from water will always be horizontal, or, at most, only slightly inclined. But there is often evidence in the rock itself that its strata were once horizontal. It is frequently observed that vertical strata contain pebbles with their longer axes in the plane of the strata. ([Fig. 51.] When these pebbles were deposited, the longer axes would take, on an obvious mechanical principle, a horizontal position. Their present vertical position must have resulted from a change in the position of the strata in which they are enclosed. The same thing is shown by the position of a petrified forest in the south of England, known as the Portland dirt-bed. Some parts of it are inclined at an angle of forty-five degrees. The position of the vegetable remains ([Fig. 52]) shows that when they were growing the surface was horizontal.
The line b d ([Fig. 53]), on inclined strata which makes with the horizon the greatest angle, is called the direction of the dip. The angle thus formed (a b d) is the angle of inclination. When inclined strata come to the surface, the exposed edge, b c, is the outcrop, and the line of outcrop on a horizontal surface is called the strike of the strata.