If the axes of folds are absolutely horizontal, and the surface of the earth be represented as a plain, the lines of intersection of the truncated strata with the ground, or with any horizontal surface, will give the directions of continuation of the individual strata. This strike direction is usually determined at each exposure by use of a compass provided with a spirit level. When that edge of the leveled compass which is parallel to the north-south line upon the dial is held against the sloping rock stratum, the angle of strike is measured in degrees by the compass needle. If the cardinal directions have been placed in their correct positions upon the compass dial, the needle will point to the northwest when the strike is northeast, and _vice versa_ (Fig. 28 _a_). Upon the geologist’s compass it is therefore customary to reverse the initials which represent the east and west directions, in order that the correct strike may be read directly from the dial (Fig. 28 _b_). [Illustration: FIG. 28.—Diagram to illustrate the manner of determining the strike of rock beds at an outcropping. _a_, a compass which has the cardinal directions in their natural positions; _b_, a compass with the east and west initials reversed upon the dial; _c_, home-made clinometer in position to determine the dip.] By the dip is meant the inclination of the stratum at any exposure, and this must obviously be measured in a vertical plane along the steepest line in the bedding plane. The dip angle is always referred to a horizontal plane, and hence vertical beds have a dip of 90°. The device for measuring this angle of dip, the _clinometer_, is merely a simple pendulum which serves as an indicator and is centered at the corner of a graduated quadrant. A home-made variety is easily constructed from a square piece of board and an attached paper quadrant (Fig. 28 _c_), but the geologist’s compass is always provided with a clinometer attachment to the dial. [Illustration: FIG. 29.—Diagram to show the use of T symbols to indicate the dip and strike of outcroppings.]