Since the strike is the intersection of the bedding plane with a horizontal surface, and the dip is the intersection with that particular vertical plane which gives the steepest inclination, the strike and dip are perpendicular to each other. To represent them upon maps, it is more or less customary to use the so-called T symbols, the top of the T giving the direction of the strike and the shank that of the dip. If meridians are drawn upon the map, the direction or attitude of the T can be found by the use of a simple protractor; and when entered upon the map, the exact angle of the strike may be supplied by a figure near the top of the T, and the dip angle by a figure at the end of the shank. It is the custom, also, to make the length of the shank inversely proportional to the steepness of the dip, so that in a broad way the attitudes of the strata may be taken in at a glance (Fig. 29). It is further of advantage to make the top of the T a double line, so that some symbol or color may show the correlations of the different exposures. To illustrate, in Fig. 29, the symbol marked _a_ represents an outcrop of limestone, the strike of which is 50° east of north (N. 50° E.), and the dip of which is 45° southeast. In the same figure _b_ represents a shale outcrop in horizontal beds, which have in consequence a universal strike and a dip of 0°. An exposure of limestone in vertical beds which strike N. 60° E. is shown at _c_, etc. [Illustration: FIG. 30.—Diagram to show how the thickness of a formation may be obtained from the angle of the dip and the width of the exposures.] =Measurement of the thickness of formations.=—When formations still lie in horizontal beds, we may sometimes learn their thickness directly either from the depth of borings to the underlying rock, or by measurements upon steep cañon walls. If the beds stand vertically, the matter is exceedingly simple, for in this case the thickness is the width of the outcrops of the formation between the beds which bound it upon either side. In the general case, in which the beds are neither horizontal nor vertical, the thickness must be obtained indirectly from the width of the exposures and the angle of the dip. The factor by which the exposure width must be multiplied is known as the sine of the dip angle (Fig. 30), which is given with sufficient accuracy for most purposes in the following table. It is obvious that in order to obtain the full thickness of a formation it is necessary to measure from the contact with the adjacent formation upon the one side to a similar contact with the nearest formation upon the other. _Natural Sines_ 0° .00 35° .57 70° .94 5° .09 40° .64 75° .97 10° .17 45° .71 80° .98 15° .26 50° .77 85° 1.00 20° .34 55° .82 90° 1.00 25° .42 60° .87 30° .50 65° .91