RELATION OF DIP, STRIKE AND OUTCROP 805 



The following method is given by Keilhack (12:65, ^^) foi* the 

 determination of the true dip and strike when observations are pos- 

 sible only on vertical clififs or quarry walls (Fig. 189) : 



Given two dip observations on vertical quarry walls, one of 65°, 

 on a wall, the compass alignment of which is N. 45° W., and one 

 of 45° on a wall, the alignment of which is N. 65° E. Draw two 

 lines at a b and a c, the former at an angle of N. 45° W., and the 

 latter N. 65° E., so that they intersect in the point a. At the point a 

 erect perpendiculars to a b and a c. Lay off equal distances on these 

 from a, locating the points e and d, respectively. At d lay off the 



Fig. 189. 



complement of the angle observed on the wall represented by a c, 

 that is, the complement of 45°, which is 45". At c lay off the com- 

 plement of the angle observed on the wall represented by the line 

 a b, that is, the complement of 65°, which is 25°. Complete the 

 right angle triangles by continuing the hypothenuses until they meet 

 the lines a c and a b, at f and g, respectively. Join f g hy a. line 

 which represents the true strike of the strata, which if a c and a b 

 are properly oriented can be readily measured. Drop a perpendicu- 

 lar a h from a to f g. This is the direction of dip toward either 

 a or h, as the case may be. Erect a perpendicular to a h at a, and 

 lay off the length a d (^a e), on it, locating point i. Connect i and 

 //, then the angle i h a is the angle of true dip. This will be readily 

 understood if the three shaded triangles are bent at right angles to 



