802 



PRINCIPLES OF STRATIGRAPHY 



angle of deflection of the strike. Its projection on a horizontal 

 plane is the angle Y' X' Y. 



The amount of deflection of the strike, by a sloping surface, may 

 be seen by the following consideration : Given a stratum dipping 

 due west, at an agle of 45 degrees, a horizontal surface intersecting 

 this will show the true strike due north and south. Given a second 

 surface, whose intersection with the horizontal surface is at right 

 angles to the strike, i. e., due east and west in the given case, and 

 which also intersects the inclined plane. If this surface is tilted to 

 the vertical, i. c, if the beds are seen in vertical section at right 

 angles to the strike of the inclined bed, the outcrop or intersection 

 of this stratum with the surface will coincide with the dip, i. e., the 



line of outcrop as marked by compass direction has been deflected 

 90° by a tilting of the surface to the extent of 90°. If the surface 

 is tilted 45°, the line of intersection as marked by compass direc- 

 tion will evidently lie halfway between the two or N. 45° E. by S. 

 45°. (Fig. 187.) Thus in the model tlie shaded bed A C 

 H F is inclined at 45° from the horizontal, while the surface A B 

 C D is also inclined 45° from the horizontal, but in a direction at 

 right angles to that of the bed ACHE, i. ^./its intersection with 

 the horizontal surface is at right angles to the strike of the bed. 

 Thus, by construction, XY=:X C=XD, hence A X, being the 

 diagonal of the square A D X Y, makes an angle of 45° with 

 D X E and with the direction of C H. It is evident that the line of 

 intersection C H between the inclined bed and the horizontal surface 

 B C H G is the true strike. It is also the strike of the vertical bed 

 C H E D. The line C D, the intersection of this vertical plane with 



