8o4 



PRINCIPLES OF STRATIGRAPHY 



increase or decrease according as the direction varies toward or 

 away from that of the incHned strata. Thus the more nearly the 

 direction of slope of the inclined surface approaches that of the 

 inclined strata, the more nearly will the amount of deflection ap- 

 proach 90 degrees,, while the more the direction of slope approaches 

 the opposite of that of the strata the more nearly will the true strike 

 be approached. The following formulas will serve in such a case. 

 In Fig 188. 



Let A D represent the outcrop of a stratum along a sloping hill- 

 side: 



D B represent the dip of the stratum. 



a :=outcrop dip, or the angle CAD made by the line of out- 

 crop A D with a horizontal plane obtained by placing the 

 clinometer on the line of outcrop A D. 



j8 = the angle of dip of the stratum, B D K or C B D in 

 diagram. 



y = the apgle by which the outcrop is shifted by the slope of the 

 hill (ACE). . 



„, . tan a 



ihen sm 7 = ^ ^ or y 



tan /? 



sm 



^ / tan a\ 

 ~ \ tan /?/ 



tan a 



, „ tan a ^, , 



and tan p = — or p = tan — * 



sm y \ sm y 



* It should be noted that, as viewed from above, the deflection is in the 

 direction of the dip, but, as viewed from below, looking up the plane, the 

 deflection is in the opposite direction. This must ])e borne in mind when 

 the compass direction is read ; that on a northward sloping plane will be 

 read from above, that on a southward sloping plane from below. 



