444 Wright: a new dip chart 



diagram a (fig. 2) must be used, while for C > 45° diagram b 

 (fig. 2) must be employed, and also the angles indicated in 

 parenthesis (complements of the first) . This change results from 

 the fact that the tangent values range from zero to infinity, 

 whereas the chart extends only to unity and reciprocal values 

 have to be taken for values above unity. Thus for the first 

 case (C < 45°, tan C < 1) the equation employed is 



tan C tan A 



— — — = — ; — or tan C = sin 5. tan A 



sm B 1 



while in the second case (C > 45°, tan C > 1) the equation is 

 transformed to read 



tan C tan A tan (90 - C) tan (90 - A) 



= or = — - 



1 sin 5 1 sin B 



The subtraction indicated in the last equation is accomplished 

 by reading the numbers in parenthesis on the chart. 



Two examples will suffice to indicate the method of using the 

 chart : 



(1) Find the apparent dip of a stratum, striking N 43.3° E 

 and dipping 22.2°SE, on a vertical A^-S section. In this case 

 B = 43.3°, A = 22.2°. C, which is always less than A, is accord- 

 ingly < 45° and diagram a (fig. 2) is the correct one to use. 

 From the chart we read off directly C = 15.6.° 



(2) A vein striking A^ 23° W and dipping 63° NE is crosscut 

 on a vertical E-W section. Find the inchnation, from the hori- 

 zontal, of the its trace on this section. In this instance B = 67°, 

 A =63°; from the chart we find by use of diagram b (fig. 2) 

 C = 61°. 



The chart has been found satisfactory and of great value in 

 the graphical solution of transformation equations for projec- 

 tion work in optical crystallography. It is here presented in 

 the hope that it may prove of equal service in the solution of the 

 problem to find, on a given vertical section, the apparent dip 

 of a bed or vein. 



