46 FIELD GEOLOGY. 



found by Eules A and B may be taken figs. 11 and 

 12. 



Construct (fig. 15) a right-angled triangle, which shall 



Fig. 15. 

 represent the apparent dip on the part of an exposed 



" Draw from any point M in the 

 quarry, M N parallel to the face OB, 

 and set off M N so that dip in O B : 

 dip in O A : : O M : M N. Then 

 O N is the direction of the strike" 



In the "Geol. Magazine" for June* 

 1876, Mr. H. G. Day suggests, "In 

 the first rule (A), instead of 'the 

 number of degrees of dip/ write * the 

 tangent of the angle of apparent 

 dip.' In the second rule (B), proceed 

 as before, but measure the length 

 along one of the lines produced backwards." 



There is a slight error arising from the difference between the 

 circular measure of the angle and the tangent of that angle but 

 in most cases it may be disregarded. And instead of adopting 

 Eule B when " one observed dip inclines from, and the other to- 

 wards the angle enclosed by their lines/' it will be well to work 

 out the dip from a horizontal triangle constructed on the other 

 side of one of the lines. Both apparent dips will then run either 

 from or towards the angle, and Eule A can be applied ; as, for 

 instance, B C in the figure, instead of BOA, the angle of the 

 quarry. 



