HORIZONTAL SECTIONS. 



85 



quired section, from the Ml dip and deviation of its 

 direction from that of the section (fig. 20). 



Fig. 20. 



" Construct the right-angled triangle ABC, with ABC 

 equal to the full dip ; also the right-angled triangle BCD 

 with BCD equal to the deviation ; lastly the right- 

 angled triangle c D E, in which c E is equal to A c. 



" Then c D E is the required apparent angle. 



" Proof. If A B c be- a vertical plane along the full 

 dip, and c D E the vertical plane of section, BCD will be 

 a horizontal plane, and A c, c E will coincide, so that B D, 

 A E will be the plane of stratification, giving the ap- 

 parent angle c D E along the section." 



In the section, fig. 19, illustrating the geological 

 structure of the area surveyed, the boundary lines of the 

 chalk and upper green-sand would be scaled from the 

 map, or their position ascertained while chaining as the 



