GEOLOGICAL SECTIONS. 49 



The following method of finding by diagram, " from 

 two apparent dips the full dip and, its direction," was 

 published by Mr. W. H. Dalton, of H.M. Geol. Survey, 

 in the " Geological Magazine."* 



Problem. From two apparent dips to find the full 

 dip and its direction (fig. 17). 



Suppose two apparent dips, 41 N.W. and 33 N". 

 30 E. 



Eesult, 44 J K 18W. 



Draw B A, B c in the directions of the apparent dips ; 

 erect B D, B E vertical to B A, B c respectively, and equal 

 to each other. From D, E, draw D A, E c, making BAD, 

 B c E equal to the apparent angles whose direction is 

 shown by B A, B c respectively. Join A c, and draw B F 

 vertical to it. Draw B G parallel to A c, and equal to 

 B E ; join F G. 



Then B F is the direction of the full dip, and G F B 

 its amount. 



Proof. If A B c be placed horizontally and A B D, 

 B c E, B F G vertically, D G and E will coincide, and D A, 

 <2 F, E c and A c will be in the plane of stratification, 

 giving the apparent angles at A and c and the full dip 

 at F. In practice, the triangle B G F might be more 

 cxpeditiously constructed between B F and A c. 



From any two observed dips the amount and direc- 

 tion of the true dip may be ascertained by calculation ; 

 but for all practical purposes the results obtained by 

 diagram are sufficiently accurate and much more expe- 

 ditious. 



* Vol. x., No. 7, p. 332. 



