Position of Strata in Stratified Rocks. 369 



first to be projected vertically as well as the angle Gab, which 

 the seam having for its horizontal projection the line A B makes 

 with the horizon. 



The known angle ECF, which is seen drawn on the hori- 

 zontal plane, is to be projected vertically ; for which operation it 

 is necessary that it should be made to turn upon DC till it 

 should be returned in its true position, then the point E will be 

 projected horizontally in F and vertically in e. The line ef will 

 be the vertical projection of EF, and the angle e c/will be the 

 vertical projection of the angle ECF. 



Two vertical planes are then drawn through the horizontal 

 projections of the seams, and the intersection of these two planes, 

 which is a line perpendicular to the horizontal plane, is vertically 

 projected according to the line m n. The point where the line 

 m n meets with the line a b will be the vertical projection of the 

 point where the two horizontal projections of the seams meet. 



The point s being common to the two lines, the angle e cf 

 is made to slide till the line b c meets the point *. The angles 

 will be then in the same plane, without their size being altered. 



The point G being projected in g, the point where the seam, 

 having for its horizontal projection the line DC, cuts the horizon- 

 tal plane, will be found, and Ag will be a horizontal line drawn 

 upon the plane passing through the two seams, or, in other words, 

 will be the true direction of the strata. 



Nothing remains now but to find the angle of dip, which 

 will be easily obtained in drawing a line perpendicular to the 

 direction or line Ag, and passing through the line of intersection. 

 Its horizontal projection will be found in drawing from the point 

 N the perpendicular NP, so that a triangle will be formed, of 

 which two sides are known, viz. SN and PN, which will be 

 brought to np, and the angle Sp n will be the angle made with 

 the horizon by the plane passing through the two seams. PN 

 will be the horizontal projection of the dip. 



