., Position of Strata in Stratified Rocks. 375 



\ 

 meter, and in consequence to the direction of the stratum ; so that 



in this case the given line, instead of rising 45 above the hori- 

 zon, ought to be horizontal. The position of the ring does not 

 then correspond to the position of the required plane. This 

 first operation teaches us two things, 1st, That the angle of dip 

 is greater than 56, so that the half ring may be made to turn up- 

 wards upon its hinge. 2d, That the semicircle ought to turn 

 upon its centre, in such a way that the index h'ne shall move 

 gradually towards the west, and afterwards towards the north, in 

 order to bring the N. b. W. line nearer to that of the true dip, 

 to get a line or seam of a greater angle of inclination. 



Let us then raise the half ring two degrees more, and turn 

 the semicircle two points of the compass towards the N.W., so 

 that the index line will correspond to the W. b. N. The dip of 

 the line corresponding to the W. b. S. will still be 56 ; and if that 

 of the line corresponding to the N. b. W. be 45, we shall have 

 found the true plane, directed from N. b, E. to S. b, W., and 

 rising 58 towards the W. b. N. Let then the protractor be 

 placed vertically on the line N. b. W., the angle found being 34 

 instead of 45, we shall have still to raise the half ring, and move 

 the semicircle in the same direction as before. 



In raising the half ring to 60, and pointing the index line 

 to W.N.W., we shall find a dip of 45 on the N. b. W. line, 

 and of 56 on the W. b. S. one, corresponding with our data. So 

 that we shall have determined the direction of the stratum to be 

 S.S.W. and N.N,E N , and its inclination 60 P rising to the W. 

 N.W. or dipping to the E.N.E. 



If the direction of the plane, or that of a horizontal seam 

 S.S.W. and N.N.E., had been one of the given lines, the opera- 

 tion would have been much simplified ; it would have been re- 

 quired only to bring the hinge in that direction, remembering, 

 however, that the other given point of the compass should be 

 comprised in the points embraced in the semicircle ; then to 



