CONTORTIONS OF THE STRATA 49 



thickness directly either from the depth of borings to the under- 

 lying rock, or by measurements upon steep canon walls. If the 

 beds stand vertically, the matter is exceedingly simple, for in this 

 case the thickness is the width of the outcrops of the formation 

 between the beds which bound it upon either side. In the general 

 case, in which the beds are 

 neither horizontal nor ver- 

 tical, the thickness must be 

 obtained indirectly from the 

 width of the exposures and 

 the angle of the dip. The 

 factor by which the ex- 

 posure width must be mul- 

 tiplied is known as the sine F IG . 30. Diagram to show how the thickness 

 of the dip angle (Fig. 30), of a formation may be obtained from the 



which is given with sufficient angle of the dip and the width of the ex ~ 



posures. 



accuracy for most purposes 



in the following table. It is obvious that in order to obtain 

 the full thickness of a formation it is necessary to measure from 

 the contact with the adjacent formation upon the one side to a 

 similar contact with the nearest formation upon the other. 



Natural Sines 



.00 35 .57 70 .94 



5 .09 40 .64 75 .97 



10 .17 45 .71 80 .98 



15 .26 50 .77 85 1.00 



20 .34 55 .82 90 1.00 



25 .42 60 '.87 



30 .50 65 .91 



The detection of plunging folds. When the axis of a fold is 

 horizontal, its outcrops upon a plain will continue to have the same 

 strike until the formation comes to an end. Upon a generally 

 level surface, therefore, any regular progressive variation in the 

 strike direction is an indication that the folds have a plunging 

 or pitching character. Many serious mistakes of interpretation 

 have been made because of a failure to recognize this evidence of 

 plunging folds. The way in which the strikes are progressively 

 modified will be made clear by the diagrams of Figs. 31 and 32, 



