108 



LITHOLOGICAL GEOLOGY. 



dip as dp, but on 2, 3, and 4 the angle as obtained from the ex- 

 posed edges would be different ; and on the last the edges would 

 be horizontal, or nearly so. Thus all sections except the one in the 

 direction of the true line of dip (or at right angles to the strike) 

 would give a false dip. By finding the surface of a layer exposed to 



view, the true direction of the dip or slope may be ascertained and 

 the error avoided. 



117. The following figures (fig. 108) still further illustrate this subject, by 

 showing the variations of direction that may be obtained from the sections of a 

 single folded ridge. For simplicity of explanation, the fold is supposed to be a 

 symmetrical one, though with the ridge-line or anticlinal axis (a b in A) inclined. 

 In A the section is vertical; but to obtain from the measurement of the exposed 



Fig. 108. 



3S the true dip, it should have the direction of the arrows, that is, be at 

 right angles to the strike ; for the layers fold over the ridge in this direction. 

 In B the section is very obliquely inclined; in C it is horizontal, and the edges 

 show nothing of the actual dip ; in D the section follows the line of strike ; in 

 E it is oblique behind ; in F it is an oblique section on one side ; and in G a 

 vertical section in the axial plane. All of these sections give wrong results to 

 the clinometer, — a section in the direction of the arrows in fig. A being the only 

 one in which the dip of the exposed edges is the dip of the layers or strata. 



