98 



LITHOLOGICAL GEOLOGY. 



with the ridge-line or anticlinal axis (ab in A) inclined. In A the section is vertical; 

 but, to obtain from the measurement of the exposed edges the true dip, it should have 



Fig. 108. 



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. 



If the axis of the fold make a very small angle with the horizon, then the two sides 

 in a horizontal section (such as may result from denudation) will be much elongated as 

 in Fig. 108 I, instead of short as in Fig. C ; and if the axis is horizontal the two sides 

 will not meet at all, and the fact of the existence of a fold is not apparent. Even in 

 the former case there might be difficulty in determining the fact of a fold, if the part 



Fig. 108 I. 



where the sides unite were concealed from view by the soil or otherwise. But in each 

 case there may be evidence of a fold in the order of the beds on the two sides; for this 

 order on one side would be just the reverse of that on the other. »If, in Fig. 108 I, 

 c c represent a coal or iron-ore bed having its border d more impure than the rest, this 

 border, if it were on the east side in one half of the fold, would be on the west side in 

 the other half. 



The difficulties in the way of correct observation on folded rocks are further en- 

 hanced when the axial plane of the fold is inclined, — especially when it is so inclined 

 that both sides of the fold have the same dip (Fig. 106 a). Still closer study is required 

 when several folds are irregularly combined, as is common in nature. 



This important subject maybe further studied by uniting sheets of differently-colored 

 card-board together, bending them into a fold, and then cutting them through in 

 different directions. 



Distortions of fossils. — Uplifts of the rocks, besides disturbing the 

 strata themselves, cause distortion in imbedded fossils, — either (1) a 

 flattening from simple pressure ; or (2) an obliquity of form ; or 

 (3) a shortening ; or (4) an elongation. 



