DISLOCATIONS OF STRATA. 109 



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 (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 deter- 

 mining the fact of a fold, if the part 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 in the two sides ; for this order on one side would 



Fig-. 108 I. 



be just the reverse of that on the other. If, in fig. 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 

 enhanced 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 com- 

 mon in nature. 



This important subject may be further studied by uniting sheets of different- 

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

 through in different directions. 



118. Distortions of fossils. — These uplifts of the rocks, besides 

 disturbing the strata themselves, cause distortion also in im- 

 bedded fossils, — either (1) a flattening from simple pressure, or, 

 in addition (2), an obliquity of form, or else (3) a shortening, or (4) 

 an elongation. 



The following figures, from a paper by D. Sharpe, illustrate some of these dis- 

 tortions occurring in a slate rock in Wales. They represent two species of shells, 

 the Spirifer disjunctus (figs. 1 to 4) and the Spirifer giganteus (figs. 5 to 8). 

 Fig. 1 is the natural form of S. disjunctus ; the others are distorted. The lines 

 z z show the lines of cleavage in the slate; 2 lies in the rock inclined 60° to the 

 planes of cleavage, and is shortened one-half; 3 lay obliquely at an angle of 10° 

 or 15° ; it is shortened above the middle and lengthened below it; 4 is a cast, 

 the upper part pressed beneath that shown, while the lower is much drawn out; 

 5 is like 3, the angle with the cleavage-plane being less than 5°; the lower part 

 has lost its plications by the pressure and extension ; 6 has a similar angle to the 

 cleavage-plane, but a different position ; 7 intersects the cleavage-plane at only 

 1°, and its lower part is very much prolonged. Compression, a sliding of the 

 rock at the cleavage-planes, and more especially a spreading of the rock itself 



