DISLOCATION OF STRATA. 97 



(Fig. 106 a). If the part above the line a b were absent, the five 

 layers would seem to be a single regular series, with 1 as the top layer, 

 3,3' the middle,, and 1' the bottom one ; while the fact is that 1 and V 

 are the same layer, and 3,3' is actually a double one. In a number of 

 such folds, the same layer which is made two in one fold would be 

 doubled in every other, so that in a dozen folds there would seem to 

 be twenty-four, when in fact but one. A mistake as to the order of 

 succession would therefore be likely to be made, also as to the number 

 of distinct layers of a kind, and also as to the actual thickness of the 

 middle layer. Instances of a coal-layer doubled upon itself, like 3,3', 

 and of others made to appear like many distinct layers, occur in Penn- 

 sylvania. On this point special facts are mentioned in the section on 

 the Coal formation. 



Other effects of denudation are exemplified in Fig. 98, page 93. The stratum No. 

 III. is a folded one, with its top partly removed ; the layers within a short distance dip 

 in opposite directions. The layer No. IV., as has been explained, is widely disjoined. 

 Again, V. lies upon the top of the highest summit, nearly horizontally, and in a shallow 

 basin : yet it is part of the stratum V. to the left, which is obviously much folded. The 

 observer finds it necessary to study the alternations of the beds with great care, in 

 order to succeed in throwing into system all the facts in such a region. The coal-regions 

 of Pennsylvania, the whole Appalachians, all New England, and much of Great Britain 

 and Europe, illustrate these complexities arising from flexures and denudation. 



There is difficulty also in ascertaining the true dip of strata from ex- 

 posed sections. In Fig. 107, stuv is the upper layer of an out- 



cropping ledge of rock, dp the line of dip, s t the strike. The ledge 

 shows four sections 1, 2, 3, 4. On 1, the edges have the same dip as 

 dp ; but, on 2, 3, and 4, the angle as obtained from the exposed 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. 



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 



7 



