DISLOCATIONS OF STRATA. 



107 



One of the simplest of these effects is the entire removal of the 

 rocks over wide intervals, so that the continuation of a stratum is 

 met with many miles distant, as in figs. 103, 104. 



The result is more troublesome among the flexed or folded strata. 

 A series of close flexures, like fig. 105, worn off at top down to the 

 line a b, loses all appearance of folds, and seems like a series of layers^ 



Fig. 105. 



Fig;. 106 a. 



Fig. 106 b. 



12 3 3'2' V 1 2 33' 2' V 



dipping in a common direction. This is best seen from a single fold 

 (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 1' 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 num- 

 ber 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 Pennsylvania. On this point special facts are mentioned in 

 the chapter on the Coal formation. 



Other effects of denudation are exemplified in the sketch fig. 98. 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. to the left is 

 the same with IV. to the right; but they are widely disjoined and very different 

 in direction. Again, V. lies upon the top of the highest summit, nearly hori- 

 zontally, 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. 



116. There is difficulty also in ascertaining the true dip of strata 

 from exposed sections. In fig. 107, stur is the upper layer of an 

 outcropping 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 



