MOUNTAIN SCULPTURE. 



269 



in the subjoined section of the Appalachian coal-field in Pennsylvania 

 (Fig. 238). This is usually explained by supposing that the backs of 



Fig. 238.— Section of Coal-Field of Pennsylvania (after Lesley). 



the anticlinals have been broken or loosened by tension in bending ; 

 while the synclinals have been hardened by lateral pressure — and there- 

 fore the anticlinals have yielded more easily to erosion. But Prof. 

 Davis has shown * that such a supposition is at least not necessary. 

 For example : if we have a series of undulating strata, some hard 

 and some soft (Fig. 239), the erosion will be most rapid on the anti- 



Fig. 239.— Ideal Diagram showing how, according to Davis, Synclinal Ridges are formed: full 

 lines, actual surfaces and structure; dotted lines, original surfaces and structure; broken lines, 

 former erosion-surfaces. 



clines and the hard stratum (a) will be reached and cut through first 

 there. As soon as the soft stratum beneath is reached the erosion will be 

 still more rapid, and valleys will be formed. This will be understood 

 by careful inspection of the figure. 



3. Strongly-folded or Highly -inclined Outcropping Strata. — These 

 give rise to sharp ridges and valleys, the ridges being determined by 



Fig. 240.— Ideal Section across an Eroded Fold, consisting of Alternating Soft and Hard Strata 

 (after NSe and De Margerie). 



the outcrop of a hard stratum. Fig. 240 is an ideal diagram, showing 

 how such ridges are formed by erosion. In the Eocky Mountains, where 

 they are finely shown on the flanks of the mountains, they are called 

 " hog-lacks." Fig. 241 represents this form of sculpture as it often 

 appears. It is seen that every ridge is formed by outcrop of a hard 

 sandstone, which has resisted erosion more than the intervening strata. 

 Beautiful examples of this form are seen in parts of the Appalachian. 



Science, vol. xii, p. 320. 



