104 



LITHOLOGICAL GEOLOGY. 



coal-bed. It is broken by the line 1 1, which is here a wide fissure 

 filled by rock, and also by r r, another fissure filled by earth from 

 above. Fig. 98 is an actual section of a part of the Appalachians, 



Fig. 98. 



I S.E. 



\i. WIYIV EI E~ 



six miles in length, showing the foldings and contortions of the 

 strata in those mountains. 



Some of the kinds of flexures and curvatures are shown in the 



Fig. 99. 



annexed figures a-e, to appreciate which it must be understood 

 that these flexures may be each from a few feet to scores of miles 

 in extent, that they form undulations over vast regions, and some- 

 times make lofty mountains. 



The two slopes of a fold may be alike; or, as in b, c, d, one 

 may be much steeper than the other. The line a x shows the 

 position of the axial plane of the fold in each case. The 

 ridge-line of a fold may be horizontal, but more commonly it is 

 inclined and reaches gradually its greatest elevation. Moreover, 

 one fold or flexure in the rocks may succeed to another, or they 

 may form interrupted series. Such are some of the various con- 

 Fig. 100. 



ditions which have been observed, especially in mountainous re- 

 gions. Fig. 100 represents a section, by Logan, from the Azoie 



