334 MOUNTAIN RANGES 



little or no loss from denudation, though in some cases the 

 region which subsequently was upheaved into the range had its 

 oscillations of level, recorded now in unconformities. This may- 

 be seen, for example, in the Ouachita range of Arkansas. 



Another well-nigh universal fact concerning the structure of 

 mountain ranges is the intense folding or plication of their strata, 

 often accompanied by great thrust faults. The degree of plica- 

 tion varies much in different ranges. The Uinta Mountains are 

 formed by a single great and gently swelling arch of strata, faulted 

 along its northern slope. So gentle is the curvature of the beds 

 that in a single view they often seem to be quite horizontal. Much 

 more commonly the strata are thrown into a series of parallel 

 folds, which sometimes are open, upright, and symmetrical, as in 

 the Jura Mountains of Switzerland ; its folds are so symmetrical 

 and regular that a section across the parallel ridges looks like a 

 diagram. This comparatively gentle folding is, however, not the 

 rule, but rather an intense compression and plication. The Appa- 

 lachians are thrown into closed, asymmetrical, and overturned folds, 

 with frequent great thrust faults (see Fig. 93, p. 238). The Sierra 

 Nevada is so intensely plicated that the thickness of its strata has 

 not yet been estimated. The Alps have undergone such enormous 

 compression that many of the ridges are in the form of fan folds 

 {i.e. the anticlines are broader at the crest than at the base), 

 while others have been pushed over to an inverted position. 

 The combination of this violent contortion with faults and thrusts 

 often results in an indescribable confusion and chaos of forms, 

 which it is exceedingly difficult to comprehend. 



The two main characteristic features of mountain ranges are, 

 then, the immense thickness of the strata of which they are made, 

 and the compression and folding or faulting which they have un- 

 dergone. Certain minor structures which accompany these more 

 striking features should, however, not be overlooked. In the first 

 place, the folded strata of mountain ranges are very generally 

 cleaved, or fissile, or both, the planes of cleavage or fissility 

 running parallel with the axes of the folds. (2) The major 

 folds are themselves composed of successive series of minor folds 



