278 



PHYSICAL GEOLOGY 



which may be of microscopic size. Compression tends also 

 to metamorphose the rocks, and usually the amount of 

 change which they have undergone corresponds to the in- 

 tensity of the folding. 



The present topography of most folded mountains for 

 example, of the Juras and Appalachians is not controlled by 



FIG. 292. Structure section in the Appalachian Mountains. Eastern 

 Tennessee, western North Carolina. (From Greeneville, Tenn. N. C., 

 Geologic Folio, U.S. Geol. Sure.') 



the original folding and faulting, but by subsequent erosion 

 and diastrophism. In many cases the crests of the anticlines 

 were weaker than the synclines, for their rocks were stretched 

 by the folding, and joints and other openings were widened, 

 while the rocks of the synclines were compressed and strength- 

 ened ; and so the anticlines have often come to form the valleys, 

 while the synclines, originally the valleys, constitute the 



FIG. 293. Cross section of a portion of the Appalachian Mountains, showing 

 synclinal ridges and anticlinal valleys. (Rogers.) 



ridges (Fig. 293). The development of the present topog- 

 raphy of the Appalachian Mountains (Plate XVI) was dis- 

 cussed in a general way on page 153 in connection with cycles 

 of erosion. 



It is particularly to be noted that the formation of folded 

 mountains, and indeed of all mountains, is an extremely 

 slow process, probably occupying, in the case of the greater 

 ranges, hundreds of thousands or even millions of years. 

 Many mountains appear to be growing now, for example 

 the Sierras and the St. Elias Range. 



