TJieories of the Formation of Mountains, 411 



plicated or folded by the force, whatever it may have been, which eleva- 

 ted the mountains. The three folds on the right, marked a, are of the 

 type of these that Prof. Rogers calls Folded Jiexures ; 6, is an example 

 of a Normal ^e.rwre— steeper on one side than on the other ; c, is a 

 Symmetrical Jiexure^ or one which slopes equally in both directions. 

 The hollows are generally called by geologists synclinal axes and the 

 ridges anticlinal axes. 



Prof. Rogers shows tlaat on the south western side of that 

 part of the disturbed region 'now occupied by the western 

 States the flexures are broad, flat swells not sufficiently abrupt 

 or elevated to constitute mountain chains. But proceeding 

 south easterly or towards the present Atlantic ocean they become 

 more and more lofty and more closely crowded together. The 

 western undulations are symmetrical, that is to say, they exhibit 

 an equal slope on both sides but towards the east they gradually 

 become steeper on one side — then vertical and even overhanging 

 or overthrown. The steep sides are always towards the west 

 and the overthrows are also all in the same direction. 



The question to be answered is : what caused this wonderful 

 folding up of the earth's crust ? But before proceeding to give an 

 account of the various solutions of the problem that have been 

 proposed, we may state for the benefit of the non-geological 

 reader that although at the present day these mountains consist 

 of long parallel ridges they are not always the original ones. In 

 the process of curvation the strata must have been fractured 

 along the crest of each wave, and the rocks being thus broken up 

 rendered more easily operated upon by atmospheric or aqueous 

 agencies or by the action of both combined than those which formed 

 the bottoms of the valleys. In many instances it can be shown 

 that in consequence of the enormous denudation to which they 

 have been subjected the original mountains have been completely 

 worn away down to their very bases ; and further that many of 

 the finest and most fruitful valleys of the South are scooped 

 out of the foundations of the ancient hills. On the other 

 hand the bottoms of the hollows not having been so much fissured 

 have been enabled to withstand the wear and tear of nature's 

 forces until at length they constitute the crests of the ridges of 

 the present day. We have thus mountains where once there 

 were valleys and we have also valleys where of old, the moun- 

 tains stood. The general aspect of the whole region has been so 

 much changed during the long ages that have passed away since 



