VALLEYS OF ELEVATION. 



valleys of elevation, though they differ in [nothing except their form 

 from craters of elevation. 



161. Elevations of the superficial crust often take place in a 

 series of parallel lines forming so many parallel ridges with inter- 

 mediate hollows, as if an upheaving force had been exerted by 

 the subjacent strata in a series of parallel directions vertically 

 under the ridges thus produced. The Jura mountains present 

 great numbers of examples of this (fig. 74). 



Fig. 74. Section ot ridges in the Jura Mountains. 



162. The changes in the condition of the earth's surface 

 attending the undulations and disruptions of its crust, which 

 have been noticed above, excite attention because of the sudden 

 catastrophes which often attend them, and the wide devastation 

 which they sometimes spread. There are, however, other agencies 

 exterior to the surface, of which the accumulated effects, produced 

 in long intervals of time, are not less important. The principal of 

 these are air, water, and heat, acting separately or together, upon 

 the solid matter of the external surface of the terrestrial crust. 

 These operate mechanically by fracture and abrasion, physically by 

 dissolution and disintegration, and chemically by decomposition. 



117 



