196 STJRUCTUKAL GEOLOGY. 



finely-comminuted load to that high degree of declivity which permits, the 

 load to be moved by its own gravity, when the effect of declivity is again 

 multiplied at a still higher rate until vertically is reached and undermining 

 begins and another multiplication of the effect of declivity ensues. 



There are conditions of degradation in the extremes of declivity worthy 

 of mention. In high degrees of declivity transportation in a horizontal 

 direction is limited, the cliffs soon tumble down, and degradation by this 

 process ceases. In a very low degree of declivity approaching horizontally 

 the power of transporting material is also very small. The degradation of 

 the last few inches of a broad area of land above the level of the sea would 

 require a longer time than all the thousands of feet which might have been 

 above it, so far as this degradation depends on mechanical processes that 

 is, driving or flotation; but here the disintegration by s.olution and the 

 transportation of the material by the agency of fluidity come in to assist 

 the slow processes of mechanical degradation, and finally perform the chief 

 part of the task. 



We may now conclude that the higher the mountain, the more rapid its 

 degradation ; that high mountains cannot live much longer than low mount- 

 ains, and that mountains cannot remain long as mountains ; they are ephem- 

 eral Topographic forms. Geologically all existing mountains are recent; 

 the ancient mountains are gone. But existing mountains may be old or 

 young as compared with other existing mountains. We may speak of the 

 age of mountains, referring to the age of the rocks of which they are com- 

 posed, but this will have no reference to the age of the mountain form. We 

 may speak of the age of a mountain with respect to the inception of the up- 

 heaval which exposed the rocks to that degradation which has produced the 

 mountain form; and this epoch will not be very long ago, geologically, for 

 the rate of upheaval must be greater than the rate of degradation, else 

 mountain forms will not be produced. We may speak of the age of mount- 

 ains, referring to the completion of the upheaval by which the mountain 

 forms were produced through degradation ; the time which has elapsed since 

 the epoch to which we then refer must be short indeed; but if, in speaking 

 of the age of mountains, we refer to the time when those topographic forms 

 were produced, they are all newly born. 



