TIME AND CHANGE 



the force that cuts downward. There is convincing 

 evidence that the whole region has been many times 

 Hfted up since the cutting began, so that the river 

 has had its active and passive stages. As its channel 

 approached the sea level, its current would be much 

 less rapid, and the downward cutting would prac- 

 tically cease, till the section was elevated again. But 

 all the time the forces working laterally would be at 

 work without interruption, and would thus gain on 

 their checked brethren of the river bottom. 



There is probably another explanation of what we 

 see here. Apart from the mechanical weathering of 

 the rocks as a result of the arid climate, wherein 

 rapid and often extreme changes of temperature 

 take place, causing the surface of the rocks to flake 

 or scale off, there has doubtless been unusual chem- 

 ical weathering, and this has been largely brought 

 about by the element of iron that all these rocks 

 possess. Their many brilliant colors are imparted 

 to them by the various compounds of iron which 

 enter into their composition. And iron, though the 

 symbol of hardness and strength, is an element of 

 weakness in rocks, as it causes them to oxidize or 

 disintegrate more rapidly. In the marble canon, 

 where apparently the rock contains no iron, the 

 lateral erosion has been very little, though the river 

 has cut a trench as deep as it has in other parts of its 

 course. 



How often I thought during those days at the 



64 



