TIME AND CHANGE 
the force that cuts downward. There is convincing 
evidence that the whole region has been many times 
lifted 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 cafion, 
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 
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