THE DIVINE ABYSS 
Two forces, or kinds of forces, have worked to- 
gether in excavating the cafion: the river, which is 
the primary factor, and the meteoric forces, which 
may be called the secondary, as they follow in the 
wake of the former. The river starts the gash down- 
ward, then the aerial forces begin to eat into the 
sides. Acting alone, the river would cut a trench its 
own width, and were the rocks through which it saws 
one homogeneous mass, or of uniform texture and 
hardness, the width of the trench would probably 
have been very uniform and much less than it is 
now. The condition that has contributed to its 
great width is the heterogeneity of the different 
formations — some hard and some soft. The softer 
bands, of course, introduce the element of weakness. 
They decay and crumble the more rapidly, and thus 
undermine the harder bands overlying them, which, 
by reason of their vertical fractures, break off and 
fall to the bottom, where they are exposed to the 
action of floods and are sooner or later ground up 
in the river’s powerful maw. Hence the recession of 
the banks of the cafion has gone steadily on with the 
downward cutting of the-river. Where the rock is 
homogeneous, as it is in the inner chasm of the dark 
gneiss, the widening process seems to have gone on 
much more slowly. Geologists account for the great 
width of the main chasm when compared with the 
depth, on the theory that the forces that work later- 
ally have been more continuously active than has 
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