THE DIVINE ABYSS 



Two forces, or kinds of forces, have worked to 

 gether in excavating the canon: 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 canon 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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