PHYSICAL FEATURES OF COLORADO VALLEY. 389 



are other valleys, extending to the Green, on the south side of the 

 range. Then, why did the river run through the mountains ? 



The first explanation suggested is, that it followed a previously- 

 formed fissure through the i-ange ; but very little examination will 



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show that this explanation is unsatisfactory. The proof is abundant 

 that the river cut its own channel ; that the cailons are gorges of cor- 

 rasion. Again, the question returns to us, Why did not the stream 



