Page:Popular Science Monthly Volume 7.djvu/405

Rh 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 range; but very little examination will



show that this explanation is unsatisfactory. The proof is abundant that the river cut its own channel; that the cañons are gorges of corrasion. Again, the question returns to us, Why did not the stream