100 



GEOLOGY. 



as to destroy its canyon character. This canyon is often called a 

 gorge, a term frequently applied to small valleys of the canyon type. 



Plate X shows portions of the canyons of the Yellowstone and 

 the Colorado rivers respectively. In the first the contour interval 

 is 100 feet, and in the second, 250 feet. The horizontal scale is -i-g-^Vo^ 

 (about 2 miles to the inch) in the first, and stoVo" ir ^^ ^^^ second. These 

 scales should be borne in mind in interpreting the map. 



Falls, rapids, narrows, and other peculiar features, due primarily 

 to inequalities in the hardness of the rock affected by erosion, will be 

 considered later. 



THE STRUGGLE FOR EXISTENCE AMONG VALLEYS AND 



STREAMS. 



It is not to be inferred that every gully becomes a valley, nor that 

 every small valley becomes a large one. Among valleys, as among 

 living things, there is a struggle for existence, and fitness determines 



Fig. 84. — A surface illustrating the struggle for existence among gullies. Most of 

 the smaller gullies shown on the slope can have but little growth before being ab- 

 sorbed by their larger neighbors. A type of erosion surface common in the Bad 

 Lands. Scott's Bluff, Neb. (Darton, U. S= Geol. Surv.) 



