THE WORK OF RUNNING WATER. 



151 



reentrants being located at the joints. By working into and widening 

 joints, running water sometimes isolates masses of rock as islands 

 (Fig. 139). In a region free from mantle rock, or where the mantle rock 

 is meagre, joints often determine the courses of valleys by directing 

 the course of surface drainage. This is shown in many parts of the 

 arid west. In regions where the rocks are notably faulted, the courses 

 of the streams are sometimes controlled by the courses of the fault planes. 

 This is the case, for example, in central Washington.^ 



Fig. 137. — Shows the sagging of beds along joints. The disturbance does not extend 

 far below the surface. Cook's quarry (Niagara limestone) near La Salle, Niagara 

 Co., N. Y. (Gilbert, U. S. Geol. Surv.) 



The jointing of rocks often shows itself distinctly in the weathered 

 faces of cliffs (Figs. 140 and 141), especially in arid and semi-arid re- 

 gions, or where the slope is too steep for the accumulation of soil and 

 rock- waste on its surface. 



If a stream flowing over jointed rock has falls, the conditions are 

 sometimes afforded for the development of an exceptional and strik- 

 ing scenic feature. If above Niagara Falls, for example, there were 



* Russell. Rivers of North America, p. 280. The influence of joints on drain- 

 age is further discussed by Hobbs, Jour. Geol., Vol. IX, p. 469. 



