166 GEOLOGY 



The jointing of rocks often shows itself distinctly in the weath- 

 ered faces of cliffs (Fig. 133), especially in arid and semi-arid 

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

 and rock-waste on its surface. 



Joints in rocks may occasion the development of natural bridges. 



Fig. 131. Figure showing crenate river bank, the re-entrants being deter- 

 mined by joints. Dells of the Wisconsin River, near Kilbourn, Wis. 

 (Atwood.) 



Fig. 132. An island formed by river erosion in jointed rock; Lower Dells 

 of the Wisconsin. (Atwood.) 



