BAD LANDS 



79 



torrential showers sometimes occur ; but the absence of vegeta- 

 tion is favourable to its efficiency, and the present aridity of the 

 climate is not of very long standing, from a geological point of 

 view. The chemical action of the rain has disintegrated the 

 rocks by dissolving out the calcareous cement, and then the debris 

 so formed has been mechanically washed away. 



At the present time the action of the rain is very slow, because 

 the debris which covers the sides of the cliffs and slopes is almost 



Fig. 23. — Bad lands of South Dakota. (Photograph by Williston.) 



impervious to water, and holes left by the excavation of fossil 

 skeletons often remain visible for many years ; but where the 

 bare rock is exposed, the disintegration often proceeds with ex- 

 traordinary rapidity, and a single shower will produce notable 

 effects. The different layers of rock resist decay differently, and 

 even in the same bed some parts are much more durable than 

 others. This differential weathering has resulted in that remarka- 

 ble variety and grotesqueness of form, resembling the ruins of 

 gigantic towers and castles, for which the bad-land scenery is 

 famous. The sculpture of the rain produces this variety in ac- 



