136 



GEOLOGY. 



resistance is occasioned by unequal decay of the rock, due perhaps to 

 the rise of hot vapors which have decomposed the rock along the lines 

 of their ascent ; in the latter, a more resistant sort of igneous rock overlies 

 a less resistant. 



Structural features, such as jointing, sometimes give rise to falls, 

 or determine their distinctive features (Fig. 117), even where the forma- 

 tions involved are of uniform hardness. A joint plane has the effect 

 of a weak vertical or highly inclined bed. If an open joint is discovered 

 in a stream's bed, the water enters it. If it finds an outlet below, a 

 channel is worn along the new line of flow, with rapids or falls where 

 the water descends. Rock originally homogeneous may be much 

 fractured in some parts, while it remains unbroken in others. Where 

 a stream passes from the solid to the broken portion rapids, or even 

 falls, may develop. 



Fig. 115. — Diagram illustrating the development of falls over a vertical hard layer. 



, Sea L euel ' 



Fig. 116. — Diagram illustrating the possibility of falls where the beds dip 



down-stream. 



Falls may originate in still other ways. If for any reason a stream 

 is forced out of its valley, it may in its flow find entrance to another 

 valley, or to another part of its own valley, over a steep slope. If 

 the structure of the slope favors, a fall may speedily develop. The 

 Falls of St. Anthony are an example, the Mississippi having been turned 

 out of its earlier course by deposits of glacial drift. Again, if an ob- 

 struction of any sort, such as a flow of lava, dams a stream, rapids 

 or falls are developed where the water overflows the dam. When a 

 main valley is notably deepened by glaciation the drainage from 

 tributary valleys may fall into it, if the tributaries were not equally 

 deepened. Falls which originated in this way are common in the 

 western mountains of the United States, as well as in most mountain 

 regions recently affected by local glaciers (Fig. 118). 



