WATER. 



639 



plains along the bottom, even when the water is not too rapid; and when a 

 channel is cut in granite, lateral wear is always small. 



In the more distant part of fig. 940 there is a higher level of rock, — the 



Fig. 940. 



■■■* 





Canon of the Colorado near its junction with Green River. 



overlying gypsiferous red sandstone (Triassic or Jurassic, p. 417). It is in 

 isolated tables, and in some places in columns, needles, and towers, the greater 

 part of the formation having been swept off by erosion, due partly at least to 

 fresh waters. Still farther to the east, beyond the range of the view, another 

 still more elevated level is formed by Cretaceous strata : the existing surface- 

 features are similar to those of the older red sandstone. 



Owing to the rapid increase of ratio in the power of running 

 water attending increase of velocity, the eroding action of water 

 during freshets becomes immense. 



Many examples are on record of gorges hundreds of feet deep 

 cut out of the solid rock by two or three centuries only of work. 

 Lyell mentions the case of the Simeto in Sicily, which had been 

 dammed up by an eruption of lavas in 1603. In two and a half 



