SUMMARY. 77 



the lower Colorado ^ constantly changing not only its hed but also its numerous bends. Below 

 the month of the Gila there is but one place where the river remains unchanged, which is so 

 reuiiirkable a fact that the navigators of this river named it the " Permanent Bend." 



Coubidering such facts, we cannot doubt that the regions here spoken of have not yet passed 

 through ail the pha.ses oi their destiny. We do not, however^ believe any general and violent 

 catastrophe indispensable tor further geological developments. A long continuance and perhaps 

 imperceptible rising of the country, a simple increase of elevation, and especially an enlarge- 

 ment of the angle of grade by which the horizontalism of the quaternary main would be dis- 

 turbed, should it become subjected to these forces, would aid the torrents of the mountains and the 

 sweep of serial currents to clear the surface of the country from its desert burden. 



