158 GRAND CANON DISTRICT. 



it is soft, and there will result inequalities of grade. But so soon as there is inequal- 

 ity of grade there is inequality of velocity and inequality of capacity for corrasiou; 

 and where hard rocks have produced declivities, there the capacity for corrasion will 

 be increased. The differentiation will proceed until the capacity for corrasion is every- 

 where proportional to the resistance to be encountered; that is, until there is equi- 

 librium of action. 



In general we may say that a stream tends to equalize its work in all parts of its 

 course. Its power inheres in its fall, and each foot of fall has the same power. When 

 its work is to corrade and the resistance unequal, it concentrates its energy where the 

 resistance is great by crowding many feet of descent into a small space, and diffuses it 

 where the resistance is small by using but a small fall in a long distance. When its 

 work is to transport, the resistance is constant and the fall is evenly distributed by a 

 uniform grade. When its work includes both transportation and corrasion, as in the 

 usual case, its grades are somewhat unequal and the inequality is greatest when the 

 load is least. 



The foregoing analysis is applicable to the Colorado. It is, in respect 

 to corrasion, an exceptional river. Nearly all the large rivers of the world 

 along their lower and middle courses have either reached, or closely ap- 

 proximated to, that condition of equilibrium which Mr. Gilbert speaks of, 

 in which the transporting power is nearly adjusted without excess to the 

 load to be carried. They have little or no surplus energy to spare for 

 corrasion, and therefore neither corrade nor deposit. But the Colorado 

 is corrading rapidly, and has doubtless done so with little interruption 

 throughout the entire period of its existence. The cause may be dis- 

 cerned in one important fact already brought out. The region it tra- 

 verses has been throughout Tertiary time steadily rising, and the total 

 elevation has been enormous. This progressive elevation has antag- 

 onized the tendency of the river to reach that adjustment of its energy 

 to the work of transportation alone, and has kept alive its corrasive 

 power. There have been probably some limited periods in the history 

 of the river in which, for the time being, it had sunk its channel until it 

 reached a " base- level " — a grade below which it could not corrade. But 

 this state of affairs was afterwards subverted by a further elevation 

 which increased the declivities of the channel, restoring the corrasive 

 power. The last great upheaval, exceeding in amount 3,000 feet, was of 

 comparatively recent occurrence, and the river has not yet reached the 

 new equilibrium of action and the new adjustment of its energy to the 

 work of simple transportation. 



The reader who for the first time is brought to consider the enormous 

 depth of the gash which the Colorado has cut would naturally turn to 

 the rivers with which he is familiar to inquire whether they disclosed 

 evidence of similar and commensurate action. He would rarely find 

 any such evidence. It is only by examining the physical conditions of 

 the Colorado and comparing them with other rivers in the light of such 

 principles as Mr. Gilbert has laid down that the facts become intelligible. 

 The first and most important factor to be considered is its declivity. The 

 fall of the Colorado and its principal fork, the Green River, from Green 

 River Station, on the Pacific Railway, to the end of the Grand Canon, a 



