academy of scences] WHEELER SURVEY 41 



The similarity of the results here presented with those reached by Powell is manifest, and 

 it is explained in Gilbert's reprint of his reports by an interleaved addition to a footnote on 

 page 70, in which the localities that he visited are named as f ollows : 



I take this occasion to specify the data acquired by my own party that I may at the same time credit to the 

 observations and photographs of my friend Professor Powell and his party whatever other information is commu- 

 nicated in these pages upon the Grand Canon. We have interchanged ideas so freely in conversation that I find 

 it impossible in writing to avoid basing conclusions in part upon his unpublished material. 



The intimate relations begun between these two explorers when they were members of 

 different surveys was long continued. 



GRADED RIVERS 



The conception of a river at "grade" — to introduce at once a term that Gilbert himself 

 suggested some 30 years later — is recognized as involving a permanently maintained rather 

 than a slowly varying gradient: As "the river sinks deeper below the plateau, there will accom- 

 pany a gradual diminution of the inclination of its bed, of the velocity of its current, and, in 

 consequence, of its erosive power, until finally it can no longer clear its bottom of introduced 

 detritus, and, its downward progress being arrested, the widening of the channel will begin" 

 (75). The fact that the load of detritus to be transported will decrease in quantity and in 

 coarseness of texture with the advancing age of the river and thus permit a long-continued 

 diminution of its fall was not perceived. Indeed, in view of the statement that the amount of 

 detritus "abraded from the bottom of the canon is too insignificant to demand more than men- 

 tion " in comparison with that which is received by rock falls and slides from the walls and by 

 inwash from the side streams, Gilbert might be supposed to have thought that the"arrest of 

 downward erosion is already nearly reached by the Colorado ; but on this point his opinion was 

 clearly otherwise : 



Of the time that will elapse before this consummation we can form little conception, but it can hardly be 

 less than that consumed in the excavation already accomplished, so slowly will the work proceed as it approaches 

 completion. 



He then turns to a quantitative consideration of the duration of river erosion, a subject 

 which he had already treated at Cohoes, in New York, and to which he returned in later years 

 at Niagara. 



Of the time already consumed [in the erosion of the Colorado Canyon] we may some time have an approxi- 

 mate estimate in years, for so rapidly does the sand carve away the rock, that I believe it perfectly feasible to 

 ascertain its rate by observation, and, by considering what part of the rock-bed is exposed and what protected, 

 to assign, within reasonable limits, the present rate of degradation of the canon. To pass from this to the 

 average past rate would require the consideration of somewhat involved conditions, and the result would not 

 be so satisfactory as that obtained from the secession [recession?] of Niagara Falls, but it would be of great 

 interest to obtain even a crude estimate in centuries of a period of time commencing, as I believe, before the 

 close of the Tertiary age (75) . 



Two comments are suggested by the above extracts. First, that the brief mentions of 

 Hall and Niagara, both known in Gilbert's earlier experience, are among the few references 

 made to the studies of other observers or to the features of other rivers. Second, that several 

 of the principles concerning the erosional activities of rivers, which are enunciated as if they 

 were novelties, had been previously recognized and announced by European observers. Hence 

 in departing from a narrative of his journey and adopting a more generalized form of report, 

 the young geologist incurred responsibilities that he did not altogether meet. But it is an old 

 story that our earlier scientific explorers of the West were so engrossed with the results of 

 their own observations that they had little or no time to explore the results already gained 

 by other explorers in foreign fields. 



CLIFFS AND SLOPES IN CANTON WALLS 



Although not often quoted, Gilbert's explanatory account of the relations between the 

 stronger and weaker strata of the plateau and the cliffs and slopes in the side walls of the Colorado 

 Canyon, published in the same year, 1875, with Powell's discussion of the same subject in his 



